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Overview
Comment:Begin adding code to make the log file circular.
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SHA1: ec6dda0ade61f1511404fb0f4145d8ccbc0a5eb0
User & Date: dan 2013-10-26 19:58:34.163
Context
2013-10-28
10:47
Fix some problems with circular logs. check-in: 4b36a0245f user: dan tags: trunk
2013-10-26
19:58
Begin adding code to make the log file circular. check-in: ec6dda0ade user: dan tags: trunk
12:44
Use sqlite4_malloc instead of malloc in test fixture for num check-in: 3f6924e784 user: peterreid tags: trunk
Changes
Unified Diff Ignore Whitespace Patch
Changes to src/bt_log.c.
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#include <assert.h>
#include <stdio.h>
#include <stddef.h>

/* Magic values identifying WAL file header */
#define BT_WAL_MAGIC   0xBEE1CA62
#define BT_WAL_VERSION 0x00000001





typedef struct BtWalHdr BtWalHdr;
typedef struct BtShmHdr BtShmHdr;
typedef struct BtCkptHdr BtCkptHdr;
typedef struct BtFrameHdr BtFrameHdr;
typedef struct BtShm BtShm;








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#include <assert.h>
#include <stdio.h>
#include <stddef.h>

/* Magic values identifying WAL file header */
#define BT_WAL_MAGIC   0xBEE1CA62
#define BT_WAL_VERSION 0x00000001

/* Wrap the log around if there is a block of this many free frames at
** the start of the file.  */
#define BT_NWRAPLOG    100

typedef struct BtWalHdr BtWalHdr;
typedef struct BtShmHdr BtShmHdr;
typedef struct BtCkptHdr BtCkptHdr;
typedef struct BtFrameHdr BtFrameHdr;
typedef struct BtShm BtShm;

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  u32 iFirstFrame;                /* First frame of log (numbered from 1) */

  u32 aCksum[2];                  /* Checksum of all prior fields */
};

/*
** WAL Frame header. All fields are stored in big-endian order.





*/
struct BtFrameHdr {
  u32 pgno;                       /* Page number of this frame */
  u32 ctrl;                       /* Next frame pointer and commit bit */
  u32 aCksum[2];                  /* Frame checksum */
};

#define BT_FRAME_COMMIT 0x80000000

/*
** Shared memory header. Shared memory begins with two copies of
** this structure. All fields are stored in machine byte-order.
*/
struct BtShmHdr {
  u32 aLog[6];                    /* First/last frames for each log region */
  int nSector;                    /* Sector size assumed for WAL file */

  u32 aFrameCksum[2];             /* Checksum of previous frame */

  int padding;
  u32 aCksum[2];                  /* Object checksum */
};

/*
** A single instance of this structure follows the two BtShmHdr structures 
** in shared memory.
**







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  u32 iFirstFrame;                /* First frame of log (numbered from 1) */

  u32 aCksum[2];                  /* Checksum of all prior fields */
};

/*
** WAL Frame header. All fields are stored in big-endian order.
**
** ctrl:
**   The most-significant-bit (BT_FRAME_COMMIT) of this field is set for
**   a commit frame and clear for all others. The other 31 bits contain
**   the frame number of the next frame in the log. 
*/
struct BtFrameHdr {
  u32 pgno;                       /* Page number of this frame */
  u32 ctrl;                       /* Next frame pointer and commit bit */
  u32 aCksum[2];                  /* Frame checksum */
};

#define BT_FRAME_COMMIT 0x80000000

/*
** Shared memory header. Shared memory begins with two copies of
** this structure. All fields are stored in machine byte-order.
*/
struct BtShmHdr {
  u32 aLog[6];                    /* First/last frames for each log region */
  int nSector;                    /* Sector size assumed for WAL file */
  int iHashSide;                  /* Hash table side for region (c) of log */
  u32 aFrameCksum[2];             /* Checksum of previous frame */
  u32 iNextFrame;                 /* Location to write next log frame to */
  int padding;                    /* So that this structure is 8-byte aligned */
  u32 aCksum[2];                  /* Object checksum */
};

/*
** A single instance of this structure follows the two BtShmHdr structures 
** in shared memory.
**
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** Log handle used by bt_pager.c to access functionality implemented by
** this module. 
*/
struct BtLog {
  BtLock *pLock;                  /* Lock object associated with this log */
  bt_file *pFd;                   /* File handle open on WAL file */
  BtShmHdr snapshot;              /* Current snapshot of shm-header */

  int nShm;                       /* Size of apShm[] array */
  u8 **apShm;                     /* Array of mapped shared-memory blocks */

};

typedef u16 ht_slot;

/*
** Number of entries in each hash table bar the first. 
*/







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** Log handle used by bt_pager.c to access functionality implemented by
** this module. 
*/
struct BtLog {
  BtLock *pLock;                  /* Lock object associated with this log */
  bt_file *pFd;                   /* File handle open on WAL file */
  BtShmHdr snapshot;              /* Current snapshot of shm-header */

  int nShm;                       /* Size of apShm[] array */
  u8 **apShm;                     /* Array of mapped shared-memory blocks */
  int nWrapLog;                   /* Wrap if this many free frames at start */
};

typedef u16 ht_slot;

/*
** Number of entries in each hash table bar the first. 
*/
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** returns the value that would be produced by intepreting the 4 bytes
** of the input value as a little-endian integer.
*/
#define BYTESWAP32(x) ( \
  (((x)&0x000000FF)<<24) + (((x)&0x0000FF00)<<8)  \
  + (((x)&0x00FF0000)>>8)  + (((x)&0xFF000000)>>24) \
)









/*
** Generate or extend an 8 byte checksum based on the data in
** array aByte[] and the initial values of aIn[0] and aIn[1] (or
** initial values of 0 and 0 if aIn==NULL).
**
** The checksum is written back into aOut[] before returning.







