#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

typedef struct LogSummaryHdr LogSummaryHdr;
typedef struct LogSummary LogSummary;
typedef struct LogCheckpoint LogCheckpoint;



/*
** The following structure may be used to store the same data that
** is stored in the log-summary header.
**
** Member variables iCheck1 and iCheck2 contain the checksum for the
................................................................................
**       the required blocking file-locks.
*/
struct LogSummary {
  sqlite3_mutex *mutex;           /* Mutex used to protect this object */
  int nRef;                       /* Number of pointers to this structure */
  int fd;                         /* File descriptor open on log-summary */
  char *zPath;                    /* Path to associated WAL file */

  LogSummary *pNext;              /* Next in global list */
  int nData;                      /* Size of aData allocation/mapping */
  u32 *aData;                     /* File body */
};


/*
** List of all LogSummary objects created by this process. Protected by
** static mutex LOG_SUMMARY_MUTEX. TODO: Should have a dedicated mutex
** here instead of borrowing the LRU mutex.
*/
#define LOG_SUMMARY_MUTEX SQLITE_MUTEX_STATIC_LRU
static LogSummary *pLogSummary = 0;




















struct Log {
  LogSummary *pSummary;           /* Log file summary data */
  sqlite3_vfs *pVfs;              /* The VFS used to create pFd */
  sqlite3_file *pFd;              /* File handle for log file */
  int sync_flags;                 /* Flags to use with OsSync() */
  int isLocked;                   /* True if a snapshot is held open */
  int isWriteLocked;              /* True if this is the writer connection */
  LogSummaryHdr hdr;              /* Log summary header for current snapshot */

};


/*
** This structure is used to implement an iterator that iterates through
** all frames in the log in database page order. Where two or more frames
** correspond to the same database page, the iterator visits only the 
** frame most recently written to the log.
**
................................................................................
  struct LogSegment {
    int iNext;                    /* Next aIndex index */
    u8 *aIndex;                   /* Pointer to index array */
    u32 *aDbPage;                 /* Pointer to db page array */
  } aSegment[1];
};










/*
** Generate an 8 byte checksum based on the data in array aByte[] and the
** initial values of aCksum[0] and aCksum[1]. The checksum is written into
** aCksum[] before returning.
*/
#define LOG_CKSM_BYTES 8
static void logChecksumBytes(u8 *aByte, int nByte, u32 *aCksum){
................................................................................
  mutex = pSummary->mutex;
  if( pSummary->fd<0 ){
    rc = logSummaryInit(pSummary, pRet->pFd);
  }else{
    rc = sqlite3OsLock(pRet->pFd, SQLITE_LOCK_SHARED);
  }




 out:
  sqlite3_mutex_leave(mutex);
  sqlite3_free(zWal);
  if( rc!=SQLITE_OK ){
    assert(0);
    if( pRet ){
      sqlite3OsClose(pRet->pFd);
................................................................................
int sqlite3LogClose(
  Log *pLog,                      /* Log to close */
  sqlite3_file *pFd,              /* Database file */
  u8 *zBuf                        /* Buffer of at least page-size bytes */
){
  int rc = SQLITE_OK;
  if( pLog ){

    LogSummary *pSummary = pLog->pSummary;
    sqlite3_mutex *mutex = 0;






    if( sqlite3GlobalConfig.bCoreMutex ){
      mutex = sqlite3_mutex_alloc(LOG_SUMMARY_MUTEX);
    }
    sqlite3_mutex_enter(mutex);

    /* Decrement the reference count on the log summary. If this is the last
................................................................................
static void logLeaveMutex(Log *pLog){
  LogSummary *pSummary = pLog->pSummary;
  logSummaryUnlock(pSummary);
  sqlite3_mutex_leave(pSummary->mutex);
}

/*
** The caller must hold a SHARED lock on the database file.





































































































