Initial commit

Signed-off-by: Mavridis Philippe <mavridisf@gmail.com>

1 files changed, 3604 insertions, 0 deletions

diff --git a/src/sqlite/pager.c b/src/sqlite/pager.c
new file mode 100644
index 0000000..ef22fd4
--- /dev/null
+++ b/src/sqlite/pager.c
@@ -0,0 +1,3604 @@
+/*
+** 2001 September 15
+**
+** The author disclaims copyright to this source code.  In place of
+** a legal notice, here is a blessing:
+**
+**    May you do good and not evil.
+**    May you find forgiveness for yourself and forgive others.
+**    May you share freely, never taking more than you give.
+**
+*************************************************************************
+** This is the implementation of the page cache subsystem or "pager".
+** 
+** The pager is used to access a database disk file.  It implements
+** atomic commit and rollback through the use of a journal file that
+** is separate from the database file.  The pager also implements file
+** locking to prevent two processes from writing the same database
+** file simultaneously, or one process from reading the database while
+** another is writing.
+**
+** @(#) $Id: pager.c,v 1.1.1.1 2006/02/03 20:35:17 hoganrobert Exp $
+*/
+#ifndef SQLITE_OMIT_DISKIO
+#include "sqliteInt.h"
+#include "os.h"
+#include "pager.h"
+#include <assert.h>
+#include <string.h>
+
+/*
+** Macros for troubleshooting.  Normally turned off
+*/
+#if 0
+#define TRACE1(X)       sqlite3DebugPrintf(X)
+#define TRACE2(X,Y)     sqlite3DebugPrintf(X,Y)
+#define TRACE3(X,Y,Z)   sqlite3DebugPrintf(X,Y,Z)
+#define TRACE4(X,Y,Z,W) sqlite3DebugPrintf(X,Y,Z,W)
+#define TRACE5(X,Y,Z,W,V) sqlite3DebugPrintf(X,Y,Z,W,V)
+#else
+#define TRACE1(X)
+#define TRACE2(X,Y)
+#define TRACE3(X,Y,Z)
+#define TRACE4(X,Y,Z,W)
+#define TRACE5(X,Y,Z,W,V)
+#endif
+
+/*
+** The following two macros are used within the TRACEX() macros above
+** to print out file-descriptors. They are required so that tracing
+** can be turned on when using both the regular os_unix.c and os_test.c
+** backends.
+**
+** PAGERID() takes a pointer to a Pager struct as it's argument. The
+** associated file-descriptor is returned. FILEHANDLEID() takes an OsFile
+** struct as it's argument.
+*/
+#ifdef OS_TEST
+#define PAGERID(p) (p->fd->fd.h)
+#define FILEHANDLEID(fd) (fd->fd.h)
+#else
+#define PAGERID(p) (p->fd.h)
+#define FILEHANDLEID(fd) (fd.h)
+#endif
+
+/*
+** The page cache as a whole is always in one of the following
+** states:
+**
+**   PAGER_UNLOCK        The page cache is not currently reading or 
+**                       writing the database file.  There is no
+**                       data held in memory.  This is the initial
+**                       state.
+**
+**   PAGER_SHARED        The page cache is reading the database.
+**                       Writing is not permitted.  There can be
+**                       multiple readers accessing the same database
+**                       file at the same time.
+**
+**   PAGER_RESERVED      This process has reserved the database for writing
+**                       but has not yet made any changes.  Only one process
+**                       at a time can reserve the database.  The original
+**                       database file has not been modified so other
+**                       processes may still be reading the on-disk
+**                       database file.
+**
+**   PAGER_EXCLUSIVE     The page cache is writing the database.
+**                       Access is exclusive.  No other processes or
+**                       threads can be reading or writing while one
+**                       process is writing.
+**
+**   PAGER_SYNCED        The pager moves to this state from PAGER_EXCLUSIVE
+**                       after all dirty pages have been written to the
+**                       database file and the file has been synced to
+**                       disk. All that remains to do is to remove the
+**                       journal file and the transaction will be
+**                       committed.
+**
+** The page cache comes up in PAGER_UNLOCK.  The first time a
+** sqlite3pager_get() occurs, the state transitions to PAGER_SHARED.
+** After all pages have been released using sqlite_page_unref(),
+** the state transitions back to PAGER_UNLOCK.  The first time
+** that sqlite3pager_write() is called, the state transitions to
+** PAGER_RESERVED.  (Note that sqlite_page_write() can only be
+** called on an outstanding page which means that the pager must
+** be in PAGER_SHARED before it transitions to PAGER_RESERVED.)
+** The transition to PAGER_EXCLUSIVE occurs when before any changes
+** are made to the database file.  After an sqlite3pager_rollback()
+** or sqlite_pager_commit(), the state goes back to PAGER_SHARED.
+*/
+#define PAGER_UNLOCK      0
+#define PAGER_SHARED      1   /* same as SHARED_LOCK */
+#define PAGER_RESERVED    2   /* same as RESERVED_LOCK */
+#define PAGER_EXCLUSIVE   4   /* same as EXCLUSIVE_LOCK */
+#define PAGER_SYNCED      5
+
+/*
+** If the SQLITE_BUSY_RESERVED_LOCK macro is set to true at compile-time,
+** then failed attempts to get a reserved lock will invoke the busy callback.
+** This is off by default.  To see why, consider the following scenario:
+** 
+** Suppose thread A already has a shared lock and wants a reserved lock.
+** Thread B already has a reserved lock and wants an exclusive lock.  If
+** both threads are using their busy callbacks, it might be a long time
+** be for one of the threads give up and allows the other to proceed.
+** But if the thread trying to get the reserved lock gives up quickly
+** (if it never invokes its busy callback) then the contention will be
+** resolved quickly.
+*/
+#ifndef SQLITE_BUSY_RESERVED_LOCK
+# define SQLITE_BUSY_RESERVED_LOCK 0
+#endif
+
+/*
+** This macro rounds values up so that if the value is an address it
+** is guaranteed to be an address that is aligned to an 8-byte boundary.
+*/
+#define FORCE_ALIGNMENT(X)   (((X)+7)&~7)
+
+/*
+** Each in-memory image of a page begins with the following header.
+** This header is only visible to this pager module.  The client
+** code that calls pager sees only the data that follows the header.
+**
+** Client code should call sqlite3pager_write() on a page prior to making
+** any modifications to that page.  The first time sqlite3pager_write()
+** is called, the original page contents are written into the rollback
+** journal and PgHdr.inJournal and PgHdr.needSync are set.  Later, once
+** the journal page has made it onto the disk surface, PgHdr.needSync
+** is cleared.  The modified page cannot be written back into the original
+** database file until the journal pages has been synced to disk and the
+** PgHdr.needSync has been cleared.
+**
+** The PgHdr.dirty flag is set when sqlite3pager_write() is called and
+** is cleared again when the page content is written back to the original
+** database file.
+*/
+typedef struct PgHdr PgHdr;
+struct PgHdr {
+  Pager *pPager;                 /* The pager to which this page belongs */
+  Pgno pgno;                     /* The page number for this page */
+  PgHdr *pNextHash, *pPrevHash;  /* Hash collision chain for PgHdr.pgno */
+  PgHdr *pNextFree, *pPrevFree;  /* Freelist of pages where nRef==0 */
+  PgHdr *pNextAll;               /* A list of all pages */
+  PgHdr *pNextStmt, *pPrevStmt;  /* List of pages in the statement journal */
+  u8 inJournal;                  /* TRUE if has been written to journal */
+  u8 inStmt;                     /* TRUE if in the statement subjournal */
+  u8 dirty;                      /* TRUE if we need to write back changes */
+  u8 needSync;                   /* Sync journal before writing this page */
+  u8 alwaysRollback;             /* Disable dont_rollback() for this page */
+  short int nRef;                /* Number of users of this page */
+  PgHdr *pDirty;                 /* Dirty pages sorted by PgHdr.pgno */
+#ifdef SQLITE_CHECK_PAGES
+  u32 pageHash;
+#endif
+  /* pPager->pageSize bytes of page data follow this header */
+  /* Pager.nExtra bytes of local data follow the page data */
+};
+
+/*
+** For an in-memory only database, some extra information is recorded about
+** each page so that changes can be rolled back.  (Journal files are not
+** used for in-memory databases.)  The following information is added to
+** the end of every EXTRA block for in-memory databases.
+**
+** This information could have been added directly to the PgHdr structure.
+** But then it would take up an extra 8 bytes of storage on every PgHdr
+** even for disk-based databases.  Splitting it out saves 8 bytes.  This
+** is only a savings of 0.8% but those percentages add up.
+*/
+typedef struct PgHistory PgHistory;
+struct PgHistory {
+  u8 *pOrig;     /* Original page text.  Restore to this on a full rollback */
+  u8 *pStmt;     /* Text as it was at the beginning of the current statement */
+};
+
+/*
+** A macro used for invoking the codec if there is one
+*/
+#ifdef SQLITE_HAS_CODEC
+# define CODEC(P,D,N,X) if( P->xCodec ){ P->xCodec(P->pCodecArg,D,N,X); }
+#else
+# define CODEC(P,D,N,X)
+#endif
+
+/*
+** Convert a pointer to a PgHdr into a pointer to its data
+** and back again.
+*/
+#define PGHDR_TO_DATA(P)  ((void*)(&(P)[1]))
+#define DATA_TO_PGHDR(D)  (&((PgHdr*)(D))[-1])
+#define PGHDR_TO_EXTRA(G,P) ((void*)&((char*)(&(G)[1]))[(P)->pageSize])
+#define PGHDR_TO_HIST(P,PGR)  \
+            ((PgHistory*)&((char*)(&(P)[1]))[(PGR)->pageSize+(PGR)->nExtra])
+
+/*
+** How big to make the hash table used for locating in-memory pages
+** by page number. This macro looks a little silly, but is evaluated
+** at compile-time, not run-time (at least for gcc this is true).
+*/
+#define N_PG_HASH (\
+  (MAX_PAGES>1024)?2048: \
+  (MAX_PAGES>512)?1024: \
+  (MAX_PAGES>256)?512: \
+  (MAX_PAGES>128)?256: \
+  (MAX_PAGES>64)?128:64 \
+)
+
+/*
+** Hash a page number
+*/
+#define pager_hash(PN)  ((PN)&(N_PG_HASH-1))
+
+/*
+** A open page cache is an instance of the following structure.
+*/
+struct Pager {
+  u8 journalOpen;             /* True if journal file descriptors is valid */
+  u8 journalStarted;          /* True if header of journal is synced */
+  u8 useJournal;              /* Use a rollback journal on this file */
+  u8 noReadlock;              /* Do not bother to obtain readlocks */
+  u8 stmtOpen;                /* True if the statement subjournal is open */
+  u8 stmtInUse;               /* True we are in a statement subtransaction */
+  u8 stmtAutoopen;            /* Open stmt journal when main journal is opened*/
+  u8 noSync;                  /* Do not sync the journal if true */
+  u8 fullSync;                /* Do extra syncs of the journal for robustness */
+  u8 state;                   /* PAGER_UNLOCK, _SHARED, _RESERVED, etc. */
+  u8 errMask;                 /* One of several kinds of errors */
+  u8 tempFile;                /* zFilename is a temporary file */
+  u8 readOnly;                /* True for a read-only database */
+  u8 needSync;                /* True if an fsync() is needed on the journal */
+  u8 dirtyCache;              /* True if cached pages have changed */
+  u8 alwaysRollback;          /* Disable dont_rollback() for all pages */
+  u8 memDb;                   /* True to inhibit all file I/O */
+  u8 setMaster;               /* True if a m-j name has been written to jrnl */
+  int dbSize;                 /* Number of pages in the file */
+  int origDbSize;             /* dbSize before the current change */
+  int stmtSize;               /* Size of database (in pages) at stmt_begin() */
+  int nRec;                   /* Number of pages written to the journal */
+  u32 cksumInit;              /* Quasi-random value added to every checksum */
+  int stmtNRec;               /* Number of records in stmt subjournal */
+  int nExtra;                 /* Add this many bytes to each in-memory page */
+  int pageSize;               /* Number of bytes in a page */
+  int nPage;                  /* Total number of in-memory pages */
+  int nMaxPage;               /* High water mark of nPage */
+  int nRef;                   /* Number of in-memory pages with PgHdr.nRef>0 */
+  int mxPage;                 /* Maximum number of pages to hold in cache */
+  u8 *aInJournal;             /* One bit for each page in the database file */
+  u8 *aInStmt;                /* One bit for each page in the database */
+  char *zFilename;            /* Name of the database file */
+  char *zJournal;             /* Name of the journal file */
+  char *zDirectory;           /* Directory hold database and journal files */
+  OsFile fd, jfd;             /* File descriptors for database and journal */
+  OsFile stfd;                /* File descriptor for the statement subjournal*/
+  BusyHandler *pBusyHandler;  /* Pointer to sqlite.busyHandler */
+  PgHdr *pFirst, *pLast;      /* List of free pages */
+  PgHdr *pFirstSynced;        /* First free page with PgHdr.needSync==0 */
+  PgHdr *pAll;                /* List of all pages */
+  PgHdr *pStmt;               /* List of pages in the statement subjournal */
+  i64 journalOff;             /* Current byte offset in the journal file */
+  i64 journalHdr;             /* Byte offset to previous journal header */
+  i64 stmtHdrOff;             /* First journal header written this statement */
+  i64 stmtCksum;              /* cksumInit when statement was started */
+  i64 stmtJSize;              /* Size of journal at stmt_begin() */
+  int sectorSize;             /* Assumed sector size during rollback */
+#ifdef SQLITE_TEST
+  int nHit, nMiss, nOvfl;     /* Cache hits, missing, and LRU overflows */
+  int nRead,nWrite;           /* Database pages read/written */
+#endif
+  void (*xDestructor)(void*,int); /* Call this routine when freeing pages */
+  void (*xReiniter)(void*,int);   /* Call this routine when reloading pages */
+  void (*xCodec)(void*,void*,Pgno,int); /* Routine for en/decoding data */
+  void *pCodecArg;            /* First argument to xCodec() */
+  PgHdr *aHash[N_PG_HASH];    /* Hash table to map page number to PgHdr */
+};
+
+/*
+** If SQLITE_TEST is defined then increment the variable given in
+** the argument
+*/
+#ifdef SQLITE_TEST
+# define TEST_INCR(x)  x++
+#else
+# define TEST_INCR(x)
+#endif
+
+/*
+** These are bits that can be set in Pager.errMask.
+*/
+#define PAGER_ERR_FULL     0x01  /* a write() failed */
+#define PAGER_ERR_MEM      0x02  /* malloc() failed */
+#define PAGER_ERR_LOCK     0x04  /* error in the locking protocol */
+#define PAGER_ERR_CORRUPT  0x08  /* database or journal corruption */
+#define PAGER_ERR_DISK     0x10  /* general disk I/O error - bad hard drive? */
+
+/*
+** Journal files begin with the following magic string.  The data
+** was obtained from /dev/random.  It is used only as a sanity check.
+**
+** Since version 2.8.0, the journal format contains additional sanity
+** checking information.  If the power fails while the journal is begin
+** written, semi-random garbage data might appear in the journal
+** file after power is restored.  If an attempt is then made
+** to roll the journal back, the database could be corrupted.  The additional
+** sanity checking data is an attempt to discover the garbage in the
+** journal and ignore it.
+**
+** The sanity checking information for the new journal format consists
+** of a 32-bit checksum on each page of data.  The checksum covers both
+** the page number and the pPager->pageSize bytes of data for the page.
+** This cksum is initialized to a 32-bit random value that appears in the
+** journal file right after the header.  The random initializer is important,
+** because garbage data that appears at the end of a journal is likely
+** data that was once in other files that have now been deleted.  If the
+** garbage data came from an obsolete journal file, the checksums might
+** be correct.  But by initializing the checksum to random value which
+** is different for every journal, we minimize that risk.
+*/
+static const unsigned char aJournalMagic[] = {
+  0xd9, 0xd5, 0x05, 0xf9, 0x20, 0xa1, 0x63, 0xd7,
+};
+
+/*
+** The size of the header and of each page in the journal is determined
+** by the following macros.
+*/
+#define JOURNAL_PG_SZ(pPager)  ((pPager->pageSize) + 8)
+
+/*
+** The journal header size for this pager. In the future, this could be
+** set to some value read from the disk controller. The important
+** characteristic is that it is the same size as a disk sector.
+*/
+#define JOURNAL_HDR_SZ(pPager) (pPager->sectorSize)
+
+/*
+** The macro MEMDB is true if we are dealing with an in-memory database.
+** We do this as a macro so that if the SQLITE_OMIT_MEMORYDB macro is set,
+** the value of MEMDB will be a constant and the compiler will optimize
+** out code that would never execute.
+*/
+#ifdef SQLITE_OMIT_MEMORYDB
+# define MEMDB 0
+#else
+# define MEMDB pPager->memDb
+#endif
+
+/*
+** The default size of a disk sector
+*/
+#define PAGER_SECTOR_SIZE 512
+
+/*
+** Page number PAGER_MJ_PGNO is never used in an SQLite database (it is
+** reserved for working around a windows/posix incompatibility). It is
+** used in the journal to signify that the remainder of the journal file 
+** is devoted to storing a master journal name - there are no more pages to
+** roll back. See comments for function writeMasterJournal() for details.