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** returns the value that would be produced by intepreting the 4 bytes
** of the input value as a little-endian integer.
*/
#define BYTESWAP32(x) ( \
  (((x)&0x000000FF)<<24) + (((x)&0x0000FF00)<<8)  \
  + (((x)&0x00FF0000)>>8)  + (((x)&0xFF000000)>>24) \
)

static void btLogDebugTopology(char *zStr, u32 *aLog){
  fprintf(stderr, "%s: %d..%d  %d..%d  %d..%d\n", zStr,
      (int)aLog[0], (int)aLog[1], (int)aLog[2], 
      (int)aLog[3], (int)aLog[4], (int)aLog[5]
  );
  fflush(stderr);
}

/*
** Generate or extend an 8 byte checksum based on the data in
** array aByte[] and the initial values of aIn[0] and aIn[1] (or
** initial values of 0 and 0 if aIn==NULL).
**
** The checksum is written back into aOut[] before returning.
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      btLogChecksum(1, aBuf, pgsz, aCksum, aCksum);
      if( aCksum[0]!=fhdr.aCksum[0] || aCksum[1]!=fhdr.aCksum[1] ) break;
    }
    if( rc==SQLITE4_OK ){
      rc = xFrame(pLog, pCtx, iFrame, &fhdr);
    }

    iFrame++;
  }

  return rc;
}

/*
** Locate the iHash'th hash table in shared memory. Return it.
*/
static int btLogFindHash(
  BtLog *pLog,                    /* Log handle */

  int iHash,                      /* Hash table (numbered from 0) to find */
  ht_slot **paHash,               /* OUT: Pointer to hash slots */
  u32 **paPgno,                   /* OUT: Pointer to page number array */
  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
){
  int rc;



  rc = btLogMapShm(pLog, iHash);
  if( rc==SQLITE4_OK ){
    u8 *aChunk = pLog->apShm[iHash];
    u32 *aPgno;
    u32 iZero;

    *paHash = (ht_slot*)&aChunk[HASHTABLE_OFFSET_1];
    if( iHash==0 ){
      aPgno = (u32*)&aChunk[sizeof(BtShm)];
      iZero = 1;
    }else{
      aPgno = (u32*)aChunk;
      iZero = 1 + HASHTABLE_NFRAME_ONE + (HASHTABLE_NFRAME * (iHash-1));
    }







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      btLogChecksum(1, aBuf, pgsz, aCksum, aCksum);
      if( aCksum[0]!=fhdr.aCksum[0] || aCksum[1]!=fhdr.aCksum[1] ) break;
    }
    if( rc==SQLITE4_OK ){
      rc = xFrame(pLog, pCtx, iFrame, &fhdr);
    }

    iFrame = (fhdr.ctrl & ~BT_FRAME_COMMIT);
  }

  return rc;
}

/*
** Locate the iHash'th hash table in shared memory. Return it.
*/
static int btLogFindHash(
  BtLog *pLog,                    /* Log handle */
  int iSide,                      /* Which set of hash slots to return */
  int iHash,                      /* Hash table (numbered from 0) to find */
  ht_slot **paHash,               /* OUT: Pointer to hash slots */
  u32 **paPgno,                   /* OUT: Pointer to page number array */
  u32 *piZero                     /* OUT: Frame associated with *paPgno[0] */
){
  int rc;                         /* Return code */

  assert( iSide==0 || iSide==1 );

  rc = btLogMapShm(pLog, iHash);
  if( rc==SQLITE4_OK ){
    u8 *aChunk = pLog->apShm[iHash];
    u32 *aPgno;
    u32 iZero;

    *paHash = (ht_slot*)&aChunk[iSide?HASHTABLE_OFFSET_1:HASHTABLE_OFFSET_2];
    if( iHash==0 ){
      aPgno = (u32*)&aChunk[sizeof(BtShm)];
      iZero = 1;
    }else{
      aPgno = (u32*)aChunk;
      iZero = 1 + HASHTABLE_NFRAME_ONE + (HASHTABLE_NFRAME * (iHash-1));
    }
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*/
static int btLogHashInsert(BtLog *pLog, u32 pgno, u32 iFrame){
  int iHash;                      /* Index of hash table to update */
  int rc = SQLITE4_OK;            /* Return code */
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */


  assert( iFrame>=1 && pgno>=1 );

  /* Find the required hash table */
  iHash = btLogFrameHash(pLog, iFrame);
  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);

  /* Update the hash table */
  if( rc==SQLITE4_OK ){
    int iSlot;
    int nCollide = HASHTABLE_NSLOT*2;
    aPgno[iFrame-iZero] = pgno;





    for(iSlot=btLogHashKey(pLog,pgno); ; iSlot=btLogHashNext(pLog, iSlot)){
      if( aHash[iSlot]==0 ){
        aHash[iSlot] = (iFrame-iZero+1);
        break;
      }
      if( (nCollide--)==0 ) return btErrorBkpt(SQLITE4_CORRUPT);







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*/
static int btLogHashInsert(BtLog *pLog, u32 pgno, u32 iFrame){
  int iHash;                      /* Index of hash table to update */
  int rc = SQLITE4_OK;            /* Return code */
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */
  int iSide = pLog->snapshot.iHashSide;

  assert( iFrame>=1 && pgno>=1 );

  /* Find the required hash table */
  iHash = btLogFrameHash(pLog, iFrame);
  rc = btLogFindHash(pLog, iSide, iHash, &aHash, &aPgno, &iZero);

  /* Update the hash table */
  if( rc==SQLITE4_OK ){
    int iSlot;
    int nCollide = HASHTABLE_NSLOT*2;
    aPgno[iFrame-iZero] = pgno;

    if( iFrame==iZero ){
      memset(aHash, 0, sizeof(ht_slot) * HASHTABLE_NSLOT);
    }

    for(iSlot=btLogHashKey(pLog,pgno); ; iSlot=btLogHashNext(pLog, iSlot)){
      if( aHash[iSlot]==0 ){
        aHash[iSlot] = (iFrame-iZero+1);
        break;
      }
      if( (nCollide--)==0 ) return btErrorBkpt(SQLITE4_CORRUPT);
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** Remove everything following frame iFrame from the iHash'th hash table.
*/
static int btLogHashRollback(BtLog *pLog, int iHash, u32 iFrame){
  const int nPgno = (iHash==0 ? HASHTABLE_NFRAME_ONE : HASHTABLE_NFRAME);
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */

  int rc;

  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);
  if( rc==SQLITE4_OK ){
    int i;
    ht_slot iMax;
    iMax = (iFrame - iZero) + 1;

    for(i=0; i<HASHTABLE_NSLOT; i++){
      if( aHash[i]>iMax ) aHash[i] = 0;
    }
    memset(&aPgno[iMax], 0, (nPgno-iMax)*sizeof(u32));
  }

  return rc;
}


/*
** Return true if log is completely empty (as it is if a file zero bytes
** in size has been opened or created).
*/
static int btLogIsEmpty(BtLog *pLog){
  return (pLog->snapshot.aLog[4]==0);
}







static int btLogRecoverFrame(
  BtLog *pLog,                    /* Log module handle */
  void *pCtx,                     /* woints to type u32 - pgno of last commit*/
  u32 iFrame,                     /* Frame number */
  BtFrameHdr *pHdr                /* Frame header */
){


























  btLogHashInsert(pLog, pHdr->pgno, iFrame);
  if( pHdr->ctrl & BT_FRAME_COMMIT ){

    *(u32*)pCtx = iFrame;

    memcpy(pLog->snapshot.aFrameCksum, pHdr->aCksum, sizeof(pHdr->aCksum));
  }

#if 0
  fprintf(stderr, "recovered frame=%d pgno=%d\n", iFrame, pHdr->pgno);
  fflush(stderr);
#endif
  return 0;
}




