**
** If this call obtains a new read-lock and the database contents have been
** modified since the most recent call to LogCloseSnapshot() on this Log
** connection, then *pChanged is set to 1 before returning. Otherwise, it 
** is left unmodified. This is used by the pager layer to determine whether 
** or not any cached pages may be safely reused.
*/
int sqlite3LogOpenSnapshot(Log *pLog, int *pChanged){
  int rc = SQLITE_OK;
  if( pLog->isLocked==0 ){






























    if( SQLITE_OK==(rc = logEnterMutex(pLog)) ){
      u32 aCksum[2] = {1, 1};
      u32 aHdr[LOGSUMMARY_HDR_NFIELD+2];
      memcpy(aHdr, pLog->pSummary->aData, sizeof(aHdr));

      /* Verify the checksum on the log-summary header. If it fails,
      ** recover the log-summary from the log file.
................................................................................
       || aCksum[1]!=aHdr[LOGSUMMARY_HDR_NFIELD+1]
      ){
        rc = logSummaryRecover(pLog->pSummary, pLog->pFd);
        memcpy(aHdr, pLog->pSummary->aData, sizeof(aHdr));
        *pChanged = 1;
      }
      if( rc==SQLITE_OK ){
        pLog->isLocked = 1;
        if( memcmp(&pLog->hdr, aHdr, sizeof(LogSummaryHdr)) ){
          *pChanged = 1;
          memcpy(&pLog->hdr, aHdr, LOGSUMMARY_HDR_NFIELD*sizeof(u32));
        }
      }
      logLeaveMutex(pLog);
    }





  }
  return rc;
}

/*
** Unlock the current snapshot.
*/
void sqlite3LogCloseSnapshot(Log *pLog){




  pLog->isLocked = 0;
}



/* 
** Read a page from the log, if it is present. 
................................................................................
** This function returns SQLITE_OK if the caller may write to the database.
** Otherwise, if the caller is operating on a snapshot that has already
** been overwritten by another writer, SQLITE_OBE is returned.
*/
int sqlite3LogWriteLock(Log *pLog, int op){
  assert( pLog->isLocked );
  if( op ){







    if( memcmp(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr)) ){
      return SQLITE_BUSY;
    }
    pLog->isWriteLocked = 1;

  }else if( pLog->isWriteLocked ){

    memcpy(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr));
    pLog->isWriteLocked = 0;
  }
  return SQLITE_OK;
}

/* 
................................................................................
/* 
** Checkpoint the database. When this function is called the caller
** must hold an exclusive lock on the database file.
*/
int sqlite3LogCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  u8 *zBuf                        /* Temporary buffer to use */


){


  /* Assert() that the caller is holding an EXCLUSIVE lock on the 
  ** database file. 
  */
#ifdef SQLITE_DEBUG
  int lock;
  sqlite3OsFileControl(pFd, SQLITE_FCNTL_LOCKSTATE, &lock);
  assert( lock>=4 );
#endif
  





  return logCheckpoint(pLog, pFd, zBuf);
}

#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>

typedef struct LogSummaryHdr LogSummaryHdr;
typedef struct LogSummary LogSummary;
typedef struct LogCheckpoint LogCheckpoint;
typedef struct LogLock LogLock;


/*
** The following structure may be used to store the same data that
** is stored in the log-summary header.
**
** Member variables iCheck1 and iCheck2 contain the checksum for the
................................................................................
**       the required blocking file-locks.
*/
struct LogSummary {
  sqlite3_mutex *mutex;           /* Mutex used to protect this object */
  int nRef;                       /* Number of pointers to this structure */
  int fd;                         /* File descriptor open on log-summary */
  char *zPath;                    /* Path to associated WAL file */
  LogLock *pLock;                 /* Linked list of locks on this object */
  LogSummary *pNext;              /* Next in global list */
  int nData;                      /* Size of aData allocation/mapping */
  u32 *aData;                     /* File body */
};


/*

** The four lockable regions associated with each log-summary. A connection
** may take either a SHARED or EXCLUSIVE lock on each.
*/
#define LOG_REGION_A 0x01
#define LOG_REGION_B 0x02
#define LOG_REGION_C 0x04
#define LOG_REGION_D 0x08