+*/
+/* #define PAGER_MJ_PGNO(x) (PENDING_BYTE/((x)->pageSize)) */
+#define PAGER_MJ_PGNO(x) ((PENDING_BYTE/((x)->pageSize))+1)
+
+/*
+** The maximum legal page number is (2^31 - 1).
+*/
+#define PAGER_MAX_PGNO 2147483647
+
+/*
+** Enable reference count tracking (for debugging) here:
+*/
+#ifdef SQLITE_DEBUG
+  int pager3_refinfo_enable = 0;
+  static void pager_refinfo(PgHdr *p){
+    static int cnt = 0;
+    if( !pager3_refinfo_enable ) return;
+    sqlite3DebugPrintf(
+       "REFCNT: %4d addr=%p nRef=%d\n",
+       p->pgno, PGHDR_TO_DATA(p), p->nRef
+    );
+    cnt++;   /* Something to set a breakpoint on */
+  }
+# define REFINFO(X)  pager_refinfo(X)
+#else
+# define REFINFO(X)
+#endif
+
+/*
+** Read a 32-bit integer from the given file descriptor.  Store the integer
+** that is read in *pRes.  Return SQLITE_OK if everything worked, or an
+** error code is something goes wrong.
+**
+** All values are stored on disk as big-endian.
+*/
+static int read32bits(OsFile *fd, u32 *pRes){
+  u32 res;
+  int rc;
+  rc = sqlite3OsRead(fd, &res, sizeof(res));
+  if( rc==SQLITE_OK ){
+    unsigned char ac[4];
+    memcpy(ac, &res, 4);
+    res = (ac[0]<<24) | (ac[1]<<16) | (ac[2]<<8) | ac[3];
+  }
+  *pRes = res;
+  return rc;
+}
+
+/*
+** Write a 32-bit integer into the given file descriptor.  Return SQLITE_OK
+** on success or an error code is something goes wrong.
+*/
+static int write32bits(OsFile *fd, u32 val){
+  unsigned char ac[4];
+  ac[0] = (val>>24) & 0xff;
+  ac[1] = (val>>16) & 0xff;
+  ac[2] = (val>>8) & 0xff;
+  ac[3] = val & 0xff;
+  return sqlite3OsWrite(fd, ac, 4);
+}
+
+/*
+** Write the 32-bit integer 'val' into the page identified by page header
+** 'p' at offset 'offset'.
+*/
+static void store32bits(u32 val, PgHdr *p, int offset){
+  unsigned char *ac;
+  ac = &((unsigned char*)PGHDR_TO_DATA(p))[offset];
+  ac[0] = (val>>24) & 0xff;
+  ac[1] = (val>>16) & 0xff;
+  ac[2] = (val>>8) & 0xff;
+  ac[3] = val & 0xff;
+}
+
+/*
+** Read a 32-bit integer at offset 'offset' from the page identified by
+** page header 'p'.
+*/
+static u32 retrieve32bits(PgHdr *p, int offset){
+  unsigned char *ac;
+  ac = &((unsigned char*)PGHDR_TO_DATA(p))[offset];
+  return (ac[0]<<24) | (ac[1]<<16) | (ac[2]<<8) | ac[3];
+}
+
+
+/*
+** Convert the bits in the pPager->errMask into an approprate
+** return code.
+*/
+static int pager_errcode(Pager *pPager){
+  int rc = SQLITE_OK;
+  if( pPager->errMask & PAGER_ERR_LOCK )    rc = SQLITE_PROTOCOL;
+  if( pPager->errMask & PAGER_ERR_DISK )    rc = SQLITE_IOERR;
+  if( pPager->errMask & PAGER_ERR_FULL )    rc = SQLITE_FULL;
+  if( pPager->errMask & PAGER_ERR_MEM )     rc = SQLITE_NOMEM;
+  if( pPager->errMask & PAGER_ERR_CORRUPT ) rc = SQLITE_CORRUPT;
+  return rc;
+}
+
+#ifdef SQLITE_CHECK_PAGES
+/*
+** Return a 32-bit hash of the page data for pPage.
+*/
+static u32 pager_pagehash(PgHdr *pPage){
+  u32 hash = 0;
+  int i;
+  unsigned char *pData = (unsigned char *)PGHDR_TO_DATA(pPage);
+  for(i=0; i<pPage->pPager->pageSize; i++){
+    hash = (hash+i)^pData[i];
+  }
+  return hash;
+}
+
+/*
+** The CHECK_PAGE macro takes a PgHdr* as an argument. If SQLITE_CHECK_PAGES
+** is defined, and NDEBUG is not defined, an assert() statement checks
+** that the page is either dirty or still matches the calculated page-hash.
+*/
+#define CHECK_PAGE(x) checkPage(x)
+static void checkPage(PgHdr *pPg){
+  Pager *pPager = pPg->pPager;
+  assert( !pPg->pageHash || pPager->errMask || MEMDB || pPg->dirty || 
+      pPg->pageHash==pager_pagehash(pPg) );
+}
+
+#else
+#define CHECK_PAGE(x)
+#endif
+
+/*
+** When this is called the journal file for pager pPager must be open.
+** The master journal file name is read from the end of the file and 
+** written into memory obtained from sqliteMalloc(). *pzMaster is
+** set to point at the memory and SQLITE_OK returned. The caller must
+** sqliteFree() *pzMaster.
+**
+** If no master journal file name is present *pzMaster is set to 0 and
+** SQLITE_OK returned.
+*/
+static int readMasterJournal(OsFile *pJrnl, char **pzMaster){
+  int rc;
+  u32 len;
+  i64 szJ;
+  u32 cksum;
+  int i;
+  unsigned char aMagic[8]; /* A buffer to hold the magic header */
+
+  *pzMaster = 0;
+
+  rc = sqlite3OsFileSize(pJrnl, &szJ);
+  if( rc!=SQLITE_OK || szJ<16 ) return rc;
+
+  rc = sqlite3OsSeek(pJrnl, szJ-16);
+  if( rc!=SQLITE_OK ) return rc;
+ 
+  rc = read32bits(pJrnl, &len);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = read32bits(pJrnl, &cksum);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3OsRead(pJrnl, aMagic, 8);
+  if( rc!=SQLITE_OK || memcmp(aMagic, aJournalMagic, 8) ) return rc;
+
+  rc = sqlite3OsSeek(pJrnl, szJ-16-len);
+  if( rc!=SQLITE_OK ) return rc;
+
+  *pzMaster = (char *)sqliteMalloc(len+1);
+  if( !*pzMaster ){
+    return SQLITE_NOMEM;
+  }
+  rc = sqlite3OsRead(pJrnl, *pzMaster, len);
+  if( rc!=SQLITE_OK ){
+    sqliteFree(*pzMaster);
+    *pzMaster = 0;
+    return rc;
+  }
+
+  /* See if the checksum matches the master journal name */
+  for(i=0; i<len; i++){
+    cksum -= (*pzMaster)[i];
+  }
+  if( cksum ){
+    /* If the checksum doesn't add up, then one or more of the disk sectors
+    ** containing the master journal filename is corrupted. This means
+    ** definitely roll back, so just return SQLITE_OK and report a (nul)
+    ** master-journal filename.
+    */
+    sqliteFree(*pzMaster);
+    *pzMaster = 0;
+  }else{
+    (*pzMaster)[len] = '\0';
+  }
+   
+  return SQLITE_OK;
+}
+
+/*
+** Seek the journal file descriptor to the next sector boundary where a
+** journal header may be read or written. Pager.journalOff is updated with
+** the new seek offset.
+**
+** i.e for a sector size of 512:
+**
+** Input Offset              Output Offset
+** ---------------------------------------
+** 0                         0
+** 512                       512
+** 100                       512
+** 2000                      2048
+** 
+*/
+static int seekJournalHdr(Pager *pPager){
+  i64 offset = 0;
+  i64 c = pPager->journalOff;
+  if( c ){
+    offset = ((c-1)/JOURNAL_HDR_SZ(pPager) + 1) * JOURNAL_HDR_SZ(pPager);
+  }
+  assert( offset%JOURNAL_HDR_SZ(pPager)==0 );
+  assert( offset>=c );
+  assert( (offset-c)<JOURNAL_HDR_SZ(pPager) );
+  pPager->journalOff = offset;
+  return sqlite3OsSeek(&pPager->jfd, pPager->journalOff);
+}
+
+/*
+** The journal file must be open when this routine is called. A journal
+** header (JOURNAL_HDR_SZ bytes) is written into the journal file at the
+** current location.
+**
+** The format for the journal header is as follows:
+** - 8 bytes: Magic identifying journal format.
+** - 4 bytes: Number of records in journal, or -1 no-sync mode is on.
+** - 4 bytes: Random number used for page hash.
+** - 4 bytes: Initial database page count.
+** - 4 bytes: Sector size used by the process that wrote this journal.
+** 
+** Followed by (JOURNAL_HDR_SZ - 24) bytes of unused space.
+*/
+static int writeJournalHdr(Pager *pPager){
+
+  int rc = seekJournalHdr(pPager);
+  if( rc ) return rc;
+
+  pPager->journalHdr = pPager->journalOff;
+  if( pPager->stmtHdrOff==0 ){
+    pPager->stmtHdrOff = pPager->journalHdr;
+  }
+  pPager->journalOff += JOURNAL_HDR_SZ(pPager);
+
+  /* FIX ME: 
+  **
+  ** Possibly for a pager not in no-sync mode, the journal magic should not
+  ** be written until nRec is filled in as part of next syncJournal(). 
+  **
+  ** Actually maybe the whole journal header should be delayed until that
+  ** point. Think about this.
+  */
+  rc = sqlite3OsWrite(&pPager->jfd, aJournalMagic, sizeof(aJournalMagic));
+
+  if( rc==SQLITE_OK ){
+    /* The nRec Field. 0xFFFFFFFF for no-sync journals. */
+    rc = write32bits(&pPager->jfd, pPager->noSync ? 0xffffffff : 0);
+  }
+  if( rc==SQLITE_OK ){
+    /* The random check-hash initialiser */ 
+    sqlite3Randomness(sizeof(pPager->cksumInit), &pPager->cksumInit);
+    rc = write32bits(&pPager->jfd, pPager->cksumInit);
+  }
+  if( rc==SQLITE_OK ){
+    /* The initial database size */
+    rc = write32bits(&pPager->jfd, pPager->dbSize);
+  }
+  if( rc==SQLITE_OK ){
+    /* The assumed sector size for this process */
+    rc = write32bits(&pPager->jfd, pPager->sectorSize);
+  }
+
+  /* The journal header has been written successfully. Seek the journal
+  ** file descriptor to the end of the journal header sector.
+  */
+  if( rc==SQLITE_OK ){
+    sqlite3OsSeek(&pPager->jfd, pPager->journalOff-1);
+    rc = sqlite3OsWrite(&pPager->jfd, "\000", 1);
+  }
+  return rc;
+}
+
+/*
+** The journal file must be open when this is called. A journal header file
+** (JOURNAL_HDR_SZ bytes) is read from the current location in the journal
+** file. See comments above function writeJournalHdr() for a description of
+** the journal header format.
+**
+** If the header is read successfully, *nRec is set to the number of
+** page records following this header and *dbSize is set to the size of the
+** database before the transaction began, in pages. Also, pPager->cksumInit
+** is set to the value read from the journal header. SQLITE_OK is returned
+** in this case.
+**
+** If the journal header file appears to be corrupted, SQLITE_DONE is
+** returned and *nRec and *dbSize are not set.  If JOURNAL_HDR_SZ bytes
+** cannot be read from the journal file an error code is returned.
+*/
+static int readJournalHdr(
+  Pager *pPager, 
+  i64 journalSize,
+  u32 *pNRec, 
+  u32 *pDbSize
+){
+  int rc;
+  unsigned char aMagic[8]; /* A buffer to hold the magic header */
+
+  rc = seekJournalHdr(pPager);
+  if( rc ) return rc;
+
+  if( pPager->journalOff+JOURNAL_HDR_SZ(pPager) > journalSize ){
+    return SQLITE_DONE;
+  }
+
+  rc = sqlite3OsRead(&pPager->jfd, aMagic, sizeof(aMagic));
+  if( rc ) return rc;
+
+  if( memcmp(aMagic, aJournalMagic, sizeof(aMagic))!=0 ){
+    return SQLITE_DONE;
+  }
+
+  rc = read32bits(&pPager->jfd, pNRec);
+  if( rc ) return rc;
+
+  rc = read32bits(&pPager->jfd, &pPager->cksumInit);
+  if( rc ) return rc;
+
+  rc = read32bits(&pPager->jfd, pDbSize);
+  if( rc ) return rc;
+
+  /* Update the assumed sector-size to match the value used by 
+  ** the process that created this journal. If this journal was
+  ** created by a process other than this one, then this routine
+  ** is being called from within pager_playback(). The local value
+  ** of Pager.sectorSize is restored at the end of that routine.
+  */
+  rc = read32bits(&pPager->jfd, (u32 *)&pPager->sectorSize);
+  if( rc ) return rc;
+
+  pPager->journalOff += JOURNAL_HDR_SZ(pPager);
+  rc = sqlite3OsSeek(&pPager->jfd, pPager->journalOff);
+  return rc;
+}
+
+
+/*
+** Write the supplied master journal name into the journal file for pager
+** pPager at the current location. The master journal name must be the last
+** thing written to a journal file. If the pager is in full-sync mode, the
+** journal file descriptor is advanced to the next sector boundary before
+** anything is written. The format is:
+**
+** + 4 bytes: PAGER_MJ_PGNO.
+** + N bytes: length of master journal name.
+** + 4 bytes: N
+** + 4 bytes: Master journal name checksum.
+** + 8 bytes: aJournalMagic[].
+**
+** The master journal page checksum is the sum of the bytes in the master
+** journal name.
+*/
+static int writeMasterJournal(Pager *pPager, const char *zMaster){
+  int rc;
+  int len; 
+  int i; 
+  u32 cksum = 0; 
+
+  if( !zMaster || pPager->setMaster) return SQLITE_OK;
+  pPager->setMaster = 1;
+
+  len = strlen(zMaster);
+  for(i=0; i<len; i++){
+    cksum += zMaster[i];
+  }
+
+  /* If in full-sync mode, advance to the next disk sector before writing
+  ** the master journal name. This is in case the previous page written to
+  ** the journal has already been synced.
+  */
+  if( pPager->fullSync ){
+    rc = seekJournalHdr(pPager);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+  pPager->journalOff += (len+20);
+
+  rc = write32bits(&pPager->jfd, PAGER_MJ_PGNO(pPager));
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3OsWrite(&pPager->jfd, zMaster, len);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = write32bits(&pPager->jfd, len);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = write32bits(&pPager->jfd, cksum);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3OsWrite(&pPager->jfd, aJournalMagic, sizeof(aJournalMagic));
+  pPager->needSync = 1;
+  return rc;
+}
+
+/*
+** Add or remove a page from the list of all pages that are in the
+** statement journal.
+**
+** The Pager keeps a separate list of pages that are currently in
+** the statement journal.  This helps the sqlite3pager_stmt_commit()
+** routine run MUCH faster for the common case where there are many
+** pages in memory but only a few are in the statement journal.
+*/
+static void page_add_to_stmt_list(PgHdr *pPg){
+  Pager *pPager = pPg->pPager;
+  if( pPg->inStmt ) return;
+  assert( pPg->pPrevStmt==0 && pPg->pNextStmt==0 );
+  pPg->pPrevStmt = 0;
+  if( pPager->pStmt ){
+    pPager->pStmt->pPrevStmt = pPg;
+  }
+  pPg->pNextStmt = pPager->pStmt;
+  pPager->pStmt = pPg;
+  pPg->inStmt = 1;
+}
+static void page_remove_from_stmt_list(PgHdr *pPg){
+  if( !pPg->inStmt ) return;
+  if( pPg->pPrevStmt ){
+    assert( pPg->pPrevStmt->pNextStmt==pPg );
+    pPg->pPrevStmt->pNextStmt = pPg->pNextStmt;
+  }else{
+    assert( pPg->pPager->pStmt==pPg );
+    pPg->pPager->pStmt = pPg->pNextStmt;
+  }
+  if( pPg->pNextStmt ){
+    assert( pPg->pNextStmt->pPrevStmt==pPg );
+    pPg->pNextStmt->pPrevStmt = pPg->pPrevStmt;
+  }
+  pPg->pNextStmt = 0;
+  pPg->pPrevStmt = 0;
+  pPg->inStmt = 0;
+}
+
+/*
+** Find a page in the hash table given its page number.  Return
+** a pointer to the page or NULL if not found.
+*/
+static PgHdr *pager_lookup(Pager *pPager, Pgno pgno){
+  PgHdr *p = pPager->aHash[pager_hash(pgno)];
+  while( p && p->pgno!=pgno ){
+    p = p->pNextHash;
+  }
+  return p;
+}
+
+/*
+** Unlock the database and clear the in-memory cache.  This routine
+** sets the state of the pager back to what it was when it was first
+** opened.  Any outstanding pages are invalidated and subsequent attempts
+** to access those pages will likely result in a coredump.