/*
** Run log recovery. In other words, read the log file from disk and 
** initialize the shared-memory accordingly.
*/
static int btLogRecover(BtLog *pLog){
  bt_env *pVfs = pLog->pLock->pVfs;







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** Remove everything following frame iFrame from the iHash'th hash table.
*/
static int btLogHashRollback(BtLog *pLog, int iHash, u32 iFrame){
  const int nPgno = (iHash==0 ? HASHTABLE_NFRAME_ONE : HASHTABLE_NFRAME);
  ht_slot *aHash;                 /* Hash slots */
  u32 *aPgno;                     /* Page array for updated hash table */
  u32 iZero;                      /* Zero-offset of updated hash table */
  int iSide = pLog->snapshot.iHashSide;
  int rc;

  rc = btLogFindHash(pLog, iSide, iHash, &aHash, &aPgno, &iZero);
  if( rc==SQLITE4_OK ){
    int i;
    ht_slot iMax;
    iMax = (iFrame - iZero) + 1;

    for(i=0; i<HASHTABLE_NSLOT; i++){
      if( aHash[i]>iMax ) aHash[i] = 0;
    }
    memset(&aPgno[iMax], 0, (nPgno-iMax)*sizeof(u32));
  }

  return rc;
}


/*
** Return true if log is completely empty (as it is if a file zero bytes
** in size has been opened or created).
*/
static int btLogIsEmpty(BtLog *pLog){
  return (pLog->snapshot.aLog[4]==0 && pLog->snapshot.iNextFrame==0);
}

typedef struct FrameRecoverCtx FrameRecoverCtx;
struct FrameRecoverCtx {
  u32 iLast;                      /* Frame containing last commit flag in log */
  u32 iNextFrame;                 /* Frame that follows frame iLast */
};

static int btLogRecoverFrame(
  BtLog *pLog,                    /* Log module handle */
  void *pCtx,                     /* woints to type u32 - pgno of last commit*/
  u32 iFrame,                     /* Frame number */
  BtFrameHdr *pHdr                /* Frame header */
){
  if( btLogIsEmpty(pLog) ){
    /* This is the first frame recovered. It is therefore both the first
    ** and last frame of log region (c).  */
    pLog->snapshot.aLog[4] = iFrame;
    pLog->snapshot.aLog[5] = iFrame;
  }else{
    u32 iExpect = pLog->snapshot.aLog[5]+1;
    if( iFrame==iExpect ){
      pLog->snapshot.aLog[5] = iFrame;
    }else if( iFrame<iExpect ){
      assert( iFrame==1 );
      assert( pLog->snapshot.aLog[0]==0 && pLog->snapshot.aLog[1]==0 );
      pLog->snapshot.aLog[0] = pLog->snapshot.aLog[4];
      pLog->snapshot.aLog[1] = pLog->snapshot.aLog[5];
      pLog->snapshot.aLog[4] = iFrame;
      pLog->snapshot.aLog[5] = iFrame;
      pLog->snapshot.iHashSide = (pLog->snapshot.iHashSide + 1) % 2;
    }else{
      assert( pLog->snapshot.aLog[2]==0 && pLog->snapshot.aLog[3]==0 );
      pLog->snapshot.aLog[2] = pLog->snapshot.aLog[4];
      pLog->snapshot.aLog[3] = pLog->snapshot.aLog[5];
      pLog->snapshot.aLog[4] = iFrame;
      pLog->snapshot.aLog[5] = iFrame;
    }
  }

  btLogHashInsert(pLog, pHdr->pgno, iFrame);
  if( pHdr->ctrl & BT_FRAME_COMMIT ){
    FrameRecoverCtx *pFRC = (FrameRecoverCtx*)pCtx;
    pFRC->iLast = iFrame;
    pFRC->iNextFrame = (pHdr->ctrl & ~BT_FRAME_COMMIT);
    memcpy(pLog->snapshot.aFrameCksum, pHdr->aCksum, sizeof(pHdr->aCksum));
  }

#if 0
  fprintf(stderr, "recovered frame=%d pgno=%d\n", iFrame, pHdr->pgno);
  fflush(stderr);
#endif
  return 0;
}

/*
** This function is called as part of log recovery. The log file has 
** already been scanned and the log topology (pLog->snapshot.aLog[])
** shared-memory hash tables have been populated with data corresponding
** to the entire set of valid frames recovered from the log file -
** including uncommitted frames. This function removes the uncommitted
** frames from the log topology and shared hash tables.
*/
static int btLogRollbackRecovery(BtLog *pLog, FrameRecoverCtx *pCtx){
  u32 iLast = pCtx->iLast;        /* Last committed frame in log file */
  u32 *aLog = pLog->snapshot.aLog;/* Log file topology */

  while( iLast<aLog[4] || iLast>aLog[5] ){
    if( aLog[2] ){
      aLog[5] = aLog[3];
      aLog[4] = aLog[2];
      if( aLog[0] && aLog[0]<aLog[4] ){
        aLog[3] = aLog[1];
        aLog[2] = aLog[0];
        aLog[0] = aLog[1] = 0;
      }else{
        aLog[2] = aLog[3] = 0;
      }
    }else{
      aLog[5] = aLog[1];
      aLog[4] = aLog[0];
      aLog[0] = aLog[1] = 0;
      pLog->snapshot.iHashSide = (pLog->snapshot.iHashSide + 1) % 2;
    }
  }

  aLog[5] = iLast;
  return btLogHashRollback(pLog, btLogFrameHash(pLog, iLast), iLast);
}

/*
** Run log recovery. In other words, read the log file from disk and 
** initialize the shared-memory accordingly.
*/
static int btLogRecover(BtLog *pLog){
  bt_env *pVfs = pLog->pLock->pVfs;
521
522
523
524
525
526
527
528
529
530
531








532

533





534
535
536

537

538
539
540

541
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543
544
545
546
547
        pHdr = &hdr1;
        iSlot = 1;
      }
    }
    if( rc==SQLITE4_NOTFOUND ) rc = SQLITE4_OK;
  }