/*
** A single instance of this structure is allocated as part of each 
** connection to a database log. All structures associated with the 
** same log file are linked together into a list using LogLock.pNext
** starting at LogSummary.pLock.
**
** The mLock field of the structure describes the locks (if any) 
** currently held by the connection. If a SHARED lock is held on
** any of the four locking regions, then the associated LOG_REGION_X
** bit (see above) is set. If an EXCLUSIVE lock is held on the region,
** then the (LOG_REGION_X << 8) bit is set.
*/
struct LogLock {
  LogLock *pNext;                 /* Next lock on the same log */
  u32 mLock;                      /* Mask of locks */
};

struct Log {
  LogSummary *pSummary;           /* Log file summary data */
  sqlite3_vfs *pVfs;              /* The VFS used to create pFd */
  sqlite3_file *pFd;              /* File handle for log file */
  int sync_flags;                 /* Flags to use with OsSync() */
  int isLocked;                   /* Non-zero if a snapshot is held open */
  int isWriteLocked;              /* True if this is the writer connection */
  LogSummaryHdr hdr;              /* Log summary header for current snapshot */
  LogLock lock;                   /* Lock held by this connection (if any) */
};


/*
** This structure is used to implement an iterator that iterates through
** all frames in the log in database page order. Where two or more frames
** correspond to the same database page, the iterator visits only the 
** frame most recently written to the log.
**
................................................................................
  struct LogSegment {
    int iNext;                    /* Next aIndex index */
    u8 *aIndex;                   /* Pointer to index array */
    u32 *aDbPage;                 /* Pointer to db page array */
  } aSegment[1];
};


/*
** List of all LogSummary objects created by this process. Protected by
** static mutex LOG_SUMMARY_MUTEX. TODO: Should have a dedicated mutex
** here instead of borrowing the LRU mutex.
*/
#define LOG_SUMMARY_MUTEX SQLITE_MUTEX_STATIC_LRU
static LogSummary *pLogSummary = 0;

/*
** Generate an 8 byte checksum based on the data in array aByte[] and the
** initial values of aCksum[0] and aCksum[1]. The checksum is written into
** aCksum[] before returning.
*/
#define LOG_CKSM_BYTES 8
static void logChecksumBytes(u8 *aByte, int nByte, u32 *aCksum){
................................................................................
  mutex = pSummary->mutex;
  if( pSummary->fd<0 ){
    rc = logSummaryInit(pSummary, pRet->pFd);
  }else{
    rc = sqlite3OsLock(pRet->pFd, SQLITE_LOCK_SHARED);
  }

  pRet->lock.pNext = pSummary->pLock;
  pSummary->pLock = &pRet->lock;

 out:
  sqlite3_mutex_leave(mutex);
  sqlite3_free(zWal);
  if( rc!=SQLITE_OK ){
    assert(0);
    if( pRet ){
      sqlite3OsClose(pRet->pFd);
................................................................................
int sqlite3LogClose(
  Log *pLog,                      /* Log to close */
  sqlite3_file *pFd,              /* Database file */
  u8 *zBuf                        /* Buffer of at least page-size bytes */
){
  int rc = SQLITE_OK;
  if( pLog ){
    LogLock **ppL;
    LogSummary *pSummary = pLog->pSummary;
    sqlite3_mutex *mutex = 0;

    sqlite3_mutex_enter(pSummary->mutex);
    for(ppL=&pSummary->pLock; *ppL!=&pLog->lock; ppL=&(*ppL)->pNext);
    *ppL = pLog->lock.pNext;
    sqlite3_mutex_leave(pSummary->mutex);

    if( sqlite3GlobalConfig.bCoreMutex ){
      mutex = sqlite3_mutex_alloc(LOG_SUMMARY_MUTEX);
    }
    sqlite3_mutex_enter(mutex);

    /* Decrement the reference count on the log summary. If this is the last
................................................................................
static void logLeaveMutex(Log *pLog){
  LogSummary *pSummary = pLog->pSummary;
  logSummaryUnlock(pSummary);
  sqlite3_mutex_leave(pSummary->mutex);
}