+*/
+static void pager_reset(Pager *pPager){
+  PgHdr *pPg, *pNext;
+  if( pPager->errMask ) return;
+  for(pPg=pPager->pAll; pPg; pPg=pNext){
+    pNext = pPg->pNextAll;
+    sqliteFree(pPg);
+  }
+  pPager->pFirst = 0;
+  pPager->pFirstSynced = 0;
+  pPager->pLast = 0;
+  pPager->pAll = 0;
+  memset(pPager->aHash, 0, sizeof(pPager->aHash));
+  pPager->nPage = 0;
+  if( pPager->state>=PAGER_RESERVED ){
+    sqlite3pager_rollback(pPager);
+  }
+  sqlite3OsUnlock(&pPager->fd, NO_LOCK);
+  pPager->state = PAGER_UNLOCK;
+  pPager->dbSize = -1;
+  pPager->nRef = 0;
+  assert( pPager->journalOpen==0 );
+}
+
+/*
+** This function is used to reset the pager after a malloc() failure. This
+** doesn't work with in-memory databases. If a malloc() fails when an 
+** in-memory database is in use it is not possible to recover.
+**
+** If a transaction or statement transaction is active, it is rolled back.
+**
+** It is an error to call this function if any pages are in use.
+*/
+#ifndef SQLITE_OMIT_GLOBALRECOVER
+int sqlite3pager_reset(Pager *pPager){
+  if( pPager ){
+    if( pPager->nRef || MEMDB ){
+      return SQLITE_ERROR;
+    }
+    pPager->errMask &= ~(PAGER_ERR_MEM);
+    pager_reset(pPager);
+  }
+  return SQLITE_OK;
+}
+#endif
+
+
+/*
+** When this routine is called, the pager has the journal file open and
+** a RESERVED or EXCLUSIVE lock on the database.  This routine releases
+** the database lock and acquires a SHARED lock in its place.  The journal
+** file is deleted and closed.
+**
+** TODO: Consider keeping the journal file open for temporary databases.
+** This might give a performance improvement on windows where opening
+** a file is an expensive operation.
+*/
+static int pager_unwritelock(Pager *pPager){
+  PgHdr *pPg;
+  int rc;
+  assert( !MEMDB );
+  if( pPager->state<PAGER_RESERVED ){
+    return SQLITE_OK;
+  }
+  sqlite3pager_stmt_commit(pPager);
+  if( pPager->stmtOpen ){
+    sqlite3OsClose(&pPager->stfd);
+    pPager->stmtOpen = 0;
+  }
+  if( pPager->journalOpen ){
+    sqlite3OsClose(&pPager->jfd);
+    pPager->journalOpen = 0;
+    sqlite3OsDelete(pPager->zJournal);
+    sqliteFree( pPager->aInJournal );
+    pPager->aInJournal = 0;
+    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
+      pPg->inJournal = 0;
+      pPg->dirty = 0;
+      pPg->needSync = 0;
+#ifdef SQLITE_CHECK_PAGES
+      pPg->pageHash = pager_pagehash(pPg);
+#endif
+    }
+    pPager->dirtyCache = 0;
+    pPager->nRec = 0;
+  }else{
+    assert( pPager->aInJournal==0 );
+    assert( pPager->dirtyCache==0 || pPager->useJournal==0 );
+  }
+  rc = sqlite3OsUnlock(&pPager->fd, SHARED_LOCK);
+  pPager->state = PAGER_SHARED;
+  pPager->origDbSize = 0;
+  pPager->setMaster = 0;
+  return rc;
+}
+
+/*
+** Compute and return a checksum for the page of data.
+**
+** This is not a real checksum.  It is really just the sum of the 
+** random initial value and the page number.  We experimented with
+** a checksum of the entire data, but that was found to be too slow.
+**
+** Note that the page number is stored at the beginning of data and
+** the checksum is stored at the end.  This is important.  If journal
+** corruption occurs due to a power failure, the most likely scenario
+** is that one end or the other of the record will be changed.  It is
+** much less likely that the two ends of the journal record will be
+** correct and the middle be corrupt.  Thus, this "checksum" scheme,
+** though fast and simple, catches the mostly likely kind of corruption.
+**
+** FIX ME:  Consider adding every 200th (or so) byte of the data to the
+** checksum.  That way if a single page spans 3 or more disk sectors and
+** only the middle sector is corrupt, we will still have a reasonable
+** chance of failing the checksum and thus detecting the problem.
+*/
+static u32 pager_cksum(Pager *pPager, Pgno pgno, const char *aData){
+  u32 cksum = pPager->cksumInit;
+  int i = pPager->pageSize-200;
+  while( i>0 ){
+    cksum += aData[i];
+    i -= 200;
+  }
+  return cksum;
+}
+
+/*
+** Read a single page from the journal file opened on file descriptor
+** jfd.  Playback this one page.
+**
+** If useCksum==0 it means this journal does not use checksums.  Checksums
+** are not used in statement journals because statement journals do not
+** need to survive power failures.
+*/
+static int pager_playback_one_page(Pager *pPager, OsFile *jfd, int useCksum){
+  int rc;
+  PgHdr *pPg;                   /* An existing page in the cache */
+  Pgno pgno;                    /* The page number of a page in journal */
+  u32 cksum;                    /* Checksum used for sanity checking */
+  u8 aData[SQLITE_MAX_PAGE_SIZE];  /* Temp storage for a page */
+
+  /* useCksum should be true for the main journal and false for
+  ** statement journals.  Verify that this is always the case
+  */
+  assert( jfd == (useCksum ? &pPager->jfd : &pPager->stfd) );
+
+
+  rc = read32bits(jfd, &pgno);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = sqlite3OsRead(jfd, &aData, pPager->pageSize);
+  if( rc!=SQLITE_OK ) return rc;
+  pPager->journalOff += pPager->pageSize + 4;
+
+  /* Sanity checking on the page.  This is more important that I originally
+  ** thought.  If a power failure occurs while the journal is being written,
+  ** it could cause invalid data to be written into the journal.  We need to
+  ** detect this invalid data (with high probability) and ignore it.
+  */
+  if( pgno==0 || pgno==PAGER_MJ_PGNO(pPager) ){
+    return SQLITE_DONE;
+  }
+  if( pgno>(unsigned)pPager->dbSize ){
+    return SQLITE_OK;
+  }
+  if( useCksum ){
+    rc = read32bits(jfd, &cksum);
+    if( rc ) return rc;
+    pPager->journalOff += 4;
+    if( pager_cksum(pPager, pgno, aData)!=cksum ){
+      return SQLITE_DONE;
+    }
+  }
+
+  assert( pPager->state==PAGER_RESERVED || pPager->state>=PAGER_EXCLUSIVE );
+
+  /* If the pager is in RESERVED state, then there must be a copy of this
+  ** page in the pager cache. In this case just update the pager cache,
+  ** not the database file. The page is left marked dirty in this case.
+  **
+  ** If in EXCLUSIVE state, then we update the pager cache if it exists
+  ** and the main file. The page is then marked not dirty.
+  **
+  ** Ticket #1171:  The statement journal might contain page content that is
+  ** different from the page content at the start of the transaction.
+  ** This occurs when a page is changed prior to the start of a statement
+  ** then changed again within the statement.  When rolling back such a
+  ** statement we must not write to the original database unless we know
+  ** for certain that original page contents are in the main rollback
+  ** journal.  Otherwise, if a full ROLLBACK occurs after the statement
+  ** rollback the full ROLLBACK will not restore the page to its original
+  ** content.  Two conditions must be met before writing to the database
+  ** files. (1) the database must be locked.  (2) we know that the original
+  ** page content is in the main journal either because the page is not in
+  ** cache or else it is marked as needSync==0.
+  */
+  pPg = pager_lookup(pPager, pgno);
+  assert( pPager->state>=PAGER_EXCLUSIVE || pPg!=0 );
+  TRACE3("PLAYBACK %d page %d\n", PAGERID(pPager), pgno);
+  if( pPager->state>=PAGER_EXCLUSIVE && (pPg==0 || pPg->needSync==0) ){
+    sqlite3OsSeek(&pPager->fd, (pgno-1)*(i64)pPager->pageSize);
+    rc = sqlite3OsWrite(&pPager->fd, aData, pPager->pageSize);
+    if( pPg ) pPg->dirty = 0;
+  }
+  if( pPg ){
+    /* No page should ever be explicitly rolled back that is in use, except
+    ** for page 1 which is held in use in order to keep the lock on the
+    ** database active. However such a page may be rolled back as a result
+    ** of an internal error resulting in an automatic call to
+    ** sqlite3pager_rollback().
+    */
+    void *pData;
+    /* assert( pPg->nRef==0 || pPg->pgno==1 ); */
+    pData = PGHDR_TO_DATA(pPg);
+    memcpy(pData, aData, pPager->pageSize);
+    if( pPager->xDestructor ){  /*** FIX ME:  Should this be xReinit? ***/
+      pPager->xDestructor(pData, pPager->pageSize);
+    }
+#ifdef SQLITE_CHECK_PAGES
+    pPg->pageHash = pager_pagehash(pPg);
+#endif
+    CODEC(pPager, pData, pPg->pgno, 3);
+  }
+  return rc;
+}
+
+/*
+** Parameter zMaster is the name of a master journal file. A single journal
+** file that referred to the master journal file has just been rolled back.
+** This routine checks if it is possible to delete the master journal file,
+** and does so if it is.
+**
+** The master journal file contains the names of all child journals.
+** To tell if a master journal can be deleted, check to each of the
+** children.  If all children are either missing or do not refer to
+** a different master journal, then this master journal can be deleted.
+*/
+static int pager_delmaster(const char *zMaster){
+  int rc;
+  int master_open = 0;
+  OsFile master;
+  char *zMasterJournal = 0; /* Contents of master journal file */
+  i64 nMasterJournal;       /* Size of master journal file */
+
+  /* Open the master journal file exclusively in case some other process
+  ** is running this routine also. Not that it makes too much difference.
+  */
+  memset(&master, 0, sizeof(master));
+  rc = sqlite3OsOpenReadOnly(zMaster, &master);
+  if( rc!=SQLITE_OK ) goto delmaster_out;
+  master_open = 1;
+  rc = sqlite3OsFileSize(&master, &nMasterJournal);
+  if( rc!=SQLITE_OK ) goto delmaster_out;
+
+  if( nMasterJournal>0 ){
+    char *zJournal;
+    char *zMasterPtr = 0;
+
+    /* Load the entire master journal file into space obtained from
+    ** sqliteMalloc() and pointed to by zMasterJournal. 
+    */
+    zMasterJournal = (char *)sqliteMalloc(nMasterJournal);
+    if( !zMasterJournal ){
+      rc = SQLITE_NOMEM;
+      goto delmaster_out;
+    }
+    rc = sqlite3OsRead(&master, zMasterJournal, nMasterJournal);
+    if( rc!=SQLITE_OK ) goto delmaster_out;
+
+    zJournal = zMasterJournal;
+    while( (zJournal-zMasterJournal)<nMasterJournal ){
+      if( sqlite3OsFileExists(zJournal) ){
+        /* One of the journals pointed to by the master journal exists.
+        ** Open it and check if it points at the master journal. If
+        ** so, return without deleting the master journal file.
+        */
+        OsFile journal;
+        int c;
+
+        memset(&journal, 0, sizeof(journal));
+        rc = sqlite3OsOpenReadOnly(zJournal, &journal);
+        if( rc!=SQLITE_OK ){
+          goto delmaster_out;
+        }
+
+        rc = readMasterJournal(&journal, &zMasterPtr);
+        sqlite3OsClose(&journal);
+        if( rc!=SQLITE_OK ){
+          goto delmaster_out;
+        }
+
+        c = zMasterPtr!=0 && strcmp(zMasterPtr, zMaster)==0;
+        sqliteFree(zMasterPtr);
+        if( c ){
+          /* We have a match. Do not delete the master journal file. */
+          goto delmaster_out;
+        }
+      }
+      zJournal += (strlen(zJournal)+1);
+    }
+  }
+  
+  sqlite3OsDelete(zMaster);
+
+delmaster_out:
+  if( zMasterJournal ){
+    sqliteFree(zMasterJournal);
+  }  
+  if( master_open ){
+    sqlite3OsClose(&master);
+  }
+  return rc;
+}
+
+/*
+** Make every page in the cache agree with what is on disk.  In other words,
+** reread the disk to reset the state of the cache.
+**
+** This routine is called after a rollback in which some of the dirty cache
+** pages had never been written out to disk.  We need to roll back the
+** cache content and the easiest way to do that is to reread the old content
+** back from the disk.
+*/
+static int pager_reload_cache(Pager *pPager){
+  PgHdr *pPg;
+  int rc = SQLITE_OK;
+  for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
+    char zBuf[SQLITE_MAX_PAGE_SIZE];
+    if( !pPg->dirty ) continue;
+    if( (int)pPg->pgno <= pPager->origDbSize ){
+      sqlite3OsSeek(&pPager->fd, pPager->pageSize*(i64)(pPg->pgno-1));
+      rc = sqlite3OsRead(&pPager->fd, zBuf, pPager->pageSize);
+      TRACE3("REFETCH %d page %d\n", PAGERID(pPager), pPg->pgno);
+      if( rc ) break;
+      CODEC(pPager, zBuf, pPg->pgno, 2);
+    }else{
+      memset(zBuf, 0, pPager->pageSize);
+    }
+    if( pPg->nRef==0 || memcmp(zBuf, PGHDR_TO_DATA(pPg), pPager->pageSize) ){
+      memcpy(PGHDR_TO_DATA(pPg), zBuf, pPager->pageSize);
+      if( pPager->xReiniter ){
+        pPager->xReiniter(PGHDR_TO_DATA(pPg), pPager->pageSize);
+      }else{
+        memset(PGHDR_TO_EXTRA(pPg, pPager), 0, pPager->nExtra);
+      }
+    }
+    pPg->needSync = 0;
+    pPg->dirty = 0;
+#ifdef SQLITE_CHECK_PAGES
+    pPg->pageHash = pager_pagehash(pPg);
+#endif
+  }
+  return rc;
+}
+
+/*
+** Truncate the main file of the given pager to the number of pages
+** indicated.
+*/
+static int pager_truncate(Pager *pPager, int nPage){
+  assert( pPager->state>=PAGER_EXCLUSIVE );
+  return sqlite3OsTruncate(&pPager->fd, pPager->pageSize*(i64)nPage);
+}
+
+/*
+** Playback the journal and thus restore the database file to
+** the state it was in before we started making changes.  
+**
+** The journal file format is as follows: 
+**
+**  (1)  8 byte prefix.  A copy of aJournalMagic[].
+**  (2)  4 byte big-endian integer which is the number of valid page records
+**       in the journal.  If this value is 0xffffffff, then compute the
+**       number of page records from the journal size.
+**  (3)  4 byte big-endian integer which is the initial value for the 
+**       sanity checksum.
+**  (4)  4 byte integer which is the number of pages to truncate the
+**       database to during a rollback.
+**  (5)  4 byte integer which is the number of bytes in the master journal
+**       name.  The value may be zero (indicate that there is no master
+**       journal.)
+**  (6)  N bytes of the master journal name.  The name will be nul-terminated
+**       and might be shorter than the value read from (5).  If the first byte
+**       of the name is \000 then there is no master journal.  The master
+**       journal name is stored in UTF-8.
+**  (7)  Zero or more pages instances, each as follows:
+**        +  4 byte page number.
+**        +  pPager->pageSize bytes of data.
+**        +  4 byte checksum
+**
+** When we speak of the journal header, we mean the first 6 items above.
+** Each entry in the journal is an instance of the 7th item.
+**
+** Call the value from the second bullet "nRec".  nRec is the number of
+** valid page entries in the journal.  In most cases, you can compute the
+** value of nRec from the size of the journal file.  But if a power
+** failure occurred while the journal was being written, it could be the
+** case that the size of the journal file had already been increased but
+** the extra entries had not yet made it safely to disk.  In such a case,
+** the value of nRec computed from the file size would be too large.  For
+** that reason, we always use the nRec value in the header.
+**
+** If the nRec value is 0xffffffff it means that nRec should be computed
+** from the file size.  This value is used when the user selects the
+** no-sync option for the journal.  A power failure could lead to corruption
+** in this case.  But for things like temporary table (which will be
+** deleted when the power is restored) we don't care.  
+**
+** If the file opened as the journal file is not a well-formed
+** journal file then all pages up to the first corrupted page are rolled
+** back (or no pages if the journal header is corrupted). The journal file
+** is then deleted and SQLITE_OK returned, just as if no corruption had
+** been encountered.
+**
+** If an I/O or malloc() error occurs, the journal-file is not deleted
+** and an error code is returned.
+*/
+static int pager_playback(Pager *pPager){
+  i64 szJ;                 /* Size of the journal file in bytes */
+  u32 nRec;                /* Number of Records in the journal */
+  int i;                   /* Loop counter */
+  Pgno mxPg = 0;           /* Size of the original file in pages */
+  int rc;                  /* Result code of a subroutine */
+  char *zMaster = 0;       /* Name of master journal file if any */
+
+  /* Figure out how many records are in the journal.  Abort early if
+  ** the journal is empty.
+  */
+  assert( pPager->journalOpen );
+  rc = sqlite3OsFileSize(&pPager->jfd, &szJ);
+  if( rc!=SQLITE_OK ){
+    goto end_playback;
+  }
+
+  /* Read the master journal name from the journal, if it is present.
+  ** If a master journal file name is specified, but the file is not
+  ** present on disk, then the journal is not hot and does not need to be
+  ** played back.