  /* If a header was successfully read from the file, scan the log file
  ** and populate the shared-memory hash tables.  */
  if( pHdr ){
    u32 iCommit = 0;








    rc = btLogTraverse(pLog, pHdr, btLogRecoverFrame, (void*)&iCommit);

    if( rc==SQLITE4_OK && iCommit>0 ){





      BtShm *pShm = btLogShm(pLog);
      pShm->ckpt.iFirstRead = 1;
      pShm->ckpt.iWalHdr = (iSlot<<2) + pHdr->iCnt;

      pShm->ckpt.iFirstRecover = 1;

      pLog->snapshot.aLog[4] = 1;
      pLog->snapshot.aLog[5] = iCommit;
      rc = btLogHashRollback(pLog, btLogFrameHash(pLog, iCommit), iCommit);

    }
  }

  return rc;
}

/*







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618
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645

646
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651
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658
        pHdr = &hdr1;
        iSlot = 1;
      }
    }
    if( rc==SQLITE4_NOTFOUND ) rc = SQLITE4_OK;
  }

  /* If a header was successfully read from the file, attempt to 
  ** recover frames from the log file. */
  if( pHdr ){

    /* The following iterates through all readable frames in the log file.
    ** It populates pLog->snapshot.aLog[] with the log topology and the
    ** shared hash-tables with the pgno->frame mapping. The FrameRecoverCtx
    ** object is populated with the frame number and "next frame" pointer of
    ** the last commit-frame in the log (if any). Additionally, the
    ** pLog->snapshot.aFrameCksum[] variables are populated with the checksum
    ** beloging to the frame header of the last commit-frame in the log.  */
    FrameRecoverCtx ctx = {0, 0};
    rc = btLogTraverse(pLog, pHdr, btLogRecoverFrame, (void*)&ctx);

    if( rc==SQLITE4_OK ){
      if( ctx.iLast==0 ){
        /* No transactions recovered from the log file. */
        btLogZeroSnapshot(pLog);
      }else{
        /* One or more transactions were recovered from the log file. */
        BtShm *pShm = btLogShm(pLog);

        pShm->ckpt.iWalHdr = (iSlot<<2) + pHdr->iCnt;
        pShm->ckpt.iFirstRead = pHdr->iFirstFrame;
        pShm->ckpt.iFirstRecover = pHdr->iFirstFrame;
        rc = btLogRollbackRecovery(pLog, &ctx);
        pLog->snapshot.iNextFrame = ctx.iNextFrame;


      }
    }
  }

  return rc;
}

/*
561
562
563
564
565
566
567

568
569
570
571
572
573
574
  pLog = sqlite4_malloc(pEnv, sizeof(BtLog));
  if( pLog==0 ){
    rc = SQLITE4_NOMEM;
    goto open_out;
  }
  memset(pLog, 0, sizeof(BtLog));
  pLog->pLock = (BtLock*)pPager;


  zWal = sqlite4BtPagerFilename(pPager, BT_PAGERFILE_LOG);
  rc = pVfs->xOpen(pEnv, pVfs, zWal, 0, &pLog->pFd);

  if( rc==SQLITE4_OK && bRecover ){
    rc = btLogMapShm(pLog, 0);
    if( rc==SQLITE4_OK ){







>







672
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674
675
676
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678
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680
681
682
683
684
685
686
  pLog = sqlite4_malloc(pEnv, sizeof(BtLog));
  if( pLog==0 ){
    rc = SQLITE4_NOMEM;
    goto open_out;
  }
  memset(pLog, 0, sizeof(BtLog));
  pLog->pLock = (BtLock*)pPager;
  pLog->nWrapLog = BT_NWRAPLOG;

  zWal = sqlite4BtPagerFilename(pPager, BT_PAGERFILE_LOG);
  rc = pVfs->xOpen(pEnv, pVfs, zWal, 0, &pLog->pFd);

  if( rc==SQLITE4_OK && bRecover ){
    rc = btLogMapShm(pLog, 0);
    if( rc==SQLITE4_OK ){
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634






635
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637
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647
  /* Write the object to disk */
  iOff = iHdr * pLog->snapshot.nSector;
  rc = btLogWriteData(pLog, iOff, (u8*)pHdr, sizeof(BtWalHdr));

  return rc;
}

static int btLogHashSearch(BtLog *pLog, int iHash, u32 pgno, u32 *piFrame){






  ht_slot *aHash;
  u32 *aPgno;
  u32 iZero;
  int rc;

  rc = btLogFindHash(pLog, iHash, &aHash, &aPgno, &iZero);
  if( rc==SQLITE4_OK ){
    int nCollide = HASHTABLE_NSLOT*2;
    int iSlot;
    u32 iFrame = 0;
    
    iSlot=btLogHashKey(pLog, pgno); 
    for( ; aHash[iSlot]; iSlot=btLogHashNext(pLog, iSlot)){







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|







739
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743
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759
760
761
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764
765
  /* Write the object to disk */
  iOff = iHdr * pLog->snapshot.nSector;
  rc = btLogWriteData(pLog, iOff, (u8*)pHdr, sizeof(BtWalHdr));

  return rc;
}

static int btLogHashSearch(
  BtLog *pLog,                    /* Log module handle */
  int iSide,                      /* 0 or 1 - the side of hash table to read */
  int iHash,                      /* Index of hash to query */
  u32 pgno,                       /* query for this page number */
  u32 *piFrame                    /* OUT: Frame number for matching entry */
){
  ht_slot *aHash;
  u32 *aPgno;
  u32 iZero;
  int rc;

  rc = btLogFindHash(pLog, iSide, iHash, &aHash, &aPgno, &iZero);
  if( rc==SQLITE4_OK ){
    int nCollide = HASHTABLE_NSLOT*2;
    int iSlot;
    u32 iFrame = 0;
    
    iSlot=btLogHashKey(pLog, pgno); 
    for( ; aHash[iSlot]; iSlot=btLogHashNext(pLog, iSlot)){
656
657
658
659
660
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662
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664
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668
669
670
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672
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674
675
676
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679
680

681








682
683


684
685




686
687
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689
690
691
692
      rc = SQLITE4_NOTFOUND;
    }
  }

  return rc;
}


/*
** Attempt to read data for page pgno from the log file. If successful,
** the data is written into buffer aData[] (which must be at least as
** large as a database page). In this case SQLITE4_OK is returned.
**
** If the log does not contain any version of page pgno, SQLITE4_NOTFOUND
** is returned and the contents of buffer aData[] are not modified.
**
** If any other error occurs, an SQLite4 error code is returned. The final
** state of buffer aData[] is undefined in this case.
*/
int sqlite4BtLogRead(BtLog *pLog, u32 pgno, u8 *aData){
  const int pgsz = sqlite4BtPagerPagesize((BtPager*)(pLog->pLock));
  int iHash;
  int iHashLast;
  int rc = SQLITE4_NOTFOUND;
  u32 iFrame = 0;










  iHash = btLogFrameHash(pLog, pLog->snapshot.aLog[5]);
  iHashLast = btLogFrameHash(pLog, pLog->snapshot.aLog[4]);


  for( ; rc==SQLITE4_NOTFOUND && iHash>=iHashLast && iFrame==0; iHash--){
    rc = btLogHashSearch(pLog, iHash, pgno, &iFrame);