/*
** Values for the second parameter to logLockRegion().
*/
#define LOG_UNLOCK 0
#define LOG_RDLOCK 1
#define LOG_WRLOCK 2

static int logLockRegion(Log *pLog, u32 mRegion, int op){
  LogSummary *pSummary = pLog->pSummary;
  LogLock *p;                     /* Used to iterate through in-process locks */
  u32 mNew;                       /* New locks on file */
  u32 mOld;                       /* Old locks on file */
  u32 mNewLock;                   /* New locks held by pLog */

  assert( 
       /* Writer lock operations */
          (op==LOG_WRLOCK && mRegion==(LOG_REGION_C|LOG_REGION_D))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_C|LOG_REGION_D))

       /* Reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B))
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_D))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_D))

       /* Checkpointer lock operations */
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
  );

  sqlite3_mutex_enter(pSummary->mutex);

  /* If obtaining (not releasing) a lock, check if there exist any 
  ** conflicting locks in process. Return SQLITE_BUSY in this case.
  */
  if( op ){
    u32 mConflict = (mRegion<<8) | ((op==LOG_WRLOCK) ? mRegion : 0);
    for(p=pSummary->pLock; p; p=p->pNext){
      if( p!=&pLog->lock && (p->mLock & mConflict) ){
        sqlite3_mutex_leave(pSummary->mutex);
        return SQLITE_BUSY;
      }
    }
  }

  /* Determine the new lock mask for this log connection */
  switch( op ){
    case LOG_UNLOCK: 
      mNewLock = (pLog->lock.mLock & ~(mRegion|(mRegion<<8))); 
      break;
    case LOG_RDLOCK:
      mNewLock = ((pLog->lock.mLock & ~(mRegion<<8)) | mRegion);
      break;
    default:
      assert( op==LOG_WRLOCK );
      mNewLock = (pLog->lock.mLock | (mRegion<<8) | mRegion);
      break;
  }

  /* Determine the current and desired sets of locks at the file level. */
  mNew = 0;
  for(p=pSummary->pLock; p; p=p->pNext){
    assert( (p->mLock & (p->mLock<<8))==(p->mLock & 0x00000F00) );
    if( p!=&pLog->lock ) mNew |= p->mLock;
  }
  mOld = mNew | pLog->lock.mLock;
  mNew = mNew | mNewLock;

  if( mNew!=mOld ){
    int rc;
    u32 mChange = (mNew^mOld) | ((mNew^mOld)>>8);
    struct flock f;
    memset(&f, 0, sizeof(f));
    f.l_type = (op==LOG_WRLOCK?F_WRLCK:(op==LOG_RDLOCK?F_RDLCK:F_UNLCK));
    f.l_whence = SEEK_SET;

    if(      mChange & LOG_REGION_A ) f.l_start = 12;
    else if( mChange & LOG_REGION_B ) f.l_start = 13;
    else if( mChange & LOG_REGION_C ) f.l_start = 14;
    else if( mChange & LOG_REGION_D ) f.l_start = 15;

    if(      mChange & LOG_REGION_D ) f.l_len   = 16 - f.l_start;
    else if( mChange & LOG_REGION_C ) f.l_len   = 15 - f.l_start;
    else if( mChange & LOG_REGION_B ) f.l_len   = 14 - f.l_start;
    else if( mChange & LOG_REGION_A ) f.l_len   = 13 - f.l_start;

    rc = fcntl(pSummary->fd, F_SETLK, &f);
    if( rc!=0 ){
      sqlite3_mutex_leave(pSummary->mutex);
      return SQLITE_BUSY;
    }
  }

  pLog->lock.mLock = mNewLock;
  sqlite3_mutex_leave(pSummary->mutex);
  return SQLITE_OK;
}

/*
** Lock a snapshot.
**
** If this call obtains a new read-lock and the database contents have been
** modified since the most recent call to LogCloseSnapshot() on this Log
** connection, then *pChanged is set to 1 before returning. Otherwise, it 
** is left unmodified. This is used by the pager layer to determine whether 
** or not any cached pages may be safely reused.
*/
int sqlite3LogOpenSnapshot(Log *pLog, int *pChanged){
  int rc = SQLITE_OK;
  if( pLog->isLocked==0 ){
    int nAttempt;