+  */
+  rc = readMasterJournal(&pPager->jfd, &zMaster);
+  assert( rc!=SQLITE_DONE );
+  if( rc!=SQLITE_OK || (zMaster && !sqlite3OsFileExists(zMaster)) ){
+    sqliteFree(zMaster);
+    zMaster = 0;
+    if( rc==SQLITE_DONE ) rc = SQLITE_OK;
+    goto end_playback;
+  }
+  sqlite3OsSeek(&pPager->jfd, 0);
+  pPager->journalOff = 0;
+
+  /* This loop terminates either when the readJournalHdr() call returns
+  ** SQLITE_DONE or an IO error occurs. */
+  while( 1 ){
+
+    /* Read the next journal header from the journal file.  If there are
+    ** not enough bytes left in the journal file for a complete header, or
+    ** it is corrupted, then a process must of failed while writing it.
+    ** This indicates nothing more needs to be rolled back.
+    */
+    rc = readJournalHdr(pPager, szJ, &nRec, &mxPg);
+    if( rc!=SQLITE_OK ){ 
+      if( rc==SQLITE_DONE ){
+        rc = SQLITE_OK;
+      }
+      goto end_playback;
+    }
+
+    /* If nRec is 0xffffffff, then this journal was created by a process
+    ** working in no-sync mode. This means that the rest of the journal
+    ** file consists of pages, there are no more journal headers. Compute
+    ** the value of nRec based on this assumption.
+    */
+    if( nRec==0xffffffff ){
+      assert( pPager->journalOff==JOURNAL_HDR_SZ(pPager) );
+      nRec = (szJ - JOURNAL_HDR_SZ(pPager))/JOURNAL_PG_SZ(pPager);
+    }
+
+    /* If this is the first header read from the journal, truncate the
+    ** database file back to it's original size.
+    */
+    if( pPager->state>=PAGER_EXCLUSIVE && 
+        pPager->journalOff==JOURNAL_HDR_SZ(pPager) ){
+      assert( pPager->origDbSize==0 || pPager->origDbSize==mxPg );
+      rc = pager_truncate(pPager, mxPg);
+      if( rc!=SQLITE_OK ){
+        goto end_playback;
+      }
+      pPager->dbSize = mxPg;
+    }
+
+    /* rc = sqlite3OsSeek(&pPager->jfd, JOURNAL_HDR_SZ(pPager)); */
+    if( rc!=SQLITE_OK ) goto end_playback;
+  
+    /* Copy original pages out of the journal and back into the database file.
+    */
+    for(i=0; i<nRec; i++){
+      rc = pager_playback_one_page(pPager, &pPager->jfd, 1);
+      if( rc!=SQLITE_OK ){
+        if( rc==SQLITE_DONE ){
+          rc = SQLITE_OK;
+          pPager->journalOff = szJ;
+          break;
+        }else{
+          goto end_playback;
+        }
+      }
+    }
+  }
+
+  /* Pages that have been written to the journal but never synced
+  ** where not restored by the loop above.  We have to restore those
+  ** pages by reading them back from the original database.
+  */
+  assert( rc==SQLITE_OK );
+  pager_reload_cache(pPager);
+
+end_playback:
+  if( rc==SQLITE_OK ){
+    rc = pager_unwritelock(pPager);
+  }
+  if( zMaster ){
+    /* If there was a master journal and this routine will return true,
+    ** see if it is possible to delete the master journal.
+    */
+    if( rc==SQLITE_OK ){
+      rc = pager_delmaster(zMaster);
+    }
+    sqliteFree(zMaster);
+  }
+
+  /* The Pager.sectorSize variable may have been updated while rolling
+  ** back a journal created by a process with a different PAGER_SECTOR_SIZE
+  ** value. Reset it to the correct value for this process.
+  */
+  pPager->sectorSize = PAGER_SECTOR_SIZE;
+  return rc;
+}
+
+/*
+** Playback the statement journal.
+**
+** This is similar to playing back the transaction journal but with
+** a few extra twists.
+**
+**    (1)  The number of pages in the database file at the start of
+**         the statement is stored in pPager->stmtSize, not in the
+**         journal file itself.
+**
+**    (2)  In addition to playing back the statement journal, also
+**         playback all pages of the transaction journal beginning
+**         at offset pPager->stmtJSize.
+*/
+static int pager_stmt_playback(Pager *pPager){
+  i64 szJ;                 /* Size of the full journal */
+  i64 hdrOff;
+  int nRec;                /* Number of Records */
+  int i;                   /* Loop counter */
+  int rc;
+
+  szJ = pPager->journalOff;
+#ifndef NDEBUG 
+  {
+    i64 os_szJ;
+    rc = sqlite3OsFileSize(&pPager->jfd, &os_szJ);
+    if( rc!=SQLITE_OK ) return rc;
+    assert( szJ==os_szJ );
+  }
+#endif
+
+  /* Set hdrOff to be the offset to the first journal header written
+  ** this statement transaction, or the end of the file if no journal
+  ** header was written.
+  */
+  hdrOff = pPager->stmtHdrOff;
+  assert( pPager->fullSync || !hdrOff );
+  if( !hdrOff ){
+    hdrOff = szJ;
+  }
+  
+  /* Truncate the database back to its original size.
+  */
+  if( pPager->state>=PAGER_EXCLUSIVE ){
+    rc = pager_truncate(pPager, pPager->stmtSize);
+  }
+  pPager->dbSize = pPager->stmtSize;
+
+  /* Figure out how many records are in the statement journal.
+  */
+  assert( pPager->stmtInUse && pPager->journalOpen );
+  sqlite3OsSeek(&pPager->stfd, 0);
+  nRec = pPager->stmtNRec;
+  
+  /* Copy original pages out of the statement journal and back into the
+  ** database file.  Note that the statement journal omits checksums from
+  ** each record since power-failure recovery is not important to statement
+  ** journals.
+  */
+  for(i=nRec-1; i>=0; i--){
+    rc = pager_playback_one_page(pPager, &pPager->stfd, 0);
+    assert( rc!=SQLITE_DONE );
+    if( rc!=SQLITE_OK ) goto end_stmt_playback;
+  }
+
+  /* Now roll some pages back from the transaction journal. Pager.stmtJSize
+  ** was the size of the journal file when this statement was started, so
+  ** everything after that needs to be rolled back, either into the
+  ** database, the memory cache, or both.
+  **
+  ** If it is not zero, then Pager.stmtHdrOff is the offset to the start
+  ** of the first journal header written during this statement transaction.
+  */
+  rc = sqlite3OsSeek(&pPager->jfd, pPager->stmtJSize);
+  if( rc!=SQLITE_OK ){
+    goto end_stmt_playback;
+  }
+  pPager->journalOff = pPager->stmtJSize;
+  pPager->cksumInit = pPager->stmtCksum;
+  assert( JOURNAL_HDR_SZ(pPager)<(pPager->pageSize+8) );
+  while( pPager->journalOff <= (hdrOff-(pPager->pageSize+8)) ){
+    rc = pager_playback_one_page(pPager, &pPager->jfd, 1);
+    assert( rc!=SQLITE_DONE );
+    if( rc!=SQLITE_OK ) goto end_stmt_playback;
+  }
+
+  while( pPager->journalOff < szJ ){
+    u32 nRec;
+    u32 dummy;
+    rc = readJournalHdr(pPager, szJ, &nRec, &dummy);
+    if( rc!=SQLITE_OK ){
+      assert( rc!=SQLITE_DONE );
+      goto end_stmt_playback;
+    }
+    if( nRec==0 ){
+      nRec = (szJ - pPager->journalOff) / (pPager->pageSize+8);
+    }
+    for(i=nRec-1; i>=0 && pPager->journalOff < szJ; i--){
+      rc = pager_playback_one_page(pPager, &pPager->jfd, 1);
+      assert( rc!=SQLITE_DONE );
+      if( rc!=SQLITE_OK ) goto end_stmt_playback;
+    }
+  }
+
+  pPager->journalOff = szJ;
+  
+end_stmt_playback:
+  if( rc!=SQLITE_OK ){
+    pPager->errMask |= PAGER_ERR_CORRUPT;
+    rc = SQLITE_CORRUPT;  /* bkpt-CORRUPT */
+  }else{
+    pPager->journalOff = szJ;
+    /* pager_reload_cache(pPager); */
+  }
+  return rc;
+}
+
+/*
+** Change the maximum number of in-memory pages that are allowed.
+*/
+void sqlite3pager_set_cachesize(Pager *pPager, int mxPage){
+  if( mxPage>10 ){
+    pPager->mxPage = mxPage;
+  }else{
+    pPager->mxPage = 10;
+  }
+}
+
+/*
+** Adjust the robustness of the database to damage due to OS crashes
+** or power failures by changing the number of syncs()s when writing
+** the rollback journal.  There are three levels:
+**
+**    OFF       sqlite3OsSync() is never called.  This is the default
+**              for temporary and transient files.
+**
+**    NORMAL    The journal is synced once before writes begin on the
+**              database.  This is normally adequate protection, but
+**              it is theoretically possible, though very unlikely,
+**              that an inopertune power failure could leave the journal
+**              in a state which would cause damage to the database
+**              when it is rolled back.
+**
+**    FULL      The journal is synced twice before writes begin on the
+**              database (with some additional information - the nRec field
+**              of the journal header - being written in between the two
+**              syncs).  If we assume that writing a
+**              single disk sector is atomic, then this mode provides
+**              assurance that the journal will not be corrupted to the
+**              point of causing damage to the database during rollback.
+**
+** Numeric values associated with these states are OFF==1, NORMAL=2,
+** and FULL=3.
+*/
+#ifndef SQLITE_OMIT_PAGER_PRAGMAS
+void sqlite3pager_set_safety_level(Pager *pPager, int level){
+  pPager->noSync =  level==1 || pPager->tempFile;
+  pPager->fullSync = level==3 && !pPager->tempFile;
+  if( pPager->noSync ) pPager->needSync = 0;
+}
+#endif
+
+/*
+** The following global variable is incremented whenever the library
+** attempts to open a temporary file.  This information is used for
+** testing and analysis only.  
+*/
+int sqlite3_opentemp_count = 0;
+
+/*
+** Open a temporary file.  Write the name of the file into zFile
+** (zFile must be at least SQLITE_TEMPNAME_SIZE bytes long.)  Write
+** the file descriptor into *fd.  Return SQLITE_OK on success or some
+** other error code if we fail.
+**
+** The OS will automatically delete the temporary file when it is
+** closed.
+*/
+static int sqlite3pager_opentemp(char *zFile, OsFile *fd){
+  int cnt = 8;
+  int rc;
+  sqlite3_opentemp_count++;  /* Used for testing and analysis only */
+  do{
+    cnt--;
+    sqlite3OsTempFileName(zFile);
+    rc = sqlite3OsOpenExclusive(zFile, fd, 1);
+  }while( cnt>0 && rc!=SQLITE_OK && rc!=SQLITE_NOMEM );
+  return rc;
+}
+
+/*
+** Create a new page cache and put a pointer to the page cache in *ppPager.
+** The file to be cached need not exist.  The file is not locked until
+** the first call to sqlite3pager_get() and is only held open until the
+** last page is released using sqlite3pager_unref().
+**
+** If zFilename is NULL then a randomly-named temporary file is created
+** and used as the file to be cached.  The file will be deleted
+** automatically when it is closed.
+**
+** If zFilename is ":memory:" then all information is held in cache.
+** It is never written to disk.  This can be used to implement an
+** in-memory database.
+*/
+int sqlite3pager_open(
+  Pager **ppPager,         /* Return the Pager structure here */
+  const char *zFilename,   /* Name of the database file to open */
+  int nExtra,              /* Extra bytes append to each in-memory page */
+  int flags                /* flags controlling this file */
+){
+  Pager *pPager;
+  char *zFullPathname = 0;
+  int nameLen;
+  OsFile fd;
+  int rc = SQLITE_OK;
+  int i;
+  int tempFile = 0;
+  int memDb = 0;
+  int readOnly = 0;
+  int useJournal = (flags & PAGER_OMIT_JOURNAL)==0;
+  int noReadlock = (flags & PAGER_NO_READLOCK)!=0;
+  char zTemp[SQLITE_TEMPNAME_SIZE];
+
+  *ppPager = 0;
+  memset(&fd, 0, sizeof(fd));
+  if( sqlite3_malloc_failed ){
+    return SQLITE_NOMEM;
+  }
+  if( zFilename && zFilename[0] ){
+#ifndef SQLITE_OMIT_MEMORYDB
+    if( strcmp(zFilename,":memory:")==0 ){
+      memDb = 1;
+      zFullPathname = sqliteStrDup("");
+      rc = SQLITE_OK;
+    }else
+#endif
+    {
+      zFullPathname = sqlite3OsFullPathname(zFilename);
+      if( zFullPathname ){
+        rc = sqlite3OsOpenReadWrite(zFullPathname, &fd, &readOnly);
+      }
+    }
+  }else{
+    rc = sqlite3pager_opentemp(zTemp, &fd);
+    zFilename = zTemp;
+    zFullPathname = sqlite3OsFullPathname(zFilename);
+    if( rc==SQLITE_OK ){
+      tempFile = 1;
+    }
+  }
+  if( !zFullPathname ){
+    sqlite3OsClose(&fd);
+    return SQLITE_NOMEM;
+  }
+  if( rc!=SQLITE_OK ){
+    sqlite3OsClose(&fd);
+    sqliteFree(zFullPathname);
+    return rc;
+  }
+  nameLen = strlen(zFullPathname);
+  pPager = sqliteMalloc( sizeof(*pPager) + nameLen*3 + 30 );
+  if( pPager==0 ){
+    sqlite3OsClose(&fd);
+    sqliteFree(zFullPathname);
+    return SQLITE_NOMEM;
+  }
+  TRACE3("OPEN %d %s\n", FILEHANDLEID(fd), zFullPathname);
+  pPager->zFilename = (char*)&pPager[1];
+  pPager->zDirectory = &pPager->zFilename[nameLen+1];
+  pPager->zJournal = &pPager->zDirectory[nameLen+1];
+  strcpy(pPager->zFilename, zFullPathname);
+  strcpy(pPager->zDirectory, zFullPathname);
+  for(i=nameLen; i>0 && pPager->zDirectory[i-1]!='/'; i--){}
+  if( i>0 ) pPager->zDirectory[i-1] = 0;
+  strcpy(pPager->zJournal, zFullPathname);
+  sqliteFree(zFullPathname);
+  strcpy(&pPager->zJournal[nameLen], "-journal");
+  pPager->fd = fd;
+#if OS_UNIX
+  pPager->fd.pPager = pPager;
+#endif
+  pPager->journalOpen = 0;
+  pPager->useJournal = useJournal && !memDb;
+  pPager->noReadlock = noReadlock && readOnly;
+  pPager->stmtOpen = 0;
+  pPager->stmtInUse = 0;
+  pPager->nRef = 0;
+  pPager->dbSize = memDb-1;
+  pPager->pageSize = SQLITE_DEFAULT_PAGE_SIZE;
+  pPager->stmtSize = 0;
+  pPager->stmtJSize = 0;
+  pPager->nPage = 0;
+  pPager->nMaxPage = 0;
+  pPager->mxPage = 100;
+  pPager->state = PAGER_UNLOCK;
+  pPager->errMask = 0;
+  pPager->tempFile = tempFile;
+  pPager->memDb = memDb;
+  pPager->readOnly = readOnly;
+  pPager->needSync = 0;
+  pPager->noSync = pPager->tempFile || !useJournal;
+  pPager->fullSync = (pPager->noSync?0:1);
+  pPager->pFirst = 0;
+  pPager->pFirstSynced = 0;
+  pPager->pLast = 0;
+  pPager->nExtra = FORCE_ALIGNMENT(nExtra);
+  pPager->sectorSize = PAGER_SECTOR_SIZE;
+  pPager->pBusyHandler = 0;
+  memset(pPager->aHash, 0, sizeof(pPager->aHash));
+  *ppPager = pPager;
+  return SQLITE_OK;
+}
+
+/*
+** Set the busy handler function.
+*/
+void sqlite3pager_set_busyhandler(Pager *pPager, BusyHandler *pBusyHandler){
+  pPager->pBusyHandler = pBusyHandler;
+}
+
+/*
+** Set the destructor for this pager.  If not NULL, the destructor is called
+** when the reference count on each page reaches zero.  The destructor can
+** be used to clean up information in the extra segment appended to each page.
+**
+** The destructor is not called as a result sqlite3pager_close().  
+** Destructors are only called by sqlite3pager_unref().
+*/
+void sqlite3pager_set_destructor(Pager *pPager, void (*xDesc)(void*,int)){
+  pPager->xDestructor = xDesc;
+}
+
+/*
+** Set the reinitializer for this pager.  If not NULL, the reinitializer
+** is called when the content of a page in cache is restored to its original
+** value as a result of a rollback.  The callback gives higher-level code
+** an opportunity to restore the EXTRA section to agree with the restored
+** page data.
+*/
+void sqlite3pager_set_reiniter(Pager *pPager, void (*xReinit)(void*,int)){
+  pPager->xReiniter = xReinit;
+}
+
+/*
+** Set the page size.  Return the new size.  If the suggest new page
+** size is inappropriate, then an alternative page size is selected
+** and returned.
+*/
+int sqlite3pager_set_pagesize(Pager *pPager, int pageSize){
+  assert( pageSize>=512 && pageSize<=SQLITE_MAX_PAGE_SIZE );
+  if( !pPager->memDb ){
+    pPager->pageSize = pageSize;
+  }
+  return pPager->pageSize;
+}
+
+/*
+** Read the first N bytes from the beginning of the file into memory
+** that pDest points to.  No error checking is done.