  }

  if( rc==SQLITE4_OK && iFrame!=0 ){
    bt_env *pVfs = pLog->pLock->pVfs;
    i64 iOff;
    assert( rc==SQLITE4_OK );
    iOff = btLogFrameOffset(pLog, pgsz, iFrame);







<













<
<


>

>
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774
775
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778
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780

781
782
783
784
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786
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789
790
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793


794
795
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811
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815
816
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818
819
820
821
822
      rc = SQLITE4_NOTFOUND;
    }
  }

  return rc;
}


/*
** Attempt to read data for page pgno from the log file. If successful,
** the data is written into buffer aData[] (which must be at least as
** large as a database page). In this case SQLITE4_OK is returned.
**
** If the log does not contain any version of page pgno, SQLITE4_NOTFOUND
** is returned and the contents of buffer aData[] are not modified.
**
** If any other error occurs, an SQLite4 error code is returned. The final
** state of buffer aData[] is undefined in this case.
*/
int sqlite4BtLogRead(BtLog *pLog, u32 pgno, u8 *aData){
  const int pgsz = sqlite4BtPagerPagesize((BtPager*)(pLog->pLock));


  int rc = SQLITE4_NOTFOUND;
  u32 iFrame = 0;
  int i;

  /* Loop through regions (c), (b) and (a) of the log file. In that order. */
  for(i=2; i>=0; i--){
    u32 iLo = pLog->snapshot.aLog[i*2+0];
    u32 iHi = pLog->snapshot.aLog[i*2+1];
    int iSide;
    int iHash;
    int iHashLast;

    iHash = btLogFrameHash(pLog, iHi);
    iHashLast = btLogFrameHash(pLog, iLo);
    iSide = (pLog->snapshot.iHashSide + (i==0)) % 2;

    for( ; rc==SQLITE4_NOTFOUND && iHash>=iHashLast && iFrame==0; iHash--){
      rc = btLogHashSearch(pLog, iSide, iHash, pgno, &iFrame);
      if( rc==SQLITE4_OK && (iFrame<iLo || iFrame>iHi) ){
        rc = SQLITE4_NOTFOUND;
      }
    }
  }

  if( rc==SQLITE4_OK && iFrame!=0 ){
    bt_env *pVfs = pLog->pLock->pVfs;
    i64 iOff;
    assert( rc==SQLITE4_OK );
    iOff = btLogFrameOffset(pLog, pgsz, iFrame);
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708
709
710
711
712
713


714
715
716
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718
719
720
721
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724
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728
729
730
731
732
733

734
735
736
737
738
739

























740
741
742
743
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746
747
748
749
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751
752
753
754
755




756
757
758
759
760
761



762
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764
765
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767
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771


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int sqlite4BtLogWrite(BtLog *pLog, u32 pgno, u8 *aData, int bCommit){
  const int pgsz = sqlite4BtPagerPagesize((BtPager*)(pLog->pLock));
  int rc = SQLITE4_OK;
  u32 iFrame;                     /* Write this frame (numbered from 1) */
  BtFrameHdr frame;               /* Header for new frame */
  u32 *a;                         /* Pointer to cksum of previous frame */
  i64 iOff;                       /* Offset of log file to write to */



  /* Handle a special case - if the log file is completely empty then
  ** this writer must write the first header into the WAL file. */
  if( btLogIsEmpty(pLog) ){
    BtWalHdr hdr;
    memset(&hdr, 0, sizeof(BtWalHdr));

    hdr.iMagic = BT_WAL_MAGIC;
    hdr.iVersion = BT_WAL_VERSION;
    hdr.nSector = pLog->snapshot.nSector;
    hdr.nPgsz = pgsz;
    hdr.iSalt1 = 22;
    hdr.iSalt2 = 23;
    hdr.iFirstFrame = 1;

    rc = btLogWriteHeader(pLog, 0, &hdr);
    if( rc!=SQLITE4_OK ) return rc;

    pLog->snapshot.aFrameCksum[0] = hdr.iSalt1;
    pLog->snapshot.aFrameCksum[1] = hdr.iSalt2;

  }

  /* Figure out where exactly to write the new data */
  iFrame = pLog->snapshot.aLog[5] + 1;
  iOff = btLogFrameOffset(pLog, pgsz, iFrame);


























  /* Populate the frame header object. */
  memset(&frame, 0, sizeof(frame));
  frame.pgno = pgno;
  frame.ctrl = (bCommit ? BT_FRAME_COMMIT : 0);
  a = pLog->snapshot.aFrameCksum;
  btLogChecksum(1, (u8*)&frame, offsetof(BtFrameHdr, aCksum), a, frame.aCksum);
  btLogChecksum(1, aData, pgsz, frame.aCksum, frame.aCksum);

#if 0
fprintf(stderr, "writing page %d at log offset %d (frame %d)\n", (int)pgno, (int)iOff, (int)iFrame);
fflush(stderr);
#endif


  /* Write the header and page record to the log file. */
  rc = btLogWriteData(pLog, iOff, (u8*)&frame, sizeof(frame));




  if( rc==SQLITE4_OK ){
    rc = btLogWriteData(pLog, iOff+sizeof(frame), aData, pgsz);
  }

  /* Update the wal index hash tables with the (pgno -> iFrame) record. */
  if( rc==SQLITE4_OK ){



    rc = btLogHashInsert(pLog, pgno, iFrame);
  }

  /* Update the private copy of the shm-header */
  if( rc==SQLITE4_OK ){
    pLog->snapshot.aLog[5] = iFrame;
    if( btLogIsEmpty(pLog) ){
      assert( iFrame==1 );
      pLog->snapshot.aLog[4] = iFrame;











    }


    memcpy(pLog->snapshot.aFrameCksum, frame.aCksum, sizeof(frame.aCksum));

  }

  /* If this is a COMMIT, also update the shared shm-header. */
  if( bCommit ){
    rc = btLogUpdateSharedHdr(pLog);
  }








>
>




















>


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|


>
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|


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>
>
>





|


|
>
>
>
>
>
>
>
>
>
>
>

>
>

>







837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910

911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
int sqlite4BtLogWrite(BtLog *pLog, u32 pgno, u8 *aData, int bCommit){
  const int pgsz = sqlite4BtPagerPagesize((BtPager*)(pLog->pLock));
  int rc = SQLITE4_OK;
  u32 iFrame;                     /* Write this frame (numbered from 1) */
  BtFrameHdr frame;               /* Header for new frame */
  u32 *a;                         /* Pointer to cksum of previous frame */
  i64 iOff;                       /* Offset of log file to write to */
  u32 iNextFrame;
  u32 *aLog = pLog->snapshot.aLog;