    /* Obtain a snapshot-lock on the log-summary file. The procedure
    ** for obtaining the snapshot log is:
    **
    **    1. Attempt a SHARED lock on regions A and B.
    **    2a. If step 1 is successful, drop the lock on region B.
    **    2b. If step 1 is unsuccessful, attempt a SHARED lock on region D.
    **    3. Repeat the above until the lock attempt in step 1 or 2b is 
    **       successful.
    **
    ** If neither of the locks can be obtained after 5 tries, presumably
    ** something is wrong (i.e. a process not following the locking protocol). 
    ** Return an error code in this case.
    */
    rc = SQLITE_BUSY;
    for(nAttempt=0; nAttempt<5 && rc==SQLITE_BUSY; nAttempt++){
      rc = logLockRegion(pLog, LOG_REGION_A|LOG_REGION_B, LOG_RDLOCK);
      if( rc==SQLITE_BUSY ){
        rc = logLockRegion(pLog, LOG_REGION_D, LOG_RDLOCK);
        if( rc==SQLITE_OK ) pLog->isLocked = LOG_REGION_D;
      }else{
        logLockRegion(pLog, LOG_REGION_B, LOG_UNLOCK);
        pLog->isLocked = LOG_REGION_A;
      }
    }
    if( rc!=SQLITE_OK ){
      return rc;
    }

    if( SQLITE_OK==(rc = logEnterMutex(pLog)) ){
      u32 aCksum[2] = {1, 1};
      u32 aHdr[LOGSUMMARY_HDR_NFIELD+2];
      memcpy(aHdr, pLog->pSummary->aData, sizeof(aHdr));

      /* Verify the checksum on the log-summary header. If it fails,
      ** recover the log-summary from the log file.
................................................................................
       || aCksum[1]!=aHdr[LOGSUMMARY_HDR_NFIELD+1]
      ){
        rc = logSummaryRecover(pLog->pSummary, pLog->pFd);
        memcpy(aHdr, pLog->pSummary->aData, sizeof(aHdr));
        *pChanged = 1;
      }
      if( rc==SQLITE_OK ){

        if( memcmp(&pLog->hdr, aHdr, sizeof(LogSummaryHdr)) ){
          *pChanged = 1;
          memcpy(&pLog->hdr, aHdr, LOGSUMMARY_HDR_NFIELD*sizeof(u32));
        }
      }
      logLeaveMutex(pLog);
    }

    if( rc!=SQLITE_OK ){
      /* An error occured while attempting log recovery. */
      sqlite3LogCloseSnapshot(pLog);
    }
  }
  return rc;
}

/*
** Unlock the current snapshot.
*/
void sqlite3LogCloseSnapshot(Log *pLog){
  if( pLog->isLocked ){
    assert( pLog->isLocked==LOG_REGION_A || pLog->isLocked==LOG_REGION_D );
    logLockRegion(pLog, pLog->isLocked, LOG_UNLOCK);
  }
  pLog->isLocked = 0;
}



/* 
** Read a page from the log, if it is present. 
................................................................................
** This function returns SQLITE_OK if the caller may write to the database.
** Otherwise, if the caller is operating on a snapshot that has already
** been overwritten by another writer, SQLITE_OBE is returned.
*/
int sqlite3LogWriteLock(Log *pLog, int op){
  assert( pLog->isLocked );
  if( op ){

    /* Obtain the writer lock */
    int rc = logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_WRLOCK);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    if( memcmp(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr)) ){
      return SQLITE_BUSY;
    }
    pLog->isWriteLocked = 1;

  }else if( pLog->isWriteLocked ){
    logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_UNLOCK);
    memcpy(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr));
    pLog->isWriteLocked = 0;
  }
  return SQLITE_OK;
}