+*/
+void sqlite3pager_read_fileheader(Pager *pPager, int N, unsigned char *pDest){
+  memset(pDest, 0, N);
+  if( MEMDB==0 ){
+    sqlite3OsSeek(&pPager->fd, 0);
+    sqlite3OsRead(&pPager->fd, pDest, N);
+  }
+}
+
+/*
+** Return the total number of pages in the disk file associated with
+** pPager.
+*/
+int sqlite3pager_pagecount(Pager *pPager){
+  i64 n;
+  assert( pPager!=0 );
+  if( pPager->dbSize>=0 ){
+    return pPager->dbSize;
+  }
+  if( sqlite3OsFileSize(&pPager->fd, &n)!=SQLITE_OK ){
+    pPager->errMask |= PAGER_ERR_DISK;
+    return 0;
+  }
+  n /= pPager->pageSize;
+  if( !MEMDB && n==PENDING_BYTE/pPager->pageSize ){
+    n++;
+  }
+  if( pPager->state!=PAGER_UNLOCK ){
+    pPager->dbSize = n;
+  }
+  return n;
+}
+
+/*
+** Forward declaration
+*/
+static int syncJournal(Pager*);
+
+
+/*
+** Unlink pPg from it's hash chain. Also set the page number to 0 to indicate
+** that the page is not part of any hash chain. This is required because the
+** sqlite3pager_movepage() routine can leave a page in the 
+** pNextFree/pPrevFree list that is not a part of any hash-chain.
+*/
+static void unlinkHashChain(Pager *pPager, PgHdr *pPg){
+  if( pPg->pgno==0 ){
+    /* If the page number is zero, then this page is not in any hash chain. */
+    return;
+  }
+  if( pPg->pNextHash ){
+    pPg->pNextHash->pPrevHash = pPg->pPrevHash;
+  }
+  if( pPg->pPrevHash ){
+    assert( pPager->aHash[pager_hash(pPg->pgno)]!=pPg );
+    pPg->pPrevHash->pNextHash = pPg->pNextHash;
+  }else{
+    int h = pager_hash(pPg->pgno);
+    assert( pPager->aHash[h]==pPg );
+    pPager->aHash[h] = pPg->pNextHash;
+  }
+
+  pPg->pgno = 0;
+  pPg->pNextHash = pPg->pPrevHash = 0;
+}
+
+/*
+** Unlink a page from the free list (the list of all pages where nRef==0)
+** and from its hash collision chain.
+*/
+static void unlinkPage(PgHdr *pPg){
+  Pager *pPager = pPg->pPager;
+
+  /* Keep the pFirstSynced pointer pointing at the first synchronized page */
+  if( pPg==pPager->pFirstSynced ){
+    PgHdr *p = pPg->pNextFree;
+    while( p && p->needSync ){ p = p->pNextFree; }
+    pPager->pFirstSynced = p;
+  }
+
+  /* Unlink from the freelist */
+  if( pPg->pPrevFree ){
+    pPg->pPrevFree->pNextFree = pPg->pNextFree;
+  }else{
+    assert( pPager->pFirst==pPg );
+    pPager->pFirst = pPg->pNextFree;
+  }
+  if( pPg->pNextFree ){
+    pPg->pNextFree->pPrevFree = pPg->pPrevFree;
+  }else{
+    assert( pPager->pLast==pPg );
+    pPager->pLast = pPg->pPrevFree;
+  }
+  pPg->pNextFree = pPg->pPrevFree = 0;
+
+  /* Unlink from the pgno hash table */
+  unlinkHashChain(pPager, pPg);
+}
+
+#ifndef SQLITE_OMIT_MEMORYDB
+/*
+** This routine is used to truncate an in-memory database.  Delete
+** all pages whose pgno is larger than pPager->dbSize and is unreferenced.
+** Referenced pages larger than pPager->dbSize are zeroed.
+*/
+static void memoryTruncate(Pager *pPager){
+  PgHdr *pPg;
+  PgHdr **ppPg;
+  int dbSize = pPager->dbSize;
+
+  ppPg = &pPager->pAll;
+  while( (pPg = *ppPg)!=0 ){
+    if( pPg->pgno<=dbSize ){
+      ppPg = &pPg->pNextAll;
+    }else if( pPg->nRef>0 ){
+      memset(PGHDR_TO_DATA(pPg), 0, pPager->pageSize);
+      ppPg = &pPg->pNextAll;
+    }else{
+      *ppPg = pPg->pNextAll;
+      unlinkPage(pPg);
+      sqliteFree(pPg);
+      pPager->nPage--;
+    }
+  }
+}
+#else
+#define memoryTruncate(p)
+#endif
+
+/*
+** Try to obtain a lock on a file.  Invoke the busy callback if the lock
+** is currently not available.  Repeate until the busy callback returns
+** false or until the lock succeeds.
+**
+** Return SQLITE_OK on success and an error code if we cannot obtain
+** the lock.
+*/
+static int pager_wait_on_lock(Pager *pPager, int locktype){
+  int rc;
+  assert( PAGER_SHARED==SHARED_LOCK );
+  assert( PAGER_RESERVED==RESERVED_LOCK );
+  assert( PAGER_EXCLUSIVE==EXCLUSIVE_LOCK );
+  if( pPager->state>=locktype ){
+    rc = SQLITE_OK;
+  }else{
+    int busy = 1;
+    BusyHandler *pH;
+    do {
+      rc = sqlite3OsLock(&pPager->fd, locktype);
+    }while( rc==SQLITE_BUSY && 
+        (pH = pPager->pBusyHandler)!=0 && 
+        pH->xFunc && pH->xFunc(pH->pArg, busy++)
+    );
+    if( rc==SQLITE_OK ){
+      pPager->state = locktype;
+    }
+  }
+  return rc;
+}
+
+/*
+** Truncate the file to the number of pages specified.
+*/
+int sqlite3pager_truncate(Pager *pPager, Pgno nPage){
+  int rc;
+  sqlite3pager_pagecount(pPager);
+  if( pPager->errMask!=0 ){
+    rc = pager_errcode(pPager);
+    return rc;
+  }
+  if( nPage>=(unsigned)pPager->dbSize ){
+    return SQLITE_OK;
+  }
+  if( MEMDB ){
+    pPager->dbSize = nPage;
+    memoryTruncate(pPager);
+    return SQLITE_OK;
+  }
+  rc = syncJournal(pPager);
+  if( rc!=SQLITE_OK ){
+    return rc;
+  }
+
+  /* Get an exclusive lock on the database before truncating. */
+  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
+  if( rc!=SQLITE_OK ){
+    return rc;
+  }
+
+  rc = pager_truncate(pPager, nPage);
+  if( rc==SQLITE_OK ){
+    pPager->dbSize = nPage;
+  }
+  return rc;
+}
+
+/*
+** Shutdown the page cache.  Free all memory and close all files.
+**
+** If a transaction was in progress when this routine is called, that
+** transaction is rolled back.  All outstanding pages are invalidated
+** and their memory is freed.  Any attempt to use a page associated
+** with this page cache after this function returns will likely
+** result in a coredump.
+*/
+int sqlite3pager_close(Pager *pPager){
+  PgHdr *pPg, *pNext;
+  switch( pPager->state ){
+    case PAGER_RESERVED:
+    case PAGER_SYNCED: 
+    case PAGER_EXCLUSIVE: {
+      /* We ignore any IO errors that occur during the rollback
+      ** operation. So disable IO error simulation so that testing
+      ** works more easily.
+      */
+#if defined(SQLITE_TEST) && (defined(OS_UNIX) || defined(OS_WIN))
+      extern int sqlite3_io_error_pending;
+      int ioerr_cnt = sqlite3_io_error_pending;
+      sqlite3_io_error_pending = -1;
+#endif
+      sqlite3pager_rollback(pPager);
+#if defined(SQLITE_TEST) && (defined(OS_UNIX) || defined(OS_WIN))
+      sqlite3_io_error_pending = ioerr_cnt;
+#endif
+      if( !MEMDB ){
+        sqlite3OsUnlock(&pPager->fd, NO_LOCK);
+      }
+      assert( pPager->errMask || pPager->journalOpen==0 );
+      break;
+    }
+    case PAGER_SHARED: {
+      if( !MEMDB ){
+        sqlite3OsUnlock(&pPager->fd, NO_LOCK);
+      }
+      break;
+    }
+    default: {
+      /* Do nothing */
+      break;
+    }
+  }
+  for(pPg=pPager->pAll; pPg; pPg=pNext){
+#ifndef NDEBUG
+    if( MEMDB ){
+      PgHistory *pHist = PGHDR_TO_HIST(pPg, pPager);
+      assert( !pPg->alwaysRollback );
+      assert( !pHist->pOrig );
+      assert( !pHist->pStmt );
+    }
+#endif
+    pNext = pPg->pNextAll;
+    sqliteFree(pPg);
+  }
+  TRACE2("CLOSE %d\n", PAGERID(pPager));
+  assert( pPager->errMask || (pPager->journalOpen==0 && pPager->stmtOpen==0) );
+  if( pPager->journalOpen ){
+    sqlite3OsClose(&pPager->jfd);
+  }
+  sqliteFree(pPager->aInJournal);
+  if( pPager->stmtOpen ){
+    sqlite3OsClose(&pPager->stfd);
+  }
+  sqlite3OsClose(&pPager->fd);
+  /* Temp files are automatically deleted by the OS
+  ** if( pPager->tempFile ){
+  **   sqlite3OsDelete(pPager->zFilename);
+  ** }
+  */
+
+  sqliteFree(pPager);
+  return SQLITE_OK;
+}
+
+/*
+** Return the page number for the given page data.
+*/
+Pgno sqlite3pager_pagenumber(void *pData){
+  PgHdr *p = DATA_TO_PGHDR(pData);
+  return p->pgno;
+}
+
+/*
+** The page_ref() function increments the reference count for a page.
+** If the page is currently on the freelist (the reference count is zero) then
+** remove it from the freelist.
+**
+** For non-test systems, page_ref() is a macro that calls _page_ref()
+** online of the reference count is zero.  For test systems, page_ref()
+** is a real function so that we can set breakpoints and trace it.
+*/
+static void _page_ref(PgHdr *pPg){
+  if( pPg->nRef==0 ){
+    /* The page is currently on the freelist.  Remove it. */
+    if( pPg==pPg->pPager->pFirstSynced ){
+      PgHdr *p = pPg->pNextFree;
+      while( p && p->needSync ){ p = p->pNextFree; }
+      pPg->pPager->pFirstSynced = p;
+    }
+    if( pPg->pPrevFree ){
+      pPg->pPrevFree->pNextFree = pPg->pNextFree;
+    }else{
+      pPg->pPager->pFirst = pPg->pNextFree;
+    }
+    if( pPg->pNextFree ){
+      pPg->pNextFree->pPrevFree = pPg->pPrevFree;
+    }else{
+      pPg->pPager->pLast = pPg->pPrevFree;
+    }
+    pPg->pPager->nRef++;
+  }
+  pPg->nRef++;
+  REFINFO(pPg);
+}
+#ifdef SQLITE_DEBUG
+  static void page_ref(PgHdr *pPg){
+    if( pPg->nRef==0 ){
+      _page_ref(pPg);
+    }else{
+      pPg->nRef++;
+      REFINFO(pPg);
+    }
+  }
+#else
+# define page_ref(P)   ((P)->nRef==0?_page_ref(P):(void)(P)->nRef++)
+#endif
+
+/*
+** Increment the reference count for a page.  The input pointer is
+** a reference to the page data.
+*/
+int sqlite3pager_ref(void *pData){
+  PgHdr *pPg = DATA_TO_PGHDR(pData);
+  page_ref(pPg);
+  return SQLITE_OK;
+}
+
+/*
+** Sync the journal.  In other words, make sure all the pages that have
+** been written to the journal have actually reached the surface of the
+** disk.  It is not safe to modify the original database file until after
+** the journal has been synced.  If the original database is modified before
+** the journal is synced and a power failure occurs, the unsynced journal
+** data would be lost and we would be unable to completely rollback the
+** database changes.  Database corruption would occur.
+** 
+** This routine also updates the nRec field in the header of the journal.
+** (See comments on the pager_playback() routine for additional information.)
+** If the sync mode is FULL, two syncs will occur.  First the whole journal
+** is synced, then the nRec field is updated, then a second sync occurs.
+**
+** For temporary databases, we do not care if we are able to rollback
+** after a power failure, so sync occurs.
+**
+** This routine clears the needSync field of every page current held in
+** memory.
+*/
+static int syncJournal(Pager *pPager){
+  PgHdr *pPg;
+  int rc = SQLITE_OK;
+
+  /* Sync the journal before modifying the main database
+  ** (assuming there is a journal and it needs to be synced.)
+  */
+  if( pPager->needSync ){
+    if( !pPager->tempFile ){
+      assert( pPager->journalOpen );
+      /* assert( !pPager->noSync ); // noSync might be set if synchronous
+      ** was turned off after the transaction was started.  Ticket #615 */
+#ifndef NDEBUG
+      {
+        /* Make sure the pPager->nRec counter we are keeping agrees
+        ** with the nRec computed from the size of the journal file.
+        */
+        i64 jSz;
+        rc = sqlite3OsFileSize(&pPager->jfd, &jSz);
+        if( rc!=0 ) return rc;
+        assert( pPager->journalOff==jSz );
+      }
+#endif
+      {
+        /* Write the nRec value into the journal file header. If in
+        ** full-synchronous mode, sync the journal first. This ensures that
+        ** all data has really hit the disk before nRec is updated to mark
+        ** it as a candidate for rollback. 
+        */
+        if( pPager->fullSync ){
+          TRACE2("SYNC journal of %d\n", PAGERID(pPager));
+          rc = sqlite3OsSync(&pPager->jfd);
+          if( rc!=0 ) return rc;
+        }
+        sqlite3OsSeek(&pPager->jfd, pPager->journalHdr + sizeof(aJournalMagic));
+        rc = write32bits(&pPager->jfd, pPager->nRec);
+        if( rc ) return rc;
+
+        sqlite3OsSeek(&pPager->jfd, pPager->journalOff);
+      }
+      TRACE2("SYNC journal of %d\n", PAGERID(pPager));
+      rc = sqlite3OsSync(&pPager->jfd);
+      if( rc!=0 ) return rc;
+      pPager->journalStarted = 1;
+    }
+    pPager->needSync = 0;
+
+    /* Erase the needSync flag from every page.
+    */
+    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
+      pPg->needSync = 0;
+    }
+    pPager->pFirstSynced = pPager->pFirst;
+  }
+
+#ifndef NDEBUG
+  /* If the Pager.needSync flag is clear then the PgHdr.needSync
+  ** flag must also be clear for all pages.  Verify that this
+  ** invariant is true.
+  */
+  else{
+    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
+      assert( pPg->needSync==0 );
+    }
+    assert( pPager->pFirstSynced==pPager->pFirst );
+  }
+#endif
+
+  return rc;
+}
+
+/*
+** Given a list of pages (connected by the PgHdr.pDirty pointer) write
+** every one of those pages out to the database file and mark them all
+** as clean.
+*/
+static int pager_write_pagelist(PgHdr *pList){
+  Pager *pPager;
+  int rc;
+
+  if( pList==0 ) return SQLITE_OK;
+  pPager = pList->pPager;
+
+  /* At this point there may be either a RESERVED or EXCLUSIVE lock on the
+  ** database file. If there is already an EXCLUSIVE lock, the following
+  ** calls to sqlite3OsLock() are no-ops.
+  **
+  ** Moving the lock from RESERVED to EXCLUSIVE actually involves going
+  ** through an intermediate state PENDING.   A PENDING lock prevents new
+  ** readers from attaching to the database but is unsufficient for us to
+  ** write.  The idea of a PENDING lock is to prevent new readers from
+  ** coming in while we wait for existing readers to clear.
+  **
+  ** While the pager is in the RESERVED state, the original database file
+  ** is unchanged and we can rollback without having to playback the
+  ** journal into the original database file.  Once we transition to
+  ** EXCLUSIVE, it means the database file has been changed and any rollback
+  ** will require a journal playback.
+  */
+  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
+  if( rc!=SQLITE_OK ){
+    return rc;
+  }
+
+  while( pList ){
+    assert( pList->dirty );
+    sqlite3OsSeek(&pPager->fd, (pList->pgno-1)*(i64)pPager->pageSize);
+    /* If there are dirty pages in the page cache with page numbers greater
+    ** than Pager.dbSize, this means sqlite3pager_truncate() was called to
+    ** make the file smaller (presumably by auto-vacuum code). Do not write
+    ** any such pages to the file.
+    */
+    if( pList->pgno<=pPager->dbSize ){
+      CODEC(pPager, PGHDR_TO_DATA(pList), pList->pgno, 6);
+      TRACE3("STORE %d page %d\n", PAGERID(pPager), pList->pgno);
+      rc = sqlite3OsWrite(&pPager->fd, PGHDR_TO_DATA(pList), pPager->pageSize);
+      CODEC(pPager, PGHDR_TO_DATA(pList), pList->pgno, 0);
+      TEST_INCR(pPager->nWrite);
+    }
+#ifndef NDEBUG
+    else{
+      TRACE3("NOSTORE %d page %d\n", PAGERID(pPager), pList->pgno);
+    }
+#endif
+    if( rc ) return rc;
+    pList->dirty = 0;
+#ifdef SQLITE_CHECK_PAGES
+    pList->pageHash = pager_pagehash(pList);
+#endif
+    pList = pList->pDirty;
+  }
+  return SQLITE_OK;
+}
+
+/*
+** Collect every dirty page into a dirty list and
+** return a pointer to the head of that list.  All pages are
+** collected even if they are still in use.