  /* Handle a special case - if the log file is completely empty then
  ** this writer must write the first header into the WAL file. */
  if( btLogIsEmpty(pLog) ){
    BtWalHdr hdr;
    memset(&hdr, 0, sizeof(BtWalHdr));

    hdr.iMagic = BT_WAL_MAGIC;
    hdr.iVersion = BT_WAL_VERSION;
    hdr.nSector = pLog->snapshot.nSector;
    hdr.nPgsz = pgsz;
    hdr.iSalt1 = 22;
    hdr.iSalt2 = 23;
    hdr.iFirstFrame = 1;

    rc = btLogWriteHeader(pLog, 0, &hdr);
    if( rc!=SQLITE4_OK ) return rc;

    pLog->snapshot.aFrameCksum[0] = hdr.iSalt1;
    pLog->snapshot.aFrameCksum[1] = hdr.iSalt2;
    pLog->snapshot.iNextFrame = 1;
  }

  /* Figure out the offset to write the current frame to. */
  iFrame = pLog->snapshot.iNextFrame;
  iOff = btLogFrameOffset(pLog, pgsz, iFrame);

  /* The current frame will be written to location pLog->snapshot.iNextFrame.
  ** This code determines where the following frame will be stored. There
  ** are three possibilities:
  **
  **   1) The next frame follows the current frame (this is the usual case).
  **   2) The next frame is frame 1 - the log wraps around.
  **   3) Following the current frame is a block of frames still in use.
  **      So the next frame will immediately follow this block.
  */
  iNextFrame = pLog->snapshot.iNextFrame + 1;
  if( iFrame!=1 
   && aLog[0]==0 && aLog[2]==0 
   && aLog[4]!=0 && aLog[4]>pLog->nWrapLog 
  ){
    /* Case 2) It is possible to wrap the log around */
    iNextFrame = 1;
  }else if( iNextFrame==aLog[0] ){
    /* Case 3) It is necessary to jump over some existing log. */
    iNextFrame = aLog[1]+1;
  }

  if( iNextFrame & 0x80000000 ){
    rc = SQLITE4_FULL;
  }else{

    /* Populate the frame header object. */
    memset(&frame, 0, sizeof(frame));
    frame.pgno = pgno;
    frame.ctrl = (bCommit ? BT_FRAME_COMMIT : 0) + iNextFrame;
    a = pLog->snapshot.aFrameCksum;
    btLogChecksum(1, (u8*)&frame, offsetof(BtFrameHdr,aCksum), a, frame.aCksum);
    btLogChecksum(1, aData, pgsz, frame.aCksum, frame.aCksum);

#if 0
    fprintf(stderr, "writing page %d at log offset %d (frame %d)\n", (int)pgno, (int)iOff, (int)iFrame);
    fflush(stderr);
#endif


    /* Write the frame header to the log file. */
    rc = btLogWriteData(pLog, iOff, (u8*)&frame, sizeof(frame));
  }
  pLog->snapshot.iNextFrame = iNextFrame;

  /* Write the frame contents to the log file. */
  if( rc==SQLITE4_OK ){
    rc = btLogWriteData(pLog, iOff+sizeof(frame), aData, pgsz);
  }

  /* Update the wal index hash tables with the (pgno -> iFrame) record. */
  if( rc==SQLITE4_OK ){
    if( iFrame==1 ){
      pLog->snapshot.iHashSide = (pLog->snapshot.iHashSide+1) %2;
    }
    rc = btLogHashInsert(pLog, pgno, iFrame);
  }

  /* Update the private copy of the shm-header */
  if( rc==SQLITE4_OK ){
btLogDebugTopology("log1", aLog);
    if( btLogIsEmpty(pLog) ){
      assert( iFrame==1 );
      aLog[4] = iFrame;
    }else if( iFrame==1 ){
      assert( aLog[0]==0 && aLog[1]==0 && aLog[2]==0 && aLog[3]==0 );
      aLog[0] = aLog[4];
      aLog[1] = aLog[5];
      aLog[4] = iFrame;
    }else if( iFrame!=aLog[5]+1 ){
      assert( iFrame>aLog[5] );
      assert( aLog[2]==0 && aLog[3]==0 );
      aLog[2] = aLog[4];
      aLog[3] = aLog[5];
      aLog[4] = iFrame;
    }

    aLog[5] = iFrame;
    memcpy(pLog->snapshot.aFrameCksum, frame.aCksum, sizeof(frame.aCksum));
btLogDebugTopology("log2", aLog);
  }

  /* If this is a COMMIT, also update the shared shm-header. */
  if( bCommit ){
    rc = btLogUpdateSharedHdr(pLog);
  }

833
834
835
836
837
838
839


840
841
842






843




844




845
846
847
848
849
850
851
    if( rc==SQLITE4_OK && memcmp(&shmhdr, &pLog->snapshot, sizeof(BtShmHdr)) ){
      sqlite4BtLockReaderUnlock(pLog->pLock);
      rc = SQLITE4_NOTFOUND;
    }
  }

  if( rc==SQLITE4_OK ){


    BtShm *pShm = btLogShm(pLog);
    u32 iCkpt = pShm->ckpt.iFirstRead;
    if( iCkpt>pLog->snapshot.aLog[4] ){






      pLog->snapshot.aLog[4] = iCkpt;




    }




  }

  return rc;
}

int sqlite4BtLogSnapshotClose(BtLog *pLog){
  sqlite4BtLockReaderUnlock(pLog->pLock);







>
>

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>
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>
>
>
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>
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>
>
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>
>
>