/* 
................................................................................
/* 
** Checkpoint the database. When this function is called the caller
** must hold an exclusive lock on the database file.
*/
int sqlite3LogCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
){
  int rc;




  do {
    rc = logLockRegion(pLog, LOG_REGION_B|LOG_REGION_C, LOG_WRLOCK);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );
  if( rc!=SQLITE_OK ) return rc;


  do {
    rc = logLockRegion(pLog, LOG_REGION_A, LOG_WRLOCK);
  }while( rc==SQLITE_BUSY && xBusyHandler(pBusyHandlerArg) );
  if( rc!=SQLITE_OK ) return rc;
  
  rc = logCheckpoint(pLog, pFd, zBuf);

  logLockRegion(pLog, LOG_REGION_A|LOG_REGION_B|LOG_REGION_C, LOG_UNLOCK);
  return rc;
}

/* Write a segment to the log. */
int sqlite3LogFrames(Log *pLog, int, PgHdr *, Pgno, int, int);

/* Copy pages from the log to the database file */ 
int sqlite3LogCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  u8 *zBuf                        /* Temporary buffer to use */


);

#endif /* _LOG_H_ */

/* Write a segment to the log. */
int sqlite3LogFrames(Log *pLog, int, PgHdr *, Pgno, int, int);

/* Copy pages from the log to the database file */ 
int sqlite3LogCheckpoint(
  Log *pLog,                      /* Log connection */
  sqlite3_file *pFd,              /* File descriptor open on db file */
  u8 *zBuf,                       /* Temporary buffer to use */
  int (*xBusyHandler)(void *),    /* Pointer to busy-handler function */
  void *pBusyHandlerArg           /* Argument to pass to xBusyHandler */
);

#endif /* _LOG_H_ */

  **
  ** 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.
  */

  assert( pPager->state>=PAGER_RESERVED );
  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);

  /* If the file is a temp-file has not yet been opened, open it now. It
  ** is not possible for rc to be other than SQLITE_OK if this branch
  ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
  */
................................................................................
    pPager->errCode = SQLITE_OK;
    pager_reset(pPager);
  }

  if( pagerUseLog(pPager) ){
    int changed = 0;

    /* TODO: Change the following block to grab a WAL read-lock. Or, 
    ** combine obtaining the read-lock with LogOpenSnapshot()?  */
    rc = pager_wait_on_lock(pPager, SHARED_LOCK);
    if( rc!=SQLITE_OK ){
      assert( pPager->state==PAGER_UNLOCK );
      return pager_error(pPager, rc);
    }

    rc = sqlite3LogOpenSnapshot(pPager->pLog, &changed);
    if( rc==SQLITE_OK ){
      if( changed ){
        pager_reset(pPager);
        assert( pPager->errCode || pPager->dbSizeValid==0 );
      }
      pPager->state = PAGER_SHARED;
      rc = sqlite3PagerPagecount(pPager, &changed);
    }
  }else if( pPager->state==PAGER_UNLOCK || isErrorReset ){
    sqlite3_vfs * const pVfs = pPager->pVfs;
    int isHotJournal = 0;
    assert( !MEMDB );
    assert( sqlite3PcacheRefCount(pPager->pPCache)==0 );
................................................................................
  int rc = SQLITE_OK;
  assert( pPager->state!=PAGER_UNLOCK );
  pPager->subjInMemory = (u8)subjInMemory;
  if( pPager->state==PAGER_SHARED ){
    assert( pPager->pInJournal==0 );
    assert( !MEMDB && !pPager->tempFile );

    /* Obtain a RESERVED lock on the database file. If the exFlag parameter
    ** is true, then immediately upgrade this to an EXCLUSIVE lock. The
    ** busy-handler callback can be used when upgrading to the EXCLUSIVE
    ** lock, but not when obtaining the RESERVED lock.
    */
    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 && pagerUseLog(pPager) ){
      /* Grab the write lock on the log file. If successful, upgrade to
      ** PAGER_EXCLUSIVE state. Otherwise, return an error code to the caller.
      ** The busy-handler is not invoked if another connection already
      ** holds the write-lock. If possible, the upper layer will call it.
      */
      rc = sqlite3LogWriteLock(pPager->pLog, 1);
      if( rc==SQLITE_OK ){
        pPager->dbOrigSize = pPager->dbSize;














      }
    }

    /* No need to open the journal file at this time.  It will be
    ** opened before it is written to.  If we defer opening the journal,
    ** we might save the work of creating a file if the transaction
    ** ends up being a no-op.
................................................................................