+*/
+static PgHdr *pager_get_all_dirty_pages(Pager *pPager){
+  PgHdr *p, *pList;
+  pList = 0;
+  for(p=pPager->pAll; p; p=p->pNextAll){
+    if( p->dirty ){
+      p->pDirty = pList;
+      pList = p;
+    }
+  }
+  return pList;
+}
+
+/*
+** Return TRUE if there is a hot journal on the given pager.
+** A hot journal is one that needs to be played back.
+**
+** If the current size of the database file is 0 but a journal file
+** exists, that is probably an old journal left over from a prior
+** database with the same name.  Just delete the journal.
+*/
+static int hasHotJournal(Pager *pPager){
+  if( !pPager->useJournal ) return 0;
+  if( !sqlite3OsFileExists(pPager->zJournal) ) return 0;
+  if( sqlite3OsCheckReservedLock(&pPager->fd) ) return 0;
+  if( sqlite3pager_pagecount(pPager)==0 ){
+    sqlite3OsDelete(pPager->zJournal);
+    return 0;
+  }else{
+    return 1;
+  }
+}
+
+/*
+** Acquire a page.
+**
+** A read lock on the disk file is obtained when the first page is acquired. 
+** This read lock is dropped when the last page is released.
+**
+** A _get works for any page number greater than 0.  If the database
+** file is smaller than the requested page, then no actual disk
+** read occurs and the memory image of the page is initialized to
+** all zeros.  The extra data appended to a page is always initialized
+** to zeros the first time a page is loaded into memory.
+**
+** The acquisition might fail for several reasons.  In all cases,
+** an appropriate error code is returned and *ppPage is set to NULL.
+**
+** See also sqlite3pager_lookup().  Both this routine and _lookup() attempt
+** to find a page in the in-memory cache first.  If the page is not already
+** in memory, this routine goes to disk to read it in whereas _lookup()
+** just returns 0.  This routine acquires a read-lock the first time it
+** has to go to disk, and could also playback an old journal if necessary.
+** Since _lookup() never goes to disk, it never has to deal with locks
+** or journal files.
+*/
+int sqlite3pager_get(Pager *pPager, Pgno pgno, void **ppPage){
+  PgHdr *pPg;
+  int rc;
+
+  /* The maximum page number is 2^31. Return SQLITE_CORRUPT if a page
+  ** number greater than this, or zero, is requested.
+  */
+  if( pgno>PAGER_MAX_PGNO || pgno==0 ){
+    return SQLITE_CORRUPT;
+  }
+
+  /* Make sure we have not hit any critical errors.
+  */ 
+  assert( pPager!=0 );
+  *ppPage = 0;
+  if( pPager->errMask & ~(PAGER_ERR_FULL) ){
+    return pager_errcode(pPager);
+  }
+
+  /* If this is the first page accessed, then get a SHARED lock
+  ** on the database file.
+  */
+  if( pPager->nRef==0 && !MEMDB ){
+    if( !pPager->noReadlock ){
+      rc = pager_wait_on_lock(pPager, SHARED_LOCK);
+      if( rc!=SQLITE_OK ){
+        return rc;
+      }
+    }
+
+    /* If a journal file exists, and there is no RESERVED lock on the
+    ** database file, then it either needs to be played back or deleted.
+    */
+    if( hasHotJournal(pPager) ){
+       int rc;
+
+       /* Get an EXCLUSIVE lock on the database file. At this point it is
+       ** important that a RESERVED lock is not obtained on the way to the
+       ** EXCLUSIVE lock. If it were, another process might open the
+       ** database file, detect the RESERVED lock, and conclude that the
+       ** database is safe to read while this process is still rolling it 
+       ** back.
+       ** 
+       ** Because the intermediate RESERVED lock is not requested, the
+       ** second process will get to this point in the code and fail to
+       ** obtain it's own EXCLUSIVE lock on the database file.
+       */
+       rc = sqlite3OsLock(&pPager->fd, EXCLUSIVE_LOCK);
+       if( rc!=SQLITE_OK ){
+         sqlite3OsUnlock(&pPager->fd, NO_LOCK);
+         pPager->state = PAGER_UNLOCK;
+         return rc;
+       }
+       pPager->state = PAGER_EXCLUSIVE;
+
+       /* Open the journal for reading only.  Return SQLITE_BUSY if
+       ** we are unable to open the journal file. 
+       **
+       ** The journal file does not need to be locked itself.  The
+       ** journal file is never open unless the main database file holds
+       ** a write lock, so there is never any chance of two or more
+       ** processes opening the journal at the same time.
+       */
+       rc = sqlite3OsOpenReadOnly(pPager->zJournal, &pPager->jfd);
+       if( rc!=SQLITE_OK ){
+         sqlite3OsUnlock(&pPager->fd, NO_LOCK);
+         pPager->state = PAGER_UNLOCK;
+         return SQLITE_BUSY;
+       }
+       pPager->journalOpen = 1;
+       pPager->journalStarted = 0;
+       pPager->journalOff = 0;
+       pPager->setMaster = 0;
+       pPager->journalHdr = 0;
+
+       /* Playback and delete the journal.  Drop the database write
+       ** lock and reacquire the read lock.
+       */
+       rc = pager_playback(pPager);
+       if( rc!=SQLITE_OK ){
+         return rc;
+       }
+    }
+    pPg = 0;
+  }else{
+    /* Search for page in cache */
+    pPg = pager_lookup(pPager, pgno);
+    if( MEMDB && pPager->state==PAGER_UNLOCK ){
+      pPager->state = PAGER_SHARED;
+    }
+  }
+  if( pPg==0 ){
+    /* The requested page is not in the page cache. */
+    int h;
+    TEST_INCR(pPager->nMiss);
+    if( pPager->nPage<pPager->mxPage || pPager->pFirst==0 || MEMDB ){
+      /* Create a new page */
+      pPg = sqliteMallocRaw( sizeof(*pPg) + pPager->pageSize
+                              + sizeof(u32) + pPager->nExtra
+                              + MEMDB*sizeof(PgHistory) );
+      if( pPg==0 ){
+        pPager->errMask |= PAGER_ERR_MEM;
+        return SQLITE_NOMEM;
+      }
+      memset(pPg, 0, sizeof(*pPg));
+      if( MEMDB ){
+        memset(PGHDR_TO_HIST(pPg, pPager), 0, sizeof(PgHistory));
+      }
+      pPg->pPager = pPager;
+      pPg->pNextAll = pPager->pAll;
+      pPager->pAll = pPg;
+      pPager->nPage++;
+      if( pPager->nPage>pPager->nMaxPage ){
+        assert( pPager->nMaxPage==(pPager->nPage-1) );
+        pPager->nMaxPage++;
+      }
+    }else{
+      /* Find a page to recycle.  Try to locate a page that does not
+      ** require us to do an fsync() on the journal.
+      */
+      pPg = pPager->pFirstSynced;
+
+      /* If we could not find a page that does not require an fsync()
+      ** on the journal file then fsync the journal file.  This is a
+      ** very slow operation, so we work hard to avoid it.  But sometimes
+      ** it can't be helped.
+      */
+      if( pPg==0 ){
+        int rc = syncJournal(pPager);
+        if( rc!=0 ){
+          sqlite3pager_rollback(pPager);
+          return SQLITE_IOERR;
+        }
+        if( pPager->fullSync ){
+          /* If in full-sync mode, write a new journal header into the
+	  ** journal file. This is done to avoid ever modifying a journal
+	  ** header that is involved in the rollback of pages that have
+	  ** already been written to the database (in case the header is
+	  ** trashed when the nRec field is updated).
+          */
+          pPager->nRec = 0;
+          assert( pPager->journalOff > 0 );
+          rc = writeJournalHdr(pPager);
+          if( rc!=0 ){
+            sqlite3pager_rollback(pPager);
+            return SQLITE_IOERR;
+          }
+        }
+        pPg = pPager->pFirst;
+      }
+      assert( pPg->nRef==0 );
+
+      /* Write the page to the database file if it is dirty.
+      */
+      if( pPg->dirty ){
+        assert( pPg->needSync==0 );
+        pPg->pDirty = 0;
+        rc = pager_write_pagelist( pPg );
+        if( rc!=SQLITE_OK ){
+          sqlite3pager_rollback(pPager);
+          return SQLITE_IOERR;
+        }
+      }
+      assert( pPg->dirty==0 );
+
+      /* If the page we are recycling is marked as alwaysRollback, then
+      ** set the global alwaysRollback flag, thus disabling the
+      ** sqlite_dont_rollback() optimization for the rest of this transaction.
+      ** It is necessary to do this because the page marked alwaysRollback
+      ** might be reloaded at a later time but at that point we won't remember
+      ** that is was marked alwaysRollback.  This means that all pages must
+      ** be marked as alwaysRollback from here on out.
+      */
+      if( pPg->alwaysRollback ){
+        pPager->alwaysRollback = 1;
+      }
+
+      /* Unlink the old page from the free list and the hash table
+      */
+      unlinkPage(pPg);
+      TEST_INCR(pPager->nOvfl);
+    }
+    pPg->pgno = pgno;
+    if( pPager->aInJournal && (int)pgno<=pPager->origDbSize ){
+      sqlite3CheckMemory(pPager->aInJournal, pgno/8);
+      assert( pPager->journalOpen );
+      pPg->inJournal = (pPager->aInJournal[pgno/8] & (1<<(pgno&7)))!=0;
+      pPg->needSync = 0;
+    }else{
+      pPg->inJournal = 0;
+      pPg->needSync = 0;
+    }
+    if( pPager->aInStmt && (int)pgno<=pPager->stmtSize
+             && (pPager->aInStmt[pgno/8] & (1<<(pgno&7)))!=0 ){
+      page_add_to_stmt_list(pPg);
+    }else{
+      page_remove_from_stmt_list(pPg);
+    }
+    pPg->dirty = 0;
+    pPg->nRef = 1;
+    REFINFO(pPg);
+    pPager->nRef++;
+    h = pager_hash(pgno);
+    pPg->pNextHash = pPager->aHash[h];
+    pPager->aHash[h] = pPg;
+    if( pPg->pNextHash ){
+      assert( pPg->pNextHash->pPrevHash==0 );
+      pPg->pNextHash->pPrevHash = pPg;
+    }
+    if( pPager->nExtra>0 ){
+      memset(PGHDR_TO_EXTRA(pPg, pPager), 0, pPager->nExtra);
+    }
+    if( pPager->errMask!=0 ){
+      sqlite3pager_unref(PGHDR_TO_DATA(pPg));
+      rc = pager_errcode(pPager);
+      return rc;
+    }
+    if( sqlite3pager_pagecount(pPager)<(int)pgno ){
+      memset(PGHDR_TO_DATA(pPg), 0, pPager->pageSize);
+    }else{
+      int rc;
+      assert( MEMDB==0 );
+      sqlite3OsSeek(&pPager->fd, (pgno-1)*(i64)pPager->pageSize);
+      rc = sqlite3OsRead(&pPager->fd, PGHDR_TO_DATA(pPg), pPager->pageSize);
+      TRACE3("FETCH %d page %d\n", PAGERID(pPager), pPg->pgno);
+      CODEC(pPager, PGHDR_TO_DATA(pPg), pPg->pgno, 3);
+      if( rc!=SQLITE_OK ){
+        i64 fileSize;
+        if( sqlite3OsFileSize(&pPager->fd,&fileSize)!=SQLITE_OK
+               || fileSize>=pgno*pPager->pageSize ){
+          sqlite3pager_unref(PGHDR_TO_DATA(pPg));
+          return rc;
+        }else{
+          memset(PGHDR_TO_DATA(pPg), 0, pPager->pageSize);
+        }
+      }else{
+        TEST_INCR(pPager->nRead);
+      }
+    }
+#ifdef SQLITE_CHECK_PAGES
+    pPg->pageHash = pager_pagehash(pPg);
+#endif
+  }else{
+    /* The requested page is in the page cache. */
+    TEST_INCR(pPager->nHit);
+    page_ref(pPg);
+  }
+  *ppPage = PGHDR_TO_DATA(pPg);
+  return SQLITE_OK;
+}
+
+/*
+** Acquire a page if it is already in the in-memory cache.  Do
+** not read the page from disk.  Return a pointer to the page,
+** or 0 if the page is not in cache.
+**
+** See also sqlite3pager_get().  The difference between this routine
+** and sqlite3pager_get() is that _get() will go to the disk and read
+** in the page if the page is not already in cache.  This routine
+** returns NULL if the page is not in cache or if a disk I/O error 
+** has ever happened.
+*/
+void *sqlite3pager_lookup(Pager *pPager, Pgno pgno){
+  PgHdr *pPg;
+
+  assert( pPager!=0 );
+  assert( pgno!=0 );
+  if( pPager->errMask & ~(PAGER_ERR_FULL) ){
+    return 0;
+  }
+  pPg = pager_lookup(pPager, pgno);
+  if( pPg==0 ) return 0;
+  page_ref(pPg);
+  return PGHDR_TO_DATA(pPg);
+}
+
+/*
+** Release a page.
+**
+** If the number of references to the page drop to zero, then the
+** page is added to the LRU list.  When all references to all pages
+** are released, a rollback occurs and the lock on the database is
+** removed.
+*/
+int sqlite3pager_unref(void *pData){
+  PgHdr *pPg;
+
+  /* Decrement the reference count for this page
+  */
+  pPg = DATA_TO_PGHDR(pData);
+  assert( pPg->nRef>0 );
+  pPg->nRef--;
+  REFINFO(pPg);
+
+  CHECK_PAGE(pPg);
+
+  /* When the number of references to a page reach 0, call the
+  ** destructor and add the page to the freelist.
+  */
+  if( pPg->nRef==0 ){
+    Pager *pPager;
+    pPager = pPg->pPager;
+    pPg->pNextFree = 0;
+    pPg->pPrevFree = pPager->pLast;
+    pPager->pLast = pPg;
+    if( pPg->pPrevFree ){
+      pPg->pPrevFree->pNextFree = pPg;
+    }else{
+      pPager->pFirst = pPg;
+    }
+    if( pPg->needSync==0 && pPager->pFirstSynced==0 ){
+      pPager->pFirstSynced = pPg;
+    }
+    if( pPager->xDestructor ){
+      pPager->xDestructor(pData, pPager->pageSize);
+    }
+  
+    /* When all pages reach the freelist, drop the read lock from
+    ** the database file.
+    */
+    pPager->nRef--;
+    assert( pPager->nRef>=0 );
+    if( pPager->nRef==0 && !MEMDB ){
+      pager_reset(pPager);
+    }
+  }
+  return SQLITE_OK;
+}
+
+/*
+** Create a journal file for pPager.  There should already be a RESERVED
+** or EXCLUSIVE lock on the database file when this routine is called.
+**
+** Return SQLITE_OK if everything.  Return an error code and release the
+** write lock if anything goes wrong.
+*/
+static int pager_open_journal(Pager *pPager){
+  int rc;
+  assert( !MEMDB );
+  assert( pPager->state>=PAGER_RESERVED );
+  assert( pPager->journalOpen==0 );
+  assert( pPager->useJournal );
+  assert( pPager->aInJournal==0 );
+  sqlite3pager_pagecount(pPager);
+  pPager->aInJournal = sqliteMalloc( pPager->dbSize/8 + 1 );
+  if( pPager->aInJournal==0 ){
+    rc = SQLITE_NOMEM;
+    goto failed_to_open_journal;
+  }
+  rc = sqlite3OsOpenExclusive(pPager->zJournal, &pPager->jfd,pPager->tempFile);
+  pPager->journalOff = 0;
+  pPager->setMaster = 0;
+  pPager->journalHdr = 0;
+  if( rc!=SQLITE_OK ){
+    goto failed_to_open_journal;
+  }
+  SET_FULLSYNC(pPager->jfd, pPager->fullSync);
+  SET_FULLSYNC(pPager->fd, pPager->fullSync);
+  sqlite3OsOpenDirectory(pPager->zDirectory, &pPager->jfd);
+  pPager->journalOpen = 1;
+  pPager->journalStarted = 0;
+  pPager->needSync = 0;
+  pPager->alwaysRollback = 0;
+  pPager->nRec = 0;
+  if( pPager->errMask!=0 ){
+    rc = pager_errcode(pPager);
+    goto failed_to_open_journal;
+  }
+  pPager->origDbSize = pPager->dbSize;
+
+  rc = writeJournalHdr(pPager);
+
+  if( pPager->stmtAutoopen && rc==SQLITE_OK ){
+    rc = sqlite3pager_stmt_begin(pPager);
+  }
+  if( rc!=SQLITE_OK ){
+    rc = pager_unwritelock(pPager);
+    if( rc==SQLITE_OK ){
+      rc = SQLITE_FULL;
+    }
+  }
+  return rc;
+
+failed_to_open_journal:
+  sqliteFree(pPager->aInJournal);
+  pPager->aInJournal = 0;
+  sqlite3OsUnlock(&pPager->fd, NO_LOCK);
+  pPager->state = PAGER_UNLOCK;
+  return rc;
+}
+
+/*
+** Acquire a write-lock on the database.  The lock is removed when
+** the any of the following happen:
+**
+**   *  sqlite3pager_commit() is called.
+**   *  sqlite3pager_rollback() is called.
+**   *  sqlite3pager_close() is called.