1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
    if( rc==SQLITE4_OK && memcmp(&shmhdr, &pLog->snapshot, sizeof(BtShmHdr)) ){
      sqlite4BtLockReaderUnlock(pLog->pLock);
      rc = SQLITE4_NOTFOUND;
    }
  }

  if( rc==SQLITE4_OK ){
    int iRegion;
    u32 *aLog = pLog->snapshot.aLog;
    BtShm *pShm = btLogShm(pLog);
    u32 iCkpt = pShm->ckpt.iFirstRecover;

fprintf(stderr, "iCkpt=%d\n", (int)iCkpt);
btLogDebugTopology("log3", aLog);

    for(iRegion=0; iRegion<3; iRegion++){
      if( aLog[iRegion*2] ){
        if( iCkpt>=aLog[iRegion*2] && iCkpt<=aLog[iRegion*2+1] ){
          aLog[iRegion*2] = iCkpt;
          break;
        }else{
          aLog[iRegion*2] = 0;
          aLog[iRegion*2+1] = 0;
        }
      }
    }

btLogDebugTopology("log4", aLog);
  }

  return rc;
}

int sqlite4BtLogSnapshotClose(BtLog *pLog){
  sqlite4BtLockReaderUnlock(pLog->pLock);
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/*
** Parameters iFirst and iLast are frame numbers for frames that are part 
** of the current log. This function scans the wal-index from iFirst to
** iLast (inclusive) and records the set of page numbers that occur once.
** This set is sorted in ascending order and returned via the output 
** variables *paPgno and *pnPgno.
** 
*/
static int btLogGatherPgno(
  BtLog *pLog,                    /* Log module handle */
  u32 iFirst,                     /* First frame to gather pgnos from */
  u32 iLast,                      /* Last frame to gather pgnos from */
  u32 **paPgno,                   /* OUT: s4_malloc'd array of sorted pgno */
  int *pnPgno                     /* OUT: Number of entries in *paPgno */

){

  u32 i;
  u32 *aPgno;                     /* Returned array */
  u32 *aSpace;                    /* Temporary space used by merge-sort */
  int nMax = (iLast - iFirst) + 1;
  int rc = SQLITE4_OK;











  /* Allocate space to collect all page numbers. */
  aPgno = (u32*)sqlite4_malloc(pLog->pLock->pEnv, sizeof(u32)*nMax*2);
  if( aPgno==0 ) rc = btErrorBkpt(SQLITE4_NOMEM);
  aSpace = &aPgno[nMax];

  /* Copy the required page numbers into the allocated array */




  for(i=iFirst; rc==SQLITE4_OK && i<=iLast; i++){

    u32 *aPage;
    ht_slot *aHash;
    u32 iZero;





    rc = btLogFindHash(pLog, btLogFrameHash(pLog, i), &aHash, &aPage, &iZero);
    if( rc==SQLITE4_OK ){
      aPgno[i-iFirst] = aPage[i-iZero];



    }
  }

  /* Sort the contents of the array in ascending order. This step also 
  ** eliminates any  duplicate page numbers. */
  if( rc==SQLITE4_OK ){
    btLogMergeSort(aPgno, &nMax, aSpace);







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/*
** Parameters iFirst and iLast are frame numbers for frames that are part 
** of the current log. This function scans the wal-index from iFirst to
** iLast (inclusive) and records the set of page numbers that occur once.
** This set is sorted in ascending order and returned via the output 
** variables *paPgno and *pnPgno.

*/
static int btLogGatherPgno(
  BtLog *pLog,                    /* Log module handle */


  u32 **paPgno,                   /* OUT: s4_malloc'd array of sorted pgno */
  int *pnPgno,                    /* OUT: Number of entries in *paPgno */
  u32 *piLastFrame
){
  u32 *aLog = pLog->snapshot.aLog;/* Log file topology */
  u32 i;
  u32 *aPgno;                     /* Returned array */
  u32 *aSpace;                    /* Temporary space used by merge-sort */
  int nMax;
  int rc = SQLITE4_OK;
  int iRegion;

  /* Determine an upper limit on the number of distinct page numbers. This
  ** limit is used to allocate space for the returned array.  */
  nMax = 0;
  for(iRegion=0; iRegion<3; iRegion++){
    if( aLog[iRegion*2] ){
      nMax += 1 + aLog[iRegion*2+1] - aLog[iRegion*2+0];
    }
  }

  /* Allocate space to collect all page numbers. */
  aPgno = (u32*)sqlite4_malloc(pLog->pLock->pEnv, sizeof(u32)*nMax*2);
  if( aPgno==0 ) rc = btErrorBkpt(SQLITE4_NOMEM);
  aSpace = &aPgno[nMax];

  /* Copy the required page numbers into the allocated array */
  for(iRegion=0; iRegion<3; iRegion++){
    u32 iFirst = aLog[iRegion*2];
    u32 iLast = aLog[iRegion*2+1];
    if( iFirst ){
      for(i=iFirst; rc==SQLITE4_OK && i<=iLast; i++){
        int iHash = btLogFrameHash(pLog, i);
        u32 *aPage;
        ht_slot *aHash;
        u32 iZero;

        /* It doesn't matter which 'side' of the hash table is requested here,
        ** as only the page-number array, not the aHash[] table, will be used.
        ** And it is the same for both sides. Hence the constant 0 passed as
        ** the second argument to btLogFindHash().  */
        rc = btLogFindHash(pLog, 0, iHash, &aHash, &aPage, &iZero);
        if( rc==SQLITE4_OK ){
          aPgno[i-iFirst] = aPage[i-iZero];
        }
      }
      *piLastFrame = iLast;
    }
  }

  /* Sort the contents of the array in ascending order. This step also 
  ** eliminates any  duplicate page numbers. */
  if( rc==SQLITE4_OK ){
    btLogMergeSort(aPgno, &nMax, aSpace);
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  /* Take the CHECKPOINTER lock. */
  rc = sqlite4BtLockCkpt(pLock);
  if( rc==SQLITE4_OK ){
    const int pgsz = sqlite4BtPagerPagesize((BtPager*)pLock);
    bt_env *pVfs = pLock->pVfs;
    bt_file *pFd = pLock->pFd;
    BtShm *pShm;                  /* Pointer to shared-memory region */
    u32 iFirst;                   /* First frame to checkpoint */
    u32 iLast;                    /* Last frame to checkpoint */

    u32 *aPgno = 0;               /* Array of page numbers to checkpoint */
    int nPgno;                    /* Number of entries in aPgno[] */
    int i;                        /* Used to loop through aPgno[] */
    u8 *aBuf;                     /* Buffer to load page data into */


    /* Allocate space to load log data into */
    aBuf = sqlite4_malloc(pLock->pEnv, pgsz);
    if( aBuf==0 ) rc = btErrorBkpt(SQLITE4_NOMEM);
    
    /* Figure out the set of page numbers stored in the part of the log 
    ** file being checkpointed. Remove any duplicates and sort them in 
    ** ascending order.  */
    if( rc==SQLITE4_OK ){
      pShm = btLogShm(pLog);
      iFirst = pShm->ckpt.iFirstRead;
      iLast = pLog->snapshot.aLog[5];
      rc = btLogGatherPgno(pLog, iFirst, iLast, &aPgno, &nPgno);





    }

    /* Copy data from the log file to the database file. */
    for(i=0; rc==SQLITE4_OK && i<nPgno; i++){
      u32 pgno = aPgno[i];
      rc = sqlite4BtLogRead(pLog, pgno, aBuf);
      if( rc==SQLITE4_OK ){
        i64 iOff = (i64)pgsz * (pgno-1);
        rc = pVfs->xWrite(pFd, iOff, aBuf, pgsz);
      }
    }