/*
** This function is called when the user invokes "PRAGMA checkpoint".
*/
int sqlite3PagerCheckpoint(Pager *pPager){
  int rc = SQLITE_OK;
  if( pPager->pLog ){
    rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
    if( rc==SQLITE_OK ){
      u8 *zBuf = (u8 *)pPager->pTmpSpace;
      rc = sqlite3LogCheckpoint(pPager->pLog, pPager->fd, zBuf);
    }


  }
  return rc;
}

#endif /* SQLITE_OMIT_DISKIO */

  **
  ** 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.
  */
  assert( !pagerUseLog(pList->pPager) );
  assert( pPager->state>=PAGER_RESERVED );
  rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);

  /* If the file is a temp-file has not yet been opened, open it now. It
  ** is not possible for rc to be other than SQLITE_OK if this branch
  ** is taken, as pager_wait_on_lock() is a no-op for temp-files.
  */
................................................................................
    pPager->errCode = SQLITE_OK;
    pager_reset(pPager);
  }

  if( pagerUseLog(pPager) ){
    int changed = 0;









    rc = sqlite3LogOpenSnapshot(pPager->pLog, &changed);
    if( rc==SQLITE_OK ){
      if( changed ){
        pager_reset(pPager);
        assert( pPager->errCode || pPager->dbSizeValid==0 );
      }
      pPager->state = PAGER_SHARED;         /* TODO: Is this right? */
      rc = sqlite3PagerPagecount(pPager, &changed);
    }
  }else if( pPager->state==PAGER_UNLOCK || isErrorReset ){
    sqlite3_vfs * const pVfs = pPager->pVfs;
    int isHotJournal = 0;
    assert( !MEMDB );
    assert( sqlite3PcacheRefCount(pPager->pPCache)==0 );
................................................................................
  int rc = SQLITE_OK;
  assert( pPager->state!=PAGER_UNLOCK );
  pPager->subjInMemory = (u8)subjInMemory;
  if( pPager->state==PAGER_SHARED ){
    assert( pPager->pInJournal==0 );
    assert( !MEMDB && !pPager->tempFile );














    if( pagerUseLog(pPager) ){
      /* Grab the write lock on the log file. If successful, upgrade to
      ** PAGER_EXCLUSIVE state. Otherwise, return an error code to the caller.
      ** The busy-handler is not invoked if another connection already
      ** holds the write-lock. If possible, the upper layer will call it.
      */
      rc = sqlite3LogWriteLock(pPager->pLog, 1);
      if( rc==SQLITE_OK ){
        pPager->dbOrigSize = pPager->dbSize;
        pPager->state = PAGER_RESERVED;
      }
    }else{
      /* Obtain a RESERVED lock on the database file. If the exFlag parameter
      ** is true, then immediately upgrade this to an EXCLUSIVE lock. The
      ** busy-handler callback can be used when upgrading to the EXCLUSIVE
      ** lock, but not when obtaining the RESERVED lock.
      */
      rc = sqlite3OsLock(pPager->fd, RESERVED_LOCK);
      if( rc==SQLITE_OK ){
        pPager->state = PAGER_RESERVED;
        if( exFlag ){
          rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
        }
      }
    }

    /* No need to open the journal file at this time.  It will be
    ** opened before it is written to.  If we defer opening the journal,
    ** we might save the work of creating a file if the transaction
    ** ends up being a no-op.
................................................................................

/*
** This function is called when the user invokes "PRAGMA checkpoint".
*/
int sqlite3PagerCheckpoint(Pager *pPager){
  int rc = SQLITE_OK;
  if( pPager->pLog ){


    u8 *zBuf = (u8 *)pPager->pTmpSpace;
    rc = sqlite3LogCheckpoint(pPager->pLog, pPager->fd, 

        zBuf, pPager->xBusyHandler, pPager->pBusyHandlerArg
    );
  }
  return rc;
}

#endif /* SQLITE_OMIT_DISKIO */