+**   *  sqlite3pager_unref() is called to on every outstanding page.
+**
+** The first parameter to this routine is a pointer to any open page of the
+** database file.  Nothing changes about the page - it is used merely to
+** acquire a pointer to the Pager structure and as proof that there is
+** already a read-lock on the database.
+**
+** The second parameter indicates how much space in bytes to reserve for a
+** master journal file-name at the start of the journal when it is created.
+**
+** A journal file is opened if this is not a temporary file.  For temporary
+** files, the opening of the journal file is deferred until there is an
+** actual need to write to the journal.
+**
+** If the database is already reserved for writing, this routine is a no-op.
+**
+** If exFlag is true, go ahead and get an EXCLUSIVE lock on the file
+** immediately instead of waiting until we try to flush the cache.  The
+** exFlag is ignored if a transaction is already active.
+*/
+int sqlite3pager_begin(void *pData, int exFlag){
+  PgHdr *pPg = DATA_TO_PGHDR(pData);
+  Pager *pPager = pPg->pPager;
+  int rc = SQLITE_OK;
+  assert( pPg->nRef>0 );
+  assert( pPager->state!=PAGER_UNLOCK );
+  if( pPager->state==PAGER_SHARED ){
+    assert( pPager->aInJournal==0 );
+    if( MEMDB ){
+      pPager->state = PAGER_EXCLUSIVE;
+      pPager->origDbSize = pPager->dbSize;
+    }else{
+      rc = sqlite3OsLock(&pPager->fd, RESERVED_LOCK);
+      if( rc==SQLITE_OK ){
+        pPager->state = PAGER_RESERVED;
+        if( exFlag ){
+          rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
+        }
+      }
+      if( rc!=SQLITE_OK ){
+        return rc;
+      }
+      pPager->dirtyCache = 0;
+      TRACE2("TRANSACTION %d\n", PAGERID(pPager));
+      if( pPager->useJournal && !pPager->tempFile ){
+        rc = pager_open_journal(pPager);
+      }
+    }
+  }
+  return rc;
+}
+
+/*
+** Mark a data page as writeable.  The page is written into the journal 
+** if it is not there already.  This routine must be called before making
+** changes to a page.
+**
+** The first time this routine is called, the pager creates a new
+** journal and acquires a RESERVED lock on the database.  If the RESERVED
+** lock could not be acquired, this routine returns SQLITE_BUSY.  The
+** calling routine must check for that return value and be careful not to
+** change any page data until this routine returns SQLITE_OK.
+**
+** If the journal file could not be written because the disk is full,
+** then this routine returns SQLITE_FULL and does an immediate rollback.
+** All subsequent write attempts also return SQLITE_FULL until there
+** is a call to sqlite3pager_commit() or sqlite3pager_rollback() to
+** reset.
+*/
+int sqlite3pager_write(void *pData){
+  PgHdr *pPg = DATA_TO_PGHDR(pData);
+  Pager *pPager = pPg->pPager;
+  int rc = SQLITE_OK;
+
+  /* Check for errors
+  */
+  if( pPager->errMask ){ 
+    return pager_errcode(pPager);
+  }
+  if( pPager->readOnly ){
+    return SQLITE_PERM;
+  }
+
+  assert( !pPager->setMaster );
+
+  CHECK_PAGE(pPg);
+
+  /* Mark the page as dirty.  If the page has already been written
+  ** to the journal then we can return right away.
+  */
+  pPg->dirty = 1;
+  if( pPg->inJournal && (pPg->inStmt || pPager->stmtInUse==0) ){
+    pPager->dirtyCache = 1;
+  }else{
+
+    /* If we get this far, it means that the page needs to be
+    ** written to the transaction journal or the ckeckpoint journal
+    ** or both.
+    **
+    ** First check to see that the transaction journal exists and
+    ** create it if it does not.
+    */
+    assert( pPager->state!=PAGER_UNLOCK );
+    rc = sqlite3pager_begin(pData, 0);
+    if( rc!=SQLITE_OK ){
+      return rc;
+    }
+    assert( pPager->state>=PAGER_RESERVED );
+    if( !pPager->journalOpen && pPager->useJournal ){
+      rc = pager_open_journal(pPager);
+      if( rc!=SQLITE_OK ) return rc;
+    }
+    assert( pPager->journalOpen || !pPager->useJournal );
+    pPager->dirtyCache = 1;
+  
+    /* The transaction journal now exists and we have a RESERVED or an
+    ** EXCLUSIVE lock on the main database file.  Write the current page to
+    ** the transaction journal if it is not there already.
+    */
+    if( !pPg->inJournal && (pPager->useJournal || MEMDB) ){
+      if( (int)pPg->pgno <= pPager->origDbSize ){
+        int szPg;
+        u32 saved;
+        if( MEMDB ){
+          PgHistory *pHist = PGHDR_TO_HIST(pPg, pPager);
+          TRACE3("JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno);
+          assert( pHist->pOrig==0 );
+          pHist->pOrig = sqliteMallocRaw( pPager->pageSize );
+          if( pHist->pOrig ){
+            memcpy(pHist->pOrig, PGHDR_TO_DATA(pPg), pPager->pageSize);
+          }
+        }else{
+          u32 cksum;
+          CODEC(pPager, pData, pPg->pgno, 7);
+          cksum = pager_cksum(pPager, pPg->pgno, pData);
+          saved = *(u32*)PGHDR_TO_EXTRA(pPg, pPager);
+          store32bits(cksum, pPg, pPager->pageSize);
+          szPg = pPager->pageSize+8;
+          store32bits(pPg->pgno, pPg, -4);
+          rc = sqlite3OsWrite(&pPager->jfd, &((char*)pData)[-4], szPg);
+          pPager->journalOff += szPg;
+          TRACE4("JOURNAL %d page %d needSync=%d\n",
+                  PAGERID(pPager), pPg->pgno, pPg->needSync);
+          CODEC(pPager, pData, pPg->pgno, 0);
+          *(u32*)PGHDR_TO_EXTRA(pPg, pPager) = saved;
+          if( rc!=SQLITE_OK ){
+            sqlite3pager_rollback(pPager);
+            pPager->errMask |= PAGER_ERR_FULL;
+            return rc;
+          }
+          pPager->nRec++;
+          assert( pPager->aInJournal!=0 );
+          pPager->aInJournal[pPg->pgno/8] |= 1<<(pPg->pgno&7);
+          pPg->needSync = !pPager->noSync;
+          if( pPager->stmtInUse ){
+            pPager->aInStmt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
+            page_add_to_stmt_list(pPg);
+          }
+        }
+      }else{
+        pPg->needSync = !pPager->journalStarted && !pPager->noSync;
+        TRACE4("APPEND %d page %d needSync=%d\n",
+                PAGERID(pPager), pPg->pgno, pPg->needSync);
+      }
+      if( pPg->needSync ){
+        pPager->needSync = 1;
+      }
+      pPg->inJournal = 1;
+    }
+  
+    /* If the statement journal is open and the page is not in it,
+    ** then write the current page to the statement journal.  Note that
+    ** the statement journal format differs from the standard journal format
+    ** in that it omits the checksums and the header.
+    */
+    if( pPager->stmtInUse && !pPg->inStmt && (int)pPg->pgno<=pPager->stmtSize ){
+      assert( pPg->inJournal || (int)pPg->pgno>pPager->origDbSize );
+      if( MEMDB ){
+        PgHistory *pHist = PGHDR_TO_HIST(pPg, pPager);
+        assert( pHist->pStmt==0 );
+        pHist->pStmt = sqliteMallocRaw( pPager->pageSize );
+        if( pHist->pStmt ){
+          memcpy(pHist->pStmt, PGHDR_TO_DATA(pPg), pPager->pageSize);
+        }
+        TRACE3("STMT-JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno);
+      }else{
+        store32bits(pPg->pgno, pPg, -4);
+        CODEC(pPager, pData, pPg->pgno, 7);
+        rc = sqlite3OsWrite(&pPager->stfd,((char*)pData)-4, pPager->pageSize+4);
+        TRACE3("STMT-JOURNAL %d page %d\n", PAGERID(pPager), pPg->pgno);
+        CODEC(pPager, pData, pPg->pgno, 0);
+        if( rc!=SQLITE_OK ){
+          sqlite3pager_rollback(pPager);
+          pPager->errMask |= PAGER_ERR_FULL;
+          return rc;
+        }
+        pPager->stmtNRec++;
+        assert( pPager->aInStmt!=0 );
+        pPager->aInStmt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
+      }
+      page_add_to_stmt_list(pPg);
+    }
+  }
+
+  /* Update the database size and return.
+  */
+  if( pPager->dbSize<(int)pPg->pgno ){
+    pPager->dbSize = pPg->pgno;
+    if( !MEMDB && pPager->dbSize==PENDING_BYTE/pPager->pageSize ){
+      pPager->dbSize++;
+    }
+  }
+  return rc;
+}
+
+/*
+** Return TRUE if the page given in the argument was previously passed
+** to sqlite3pager_write().  In other words, return TRUE if it is ok
+** to change the content of the page.
+*/
+int sqlite3pager_iswriteable(void *pData){
+  PgHdr *pPg = DATA_TO_PGHDR(pData);
+  return pPg->dirty;
+}
+
+#ifndef SQLITE_OMIT_VACUUM
+/*
+** Replace the content of a single page with the information in the third
+** argument.
+*/
+int sqlite3pager_overwrite(Pager *pPager, Pgno pgno, void *pData){
+  void *pPage;
+  int rc;
+
+  rc = sqlite3pager_get(pPager, pgno, &pPage);
+  if( rc==SQLITE_OK ){
+    rc = sqlite3pager_write(pPage);
+    if( rc==SQLITE_OK ){
+      memcpy(pPage, pData, pPager->pageSize);
+    }
+    sqlite3pager_unref(pPage);
+  }
+  return rc;
+}
+#endif
+
+/*
+** A call to this routine tells the pager that it is not necessary to
+** write the information on page "pgno" back to the disk, even though
+** that page might be marked as dirty.
+**
+** The overlying software layer calls this routine when all of the data
+** on the given page is unused.  The pager marks the page as clean so
+** that it does not get written to disk.
+**
+** Tests show that this optimization, together with the
+** sqlite3pager_dont_rollback() below, more than double the speed
+** of large INSERT operations and quadruple the speed of large DELETEs.
+**
+** When this routine is called, set the alwaysRollback flag to true.
+** Subsequent calls to sqlite3pager_dont_rollback() for the same page
+** will thereafter be ignored.  This is necessary to avoid a problem
+** where a page with data is added to the freelist during one part of
+** a transaction then removed from the freelist during a later part
+** of the same transaction and reused for some other purpose.  When it
+** is first added to the freelist, this routine is called.  When reused,
+** the dont_rollback() routine is called.  But because the page contains
+** critical data, we still need to be sure it gets rolled back in spite
+** of the dont_rollback() call.
+*/
+void sqlite3pager_dont_write(Pager *pPager, Pgno pgno){
+  PgHdr *pPg;
+
+  if( MEMDB ) return;
+
+  pPg = pager_lookup(pPager, pgno);
+  pPg->alwaysRollback = 1;
+  if( pPg && pPg->dirty ){
+    if( pPager->dbSize==(int)pPg->pgno && pPager->origDbSize<pPager->dbSize ){
+      /* If this pages is the last page in the file and the file has grown
+      ** during the current transaction, then do NOT mark the page as clean.
+      ** When the database file grows, we must make sure that the last page
+      ** gets written at least once so that the disk file will be the correct
+      ** size. If you do not write this page and the size of the file
+      ** on the disk ends up being too small, that can lead to database
+      ** corruption during the next transaction.
+      */
+    }else{
+      TRACE3("DONT_WRITE page %d of %d\n", pgno, PAGERID(pPager));
+      pPg->dirty = 0;
+#ifdef SQLITE_CHECK_PAGES
+      pPg->pageHash = pager_pagehash(pPg);
+#endif
+    }
+  }
+}
+
+/*
+** A call to this routine tells the pager that if a rollback occurs,
+** it is not necessary to restore the data on the given page.  This
+** means that the pager does not have to record the given page in the
+** rollback journal.
+*/
+void sqlite3pager_dont_rollback(void *pData){
+  PgHdr *pPg = DATA_TO_PGHDR(pData);
+  Pager *pPager = pPg->pPager;
+
+  if( pPager->state!=PAGER_EXCLUSIVE || pPager->journalOpen==0 ) return;
+  if( pPg->alwaysRollback || pPager->alwaysRollback || MEMDB ) return;
+  if( !pPg->inJournal && (int)pPg->pgno <= pPager->origDbSize ){
+    assert( pPager->aInJournal!=0 );
+    pPager->aInJournal[pPg->pgno/8] |= 1<<(pPg->pgno&7);
+    pPg->inJournal = 1;
+    if( pPager->stmtInUse ){
+      pPager->aInStmt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
+      page_add_to_stmt_list(pPg);
+    }
+    TRACE3("DONT_ROLLBACK page %d of %d\n", pPg->pgno, PAGERID(pPager));
+  }
+  if( pPager->stmtInUse && !pPg->inStmt && (int)pPg->pgno<=pPager->stmtSize ){
+    assert( pPg->inJournal || (int)pPg->pgno>pPager->origDbSize );
+    assert( pPager->aInStmt!=0 );
+    pPager->aInStmt[pPg->pgno/8] |= 1<<(pPg->pgno&7);
+    page_add_to_stmt_list(pPg);
+  }
+}
+
+
+#ifndef SQLITE_OMIT_MEMORYDB
+/*
+** Clear a PgHistory block
+*/
+static void clearHistory(PgHistory *pHist){
+  sqliteFree(pHist->pOrig);
+  sqliteFree(pHist->pStmt);
+  pHist->pOrig = 0;
+  pHist->pStmt = 0;
+}
+#else
+#define clearHistory(x)
+#endif
+
+/*
+** Commit all changes to the database and release the write lock.
+**
+** If the commit fails for any reason, a rollback attempt is made
+** and an error code is returned.  If the commit worked, SQLITE_OK
+** is returned.
+*/
+int sqlite3pager_commit(Pager *pPager){
+  int rc;
+  PgHdr *pPg;
+
+  if( pPager->errMask==PAGER_ERR_FULL ){
+    rc = sqlite3pager_rollback(pPager);
+    if( rc==SQLITE_OK ){
+      rc = SQLITE_FULL;
+    }
+    return rc;
+  }
+  if( pPager->errMask!=0 ){
+    rc = pager_errcode(pPager);
+    return rc;
+  }
+  if( pPager->state<PAGER_RESERVED ){
+    return SQLITE_ERROR;
+  }
+  TRACE2("COMMIT %d\n", PAGERID(pPager));
+  if( MEMDB ){
+    pPg = pager_get_all_dirty_pages(pPager);
+    while( pPg ){
+      clearHistory(PGHDR_TO_HIST(pPg, pPager));
+      pPg->dirty = 0;
+      pPg->inJournal = 0;
+      pPg->inStmt = 0;
+      pPg->pPrevStmt = pPg->pNextStmt = 0;
+      pPg = pPg->pDirty;
+    }
+#ifndef NDEBUG
+    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
+      PgHistory *pHist = PGHDR_TO_HIST(pPg, pPager);
+      assert( !pPg->alwaysRollback );
+      assert( !pHist->pOrig );
+      assert( !pHist->pStmt );
+    }
+#endif
+    pPager->pStmt = 0;
+    pPager->state = PAGER_SHARED;
+    return SQLITE_OK;
+  }
+  if( pPager->dirtyCache==0 ){
+    /* Exit early (without doing the time-consuming sqlite3OsSync() calls)
+    ** if there have been no changes to the database file. */
+    assert( pPager->needSync==0 );
+    rc = pager_unwritelock(pPager);
+    pPager->dbSize = -1;
+    return rc;
+  }
+  assert( pPager->journalOpen );
+  rc = sqlite3pager_sync(pPager, 0, 0);
+  if( rc!=SQLITE_OK ){
+    goto commit_abort;
+  }
+  rc = pager_unwritelock(pPager);
+  pPager->dbSize = -1;
+  return rc;
+
+  /* Jump here if anything goes wrong during the commit process.
+  */
+commit_abort:
+  sqlite3pager_rollback(pPager);
+  return rc;
+}
+
+/*
+** Rollback all changes.  The database falls back to PAGER_SHARED mode.
+** All in-memory cache pages revert to their original data contents.
+** The journal is deleted.
+**
+** This routine cannot fail unless some other process is not following
+** the correct locking protocol (SQLITE_PROTOCOL) or unless some other
+** process is writing trash into the journal file (SQLITE_CORRUPT) or
+** unless a prior malloc() failed (SQLITE_NOMEM).  Appropriate error
+** codes are returned for all these occasions.  Otherwise,
+** SQLITE_OK is returned.