    /* Update the first field of the checkpoint-header. This tells readers
    ** that they need not consider anything that in the log before this
    ** point (since the data has already been copied into the database
    ** file).  */
    if( rc==SQLITE4_OK ){

      pShm->ckpt.iFirstRead = iLast+1;
      pVfs->xShmBarrier(pLog->pFd);
    }

    /* Write a new header into the log file. This tells any future recovery
    ** where it should start reading the log. Once this new header is synced
    ** to disk, the space cleared by this checkpoint operation can be 
    ** reused.  */
    if( rc==SQLITE4_OK ){
      int iSlot = ((pShm->ckpt.iWalHdr >> 2) + 1) % 2;
      i64 iOff;
      BtWalHdr hdr;
      BtFrameHdr fhdr;

      memset(&hdr, 0, sizeof(BtWalHdr));
      hdr.iMagic = BT_WAL_MAGIC;
      hdr.iVersion = BT_WAL_VERSION;
      hdr.iCnt = (((pShm->ckpt.iWalHdr & 0x03) + 1) % 3);
      hdr.nSector = pLog->snapshot.nSector;
      hdr.nPgsz = pgsz;
      hdr.iFirstFrame = iLast+1;

      iOff = btLogFrameOffset(pLog, pgsz, iLast);
      rc = btLogReadData(pLog, iOff, (u8*)&fhdr, sizeof(BtFrameHdr));
      if( rc==SQLITE4_OK ){
        hdr.iSalt1 = fhdr.aCksum[0];
        hdr.iSalt2 = fhdr.aCksum[1];
        rc = btLogWriteHeader(pLog, iSlot, &hdr);
      }
      if( rc==SQLITE4_OK ){
        pShm->ckpt.iWalHdr = (iSlot<<2) + hdr.iCnt;
      }
    }

    /* Update the second field of the checkpoint header. This tells future
    ** writers that it is now safe to recycle pages before this point
    ** (assuming all live readers are cleared).  */
    if( rc==SQLITE4_OK ){
      pShm->ckpt.iFirstRecover = iLast+1;
      pVfs->xShmBarrier(pLog->pFd);
    }

    /* Free the buffer and drop the checkpointer lock */
    sqlite4_free(pLock->pEnv, aBuf);
    sqlite4BtLockCkptUnlock(pLog->pLock);
  }
  return rc;
}









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  /* Take the CHECKPOINTER lock. */
  rc = sqlite4BtLockCkpt(pLock);
  if( rc==SQLITE4_OK ){
    const int pgsz = sqlite4BtPagerPagesize((BtPager*)pLock);
    bt_env *pVfs = pLock->pVfs;
    bt_file *pFd = pLock->pFd;
    BtShm *pShm;                  /* Pointer to shared-memory region */

    u32 iLast;                    /* Last frame to checkpoint */
    BtFrameHdr fhdr;              /* Frame header of frame iLast */
    u32 *aPgno = 0;               /* Array of page numbers to checkpoint */
    int nPgno;                    /* Number of entries in aPgno[] */
    int i;                        /* Used to loop through aPgno[] */
    u8 *aBuf;                     /* Buffer to load page data into */
    u32 iFirstRead;               /* First frame not checkpointed */

    /* Allocate space to load log data into */
    aBuf = sqlite4_malloc(pLock->pEnv, pgsz);
    if( aBuf==0 ) rc = btErrorBkpt(SQLITE4_NOMEM);
    
    /* Figure out the set of page numbers stored in the part of the log 
    ** file being checkpointed. Remove any duplicates and sort them in 
    ** ascending order.  */
    if( rc==SQLITE4_OK ){



      rc = btLogGatherPgno(pLog, &aPgno, &nPgno, &iLast);
    }
    if( rc==SQLITE4_OK ){
      i64 iOff = btLogFrameOffset(pLog, pgsz, iLast);
      rc = btLogReadData(pLog, iOff, (u8*)&fhdr, sizeof(BtFrameHdr));
      iFirstRead = (fhdr.ctrl & ~BT_FRAME_COMMIT);
    }

    /* Copy data from the log file to the database file. */
    for(i=0; rc==SQLITE4_OK && i<nPgno; i++){
      u32 pgno = aPgno[i];
      rc = sqlite4BtLogRead(pLog, pgno, aBuf);
      if( rc==SQLITE4_OK ){
        i64 iOff = (i64)pgsz * (pgno-1);
        rc = pVfs->xWrite(pFd, iOff, aBuf, pgsz);
      }
    }

    /* Update the first field of the checkpoint-header. This tells readers
    ** that they need not consider anything that in the log before this
    ** point (since the data has already been copied into the database
    ** file).  */
    if( rc==SQLITE4_OK ){
      pShm = btLogShm(pLog);
      pShm->ckpt.iFirstRead = iFirstRead;
      pVfs->xShmBarrier(pLog->pFd);
    }

    /* Write a new header into the log file. This tells any future recovery
    ** where it should start reading the log. Once this new header is synced
    ** to disk, the space cleared by this checkpoint operation can be 
    ** reused.  */
    if( rc==SQLITE4_OK ){
      int iSlot = ((pShm->ckpt.iWalHdr >> 2) + 1) % 2;

      BtWalHdr hdr;


      memset(&hdr, 0, sizeof(BtWalHdr));
      hdr.iMagic = BT_WAL_MAGIC;
      hdr.iVersion = BT_WAL_VERSION;
      hdr.iCnt = (((pShm->ckpt.iWalHdr & 0x03) + 1) % 3);
      hdr.nSector = pLog->snapshot.nSector;
      hdr.nPgsz = pgsz;
      hdr.iFirstFrame = iFirstRead;




      hdr.iSalt1 = fhdr.aCksum[0];
      hdr.iSalt2 = fhdr.aCksum[1];
      rc = btLogWriteHeader(pLog, iSlot, &hdr);

      if( rc==SQLITE4_OK ){
        pShm->ckpt.iWalHdr = (iSlot<<2) + hdr.iCnt;
      }
    }

    /* Update the second field of the checkpoint header. This tells future
    ** writers that it is now safe to recycle pages before this point
    ** (assuming all live readers are cleared).  */
    if( rc==SQLITE4_OK ){
      pShm->ckpt.iFirstRecover = iFirstRead;
      pVfs->xShmBarrier(pLog->pFd);
    }

    /* Free the buffer and drop the checkpointer lock */
    sqlite4_free(pLock->pEnv, aBuf);
    sqlite4BtLockCkptUnlock(pLog->pLock);
  }
  return rc;
}