+*/
+int sqlite3pager_rollback(Pager *pPager){
+  int rc;
+  TRACE2("ROLLBACK %d\n", PAGERID(pPager));
+  if( MEMDB ){
+    PgHdr *p;
+    for(p=pPager->pAll; p; p=p->pNextAll){
+      PgHistory *pHist;
+      assert( !p->alwaysRollback );
+      if( !p->dirty ){
+        assert( !((PgHistory *)PGHDR_TO_HIST(p, pPager))->pOrig );
+        assert( !((PgHistory *)PGHDR_TO_HIST(p, pPager))->pStmt );
+        continue;
+      }
+
+      pHist = PGHDR_TO_HIST(p, pPager);
+      if( pHist->pOrig ){
+        memcpy(PGHDR_TO_DATA(p), pHist->pOrig, pPager->pageSize);
+        TRACE3("ROLLBACK-PAGE %d of %d\n", p->pgno, PAGERID(pPager));
+      }else{
+        TRACE3("PAGE %d is clean on %d\n", p->pgno, PAGERID(pPager));
+      }
+      clearHistory(pHist);
+      p->dirty = 0;
+      p->inJournal = 0;
+      p->inStmt = 0;
+      p->pPrevStmt = p->pNextStmt = 0;
+
+      if( pPager->xReiniter ){
+        pPager->xReiniter(PGHDR_TO_DATA(p), pPager->pageSize);
+      }
+      
+    }
+    pPager->pStmt = 0;
+    pPager->dbSize = pPager->origDbSize;
+    memoryTruncate(pPager);
+    pPager->stmtInUse = 0;
+    pPager->state = PAGER_SHARED;
+    return SQLITE_OK;
+  }
+
+  if( !pPager->dirtyCache || !pPager->journalOpen ){
+    rc = pager_unwritelock(pPager);
+    pPager->dbSize = -1;
+    return rc;
+  }
+
+  if( pPager->errMask!=0 && pPager->errMask!=PAGER_ERR_FULL ){
+    if( pPager->state>=PAGER_EXCLUSIVE ){
+      pager_playback(pPager);
+    }
+    return pager_errcode(pPager);
+  }
+  if( pPager->state==PAGER_RESERVED ){
+    int rc2;
+    rc = pager_reload_cache(pPager);
+    rc2 = pager_unwritelock(pPager);
+    if( rc==SQLITE_OK ){
+      rc = rc2;
+    }
+  }else{
+    rc = pager_playback(pPager);
+  }
+  if( rc!=SQLITE_OK ){
+    rc = SQLITE_CORRUPT;  /* bkpt-CORRUPT */
+    pPager->errMask |= PAGER_ERR_CORRUPT;
+  }
+  pPager->dbSize = -1;
+  return rc;
+}
+
+/*
+** Return TRUE if the database file is opened read-only.  Return FALSE
+** if the database is (in theory) writable.
+*/
+int sqlite3pager_isreadonly(Pager *pPager){
+  return pPager->readOnly;
+}
+
+/*
+** This routine is used for testing and analysis only.
+*/
+int *sqlite3pager_stats(Pager *pPager){
+  static int a[11];
+  a[0] = pPager->nRef;
+  a[1] = pPager->nPage;
+  a[2] = pPager->mxPage;
+  a[3] = pPager->dbSize;
+  a[4] = pPager->state;
+  a[5] = pPager->errMask;
+#ifdef SQLITE_TEST
+  a[6] = pPager->nHit;
+  a[7] = pPager->nMiss;
+  a[8] = pPager->nOvfl;
+  a[9] = pPager->nRead;
+  a[10] = pPager->nWrite;
+#endif
+  return a;
+}
+
+/*
+** Set the statement rollback point.
+**
+** This routine should be called with the transaction journal already
+** open.  A new statement journal is created that can be used to rollback
+** changes of a single SQL command within a larger transaction.
+*/
+int sqlite3pager_stmt_begin(Pager *pPager){
+  int rc;
+  char zTemp[SQLITE_TEMPNAME_SIZE];
+  assert( !pPager->stmtInUse );
+  assert( pPager->dbSize>=0 );
+  TRACE2("STMT-BEGIN %d\n", PAGERID(pPager));
+  if( MEMDB ){
+    pPager->stmtInUse = 1;
+    pPager->stmtSize = pPager->dbSize;
+    return SQLITE_OK;
+  }
+  if( !pPager->journalOpen ){
+    pPager->stmtAutoopen = 1;
+    return SQLITE_OK;
+  }
+  assert( pPager->journalOpen );
+  pPager->aInStmt = sqliteMalloc( pPager->dbSize/8 + 1 );
+  if( pPager->aInStmt==0 ){
+    sqlite3OsLock(&pPager->fd, SHARED_LOCK);
+    return SQLITE_NOMEM;
+  }
+#ifndef NDEBUG
+  rc = sqlite3OsFileSize(&pPager->jfd, &pPager->stmtJSize);
+  if( rc ) goto stmt_begin_failed;
+  assert( pPager->stmtJSize == pPager->journalOff );
+#endif
+  pPager->stmtJSize = pPager->journalOff;
+  pPager->stmtSize = pPager->dbSize;
+  pPager->stmtHdrOff = 0;
+  pPager->stmtCksum = pPager->cksumInit;
+  if( !pPager->stmtOpen ){
+    rc = sqlite3pager_opentemp(zTemp, &pPager->stfd);
+    if( rc ) goto stmt_begin_failed;
+    pPager->stmtOpen = 1;
+    pPager->stmtNRec = 0;
+  }
+  pPager->stmtInUse = 1;
+  return SQLITE_OK;
+ 
+stmt_begin_failed:
+  if( pPager->aInStmt ){
+    sqliteFree(pPager->aInStmt);
+    pPager->aInStmt = 0;
+  }
+  return rc;
+}
+
+/*
+** Commit a statement.
+*/
+int sqlite3pager_stmt_commit(Pager *pPager){
+  if( pPager->stmtInUse ){
+    PgHdr *pPg, *pNext;
+    TRACE2("STMT-COMMIT %d\n", PAGERID(pPager));
+    if( !MEMDB ){
+      sqlite3OsSeek(&pPager->stfd, 0);
+      /* sqlite3OsTruncate(&pPager->stfd, 0); */
+      sqliteFree( pPager->aInStmt );
+      pPager->aInStmt = 0;
+    }
+    for(pPg=pPager->pStmt; pPg; pPg=pNext){
+      pNext = pPg->pNextStmt;
+      assert( pPg->inStmt );
+      pPg->inStmt = 0;
+      pPg->pPrevStmt = pPg->pNextStmt = 0;
+      if( MEMDB ){
+        PgHistory *pHist = PGHDR_TO_HIST(pPg, pPager);
+        sqliteFree(pHist->pStmt);
+        pHist->pStmt = 0;
+      }
+    }
+    pPager->stmtNRec = 0;
+    pPager->stmtInUse = 0;
+    pPager->pStmt = 0;
+  }
+  pPager->stmtAutoopen = 0;
+  return SQLITE_OK;
+}
+
+/*
+** Rollback a statement.
+*/
+int sqlite3pager_stmt_rollback(Pager *pPager){
+  int rc;
+  if( pPager->stmtInUse ){
+    TRACE2("STMT-ROLLBACK %d\n", PAGERID(pPager));
+    if( MEMDB ){
+      PgHdr *pPg;
+      for(pPg=pPager->pStmt; pPg; pPg=pPg->pNextStmt){
+        PgHistory *pHist = PGHDR_TO_HIST(pPg, pPager);
+        if( pHist->pStmt ){
+          memcpy(PGHDR_TO_DATA(pPg), pHist->pStmt, pPager->pageSize);
+          sqliteFree(pHist->pStmt);
+          pHist->pStmt = 0;
+        }
+      }
+      pPager->dbSize = pPager->stmtSize;
+      memoryTruncate(pPager);
+      rc = SQLITE_OK;
+    }else{
+      rc = pager_stmt_playback(pPager);
+    }
+    sqlite3pager_stmt_commit(pPager);
+  }else{
+    rc = SQLITE_OK;
+  }
+  pPager->stmtAutoopen = 0;
+  return rc;
+}
+
+/*
+** Return the full pathname of the database file.
+*/
+const char *sqlite3pager_filename(Pager *pPager){
+  return pPager->zFilename;
+}
+
+/*
+** Return the directory of the database file.
+*/
+const char *sqlite3pager_dirname(Pager *pPager){
+  return pPager->zDirectory;
+}
+
+/*
+** Return the full pathname of the journal file.
+*/
+const char *sqlite3pager_journalname(Pager *pPager){
+  return pPager->zJournal;
+}
+
+/*
+** Set the codec for this pager
+*/
+void sqlite3pager_set_codec(
+  Pager *pPager,
+  void (*xCodec)(void*,void*,Pgno,int),
+  void *pCodecArg
+){
+  pPager->xCodec = xCodec;
+  pPager->pCodecArg = pCodecArg;
+}
+
+/*
+** This routine is called to increment the database file change-counter,
+** stored at byte 24 of the pager file.
+*/
+static int pager_incr_changecounter(Pager *pPager){
+  void *pPage;
+  PgHdr *pPgHdr;
+  u32 change_counter;
+  int rc;
+
+  /* Open page 1 of the file for writing. */
+  rc = sqlite3pager_get(pPager, 1, &pPage);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = sqlite3pager_write(pPage);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Read the current value at byte 24. */
+  pPgHdr = DATA_TO_PGHDR(pPage);
+  change_counter = retrieve32bits(pPgHdr, 24);
+
+  /* Increment the value just read and write it back to byte 24. */
+  change_counter++;
+  store32bits(change_counter, pPgHdr, 24);
+
+  /* Release the page reference. */
+  sqlite3pager_unref(pPage);
+  return SQLITE_OK;
+}
+
+/*
+** Sync the database file for the pager pPager. zMaster points to the name
+** of a master journal file that should be written into the individual
+** journal file. zMaster may be NULL, which is interpreted as no master
+** journal (a single database transaction).
+**
+** This routine ensures that the journal is synced, all dirty pages written
+** to the database file and the database file synced. The only thing that
+** remains to commit the transaction is to delete the journal file (or
+** master journal file if specified).
+**
+** Note that if zMaster==NULL, this does not overwrite a previous value
+** passed to an sqlite3pager_sync() call.
+**
+** If parameter nTrunc is non-zero, then the pager file is truncated to
+** nTrunc pages (this is used by auto-vacuum databases).
+*/
+int sqlite3pager_sync(Pager *pPager, const char *zMaster, Pgno nTrunc){
+  int rc = SQLITE_OK;
+
+  TRACE4("DATABASE SYNC: File=%s zMaster=%s nTrunc=%d\n", 
+      pPager->zFilename, zMaster, nTrunc);
+
+  /* If this is an in-memory db, or no pages have been written to, or this
+  ** function has already been called, it is a no-op.
+  */
+  if( pPager->state!=PAGER_SYNCED && !MEMDB && pPager->dirtyCache ){
+    PgHdr *pPg;
+    assert( pPager->journalOpen );
+
+    /* If a master journal file name has already been written to the
+    ** journal file, then no sync is required. This happens when it is
+    ** written, then the process fails to upgrade from a RESERVED to an
+    ** EXCLUSIVE lock. The next time the process tries to commit the
+    ** transaction the m-j name will have already been written.
+    */
+    if( !pPager->setMaster ){
+      rc = pager_incr_changecounter(pPager);
+      if( rc!=SQLITE_OK ) goto sync_exit;
+#ifndef SQLITE_OMIT_AUTOVACUUM
+      if( nTrunc!=0 ){
+        /* If this transaction has made the database smaller, then all pages
+        ** being discarded by the truncation must be written to the journal
+        ** file.
+        */
+        Pgno i;
+        void *pPage;
+        for( i=nTrunc+1; i<=pPager->origDbSize; i++ ){
+          if( !(pPager->aInJournal[i/8] & (1<<(i&7))) ){
+            rc = sqlite3pager_get(pPager, i, &pPage);
+            if( rc!=SQLITE_OK ) goto sync_exit;
+            rc = sqlite3pager_write(pPage);
+            sqlite3pager_unref(pPage);
+            if( rc!=SQLITE_OK ) goto sync_exit;
+          }
+        } 
+      }
+#endif
+      rc = writeMasterJournal(pPager, zMaster);
+      if( rc!=SQLITE_OK ) goto sync_exit;
+      rc = syncJournal(pPager);
+      if( rc!=SQLITE_OK ) goto sync_exit;
+    }
+
+#ifndef SQLITE_OMIT_AUTOVACUUM
+    if( nTrunc!=0 ){
+      rc = sqlite3pager_truncate(pPager, nTrunc);
+      if( rc!=SQLITE_OK ) goto sync_exit;
+    }
+#endif
+
+    /* Write all dirty pages to the database file */
+    pPg = pager_get_all_dirty_pages(pPager);
+    rc = pager_write_pagelist(pPg);
+    if( rc!=SQLITE_OK ) goto sync_exit;
+
+    /* Sync the database file. */
+    if( !pPager->noSync ){
+      rc = sqlite3OsSync(&pPager->fd);
+    }
+
+    pPager->state = PAGER_SYNCED;
+  }
+
+sync_exit:
+  return rc;
+}
+
+#ifndef SQLITE_OMIT_AUTOVACUUM
+/*
+** Move the page identified by pData to location pgno in the file. 
+**
+** There must be no references to the current page pgno. If current page
+** pgno is not already in the rollback journal, it is not written there by
+** by this routine. The same applies to the page pData refers to on entry to
+** this routine.
+**
+** References to the page refered to by pData remain valid. Updating any
+** meta-data associated with page pData (i.e. data stored in the nExtra bytes
+** allocated along with the page) is the responsibility of the caller.
+**
+** A transaction must be active when this routine is called. It used to be
+** required that a statement transaction was not active, but this restriction
+** has been removed (CREATE INDEX needs to move a page when a statement
+** transaction is active).
+*/
+int sqlite3pager_movepage(Pager *pPager, void *pData, Pgno pgno){
+  PgHdr *pPg = DATA_TO_PGHDR(pData);
+  PgHdr *pPgOld; 
+  int h;
+  Pgno needSyncPgno = 0;
+
+  assert( pPg->nRef>0 );
+
+  TRACE5("MOVE %d page %d (needSync=%d) moves to %d\n", 
+      PAGERID(pPager), pPg->pgno, pPg->needSync, pgno);
+
+  if( pPg->needSync ){
+    needSyncPgno = pPg->pgno;
+    assert( pPg->inJournal );
+    assert( pPg->dirty );
+    assert( pPager->needSync );
+  }
+
+  /* Unlink pPg from it's hash-chain */
+  unlinkHashChain(pPager, pPg);
+
+  /* If the cache contains a page with page-number pgno, remove it
+  ** from it's hash chain. Also, if the PgHdr.needSync was set for 
+  ** page pgno before the 'move' operation, it needs to be retained 
+  ** for the page moved there.
+  */
+  pPgOld = pager_lookup(pPager, pgno);
+  if( pPgOld ){
+    assert( pPgOld->nRef==0 );
+    unlinkHashChain(pPager, pPgOld);
+    pPgOld->dirty = 0;
+    if( pPgOld->needSync ){
+      assert( pPgOld->inJournal );
+      pPg->inJournal = 1;
+      pPg->needSync = 1;
+      assert( pPager->needSync );
+    }
+  }
+
+  /* Change the page number for pPg and insert it into the new hash-chain. */
+  pPg->pgno = pgno;
+  h = pager_hash(pgno);
+  if( pPager->aHash[h] ){
+    assert( pPager->aHash[h]->pPrevHash==0 );
+    pPager->aHash[h]->pPrevHash = pPg;
+  }
+  pPg->pNextHash = pPager->aHash[h];
+  pPager->aHash[h] = pPg;
+  pPg->pPrevHash = 0;
+
+  pPg->dirty = 1;
+  pPager->dirtyCache = 1;
+
+  if( needSyncPgno ){
+    /* If needSyncPgno is non-zero, then the journal file needs to be 
+    ** sync()ed before any data is written to database file page needSyncPgno.
+    ** Currently, no such page exists in the page-cache and the 
+    ** Pager.aInJournal bit has been set. This needs to be remedied by loading
+    ** the page into the pager-cache and setting the PgHdr.needSync flag.
+    **
+    ** The sqlite3pager_get() call may cause the journal to sync. So make
+    ** sure the Pager.needSync flag is set too.
+    */
+    int rc;
+    void *pNeedSync;
+    assert( pPager->needSync );
+    rc = sqlite3pager_get(pPager, needSyncPgno, &pNeedSync);
+    if( rc!=SQLITE_OK ) return rc;
+    pPager->needSync = 1;
+    DATA_TO_PGHDR(pNeedSync)->needSync = 1;
+    DATA_TO_PGHDR(pNeedSync)->inJournal = 1;
+    DATA_TO_PGHDR(pNeedSync)->dirty = 1;
+    sqlite3pager_unref(pNeedSync);
+  }
+
+  return SQLITE_OK;
+}
+#endif
+
+#if defined(SQLITE_DEBUG) || defined(SQLITE_TEST)
+/*
+** Return the current state of the file lock for the given pager.
+** The return value is one of NO_LOCK, SHARED_LOCK, RESERVED_LOCK,
+** PENDING_LOCK, or EXCLUSIVE_LOCK.
+*/
+int sqlite3pager_lockstate(Pager *pPager){
+#ifdef OS_TEST
+  return pPager->fd->fd.locktype;
+#else
+  return pPager->fd.locktype;
+#endif
+}
+#endif
+
+#ifdef SQLITE_DEBUG
+/*
+** Print a listing of all referenced pages and their ref count.
+*/
+void sqlite3pager_refdump(Pager *pPager){
+  PgHdr *pPg;
+  for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
+    if( pPg->nRef<=0 ) continue;
+    sqlite3DebugPrintf("PAGE %3d addr=%p nRef=%d\n", 
+       pPg->pgno, PGHDR_TO_DATA(pPg), pPg->nRef);
+  }
+}
+#endif
+
+#endif /* SQLITE_OMIT_DISKIO */