bt_db *pBt;                     /* bt database handle */
  sqlite4_env *pEnv;              /* SQLite environment (for malloc/free) */
  bt_env *pVfs;                   /* Underlying VFS */

  /* Space for bt_fetch() results */
  u8 *aBuffer;                    /* Space to store results */
  int nBuffer;                    /* Allocated size of aBuffer[] in bytes */



  /* Stuff used for crash test simulation */
  BtFile *apFile[2];              /* Database and log files used by pBt */
  bt_env env;                     /* Private VFS for this object */
  int nCrashSync;                 /* Number of syncs until crash (see above) */
  int bCrash;                     /* True once a crash has been simulated */
};
................................................................................
  BtDb *pBt = (BtDb*)pVfs->pVfsCtx;
  int rc;

  if( pBt->bCrash ) return SQLITE4_IOERR;

  p = (BtFile*)testMalloc(sizeof(BtFile));
  if( !p ) return SQLITE4_NOMEM;

  assert( pBt->apFile[0]==0 || pBt->apFile[1]==0 );

  pBt->apFile[pBt->apFile[0]!=0] = p;



  p->pBt = pBt; rc = pBt->pVfs->xOpen(pEnv, pVfs, zFile, flags, &p->pFile);
  if( rc!=SQLITE4_OK ){
    testFree(p);
    p = 0;


  }

  *ppFile = (bt_file*)p;
  return rc;
}

static int btVfsSize(bt_file *pFile, sqlite4_int64 *piRes){
................................................................................
  return rc;
}

static int btVfsSectorSize(bt_file *pFile){
  BtFile *p = (BtFile*)pFile;
  return p->pBt->pVfs->xSectorSize(p->pFile);
}







static int btVfsClose(bt_file *pFile){
  BtFile *p = (BtFile*)pFile;
  int rc;
  assert( p->pBt->apFile[0]==p || p->pBt->apFile[1]==p );
  btFlushSectors(p, 0);
  testFree(p->apSector);
  rc = p->pBt->pVfs->xClose(p->pFile);

  testFree(p);
  return rc;
}

static int btVfsUnlink(sqlite4_env *pEnv, bt_env *pVfs, const char *zFile){
  BtDb *pBt = (BtDb*)pVfs->pVfsCtx;
  if( pBt->bCrash ) return SQLITE4_IOERR;
................................................................................
  BtFile *p = (BtFile*)pFile;
  if( p->pBt->bCrash ) return SQLITE4_IOERR;
  return p->pBt->pVfs->xShmUnmap(p->pFile, bDelete);
}

static int bt_close(TestDb *pTestDb){
  BtDb *p = (BtDb*)pTestDb;

  free(p->aBuffer);
  return sqlite4BtClose(p->pBt);


}

static int btMinTransaction(BtDb *p, int iMin, int *piLevel){
  int iLevel;
  int rc = SQLITE4_OK;

  iLevel = sqlite4BtTransactionLevel(p->pBt);
................................................................................
  if( bClear && zFilename && zFilename[0] ){
    char *zLog = sqlite3_mprintf("%s-wal", zFilename);
    unlink(zFilename);
    unlink(zLog);
    sqlite3_free(zLog);
  }
  
  rc = sqlite4BtNew(pEnv, sizeof(BtDb), &pBt);
  if( rc==SQLITE4_OK ){
    p = (BtDb*)sqlite4BtExtra(pBt);
    p->base.pMethods = &SqlMethods;
    p->pBt = pBt;
    p->pEnv = pEnv;


    p->env.pVfsCtx = (void*)p;
    p->env.xFullpath = btVfsFullpath;
    p->env.xOpen = btVfsOpen;
    p->env.xSize = btVfsSize;
    p->env.xRead = btVfsRead;
    p->env.xWrite = btVfsWrite;

  bt_db *pBt;                     /* bt database handle */
  sqlite4_env *pEnv;              /* SQLite environment (for malloc/free) */
  bt_env *pVfs;                   /* Underlying VFS */

  /* Space for bt_fetch() results */
  u8 *aBuffer;                    /* Space to store results */
  int nBuffer;                    /* Allocated size of aBuffer[] in bytes */

  int nRef;

  /* Stuff used for crash test simulation */
  BtFile *apFile[2];              /* Database and log files used by pBt */
  bt_env env;                     /* Private VFS for this object */
  int nCrashSync;                 /* Number of syncs until crash (see above) */
  int bCrash;                     /* True once a crash has been simulated */
};
................................................................................
  BtDb *pBt = (BtDb*)pVfs->pVfsCtx;
  int rc;

  if( pBt->bCrash ) return SQLITE4_IOERR;

  p = (BtFile*)testMalloc(sizeof(BtFile));
  if( !p ) return SQLITE4_NOMEM;
  if( flags & BT_OPEN_DATABASE ){
    pBt->apFile[0] = p;
  }else if( flags & BT_OPEN_LOG ){
    pBt->apFile[1] = p;
  }
  p->pBt = pBt; 

  rc = pBt->pVfs->xOpen(pEnv, pVfs, zFile, flags, &p->pFile);
  if( rc!=SQLITE4_OK ){
    testFree(p);
    p = 0;
  }else{
    pBt->nRef++;
  }

  *ppFile = (bt_file*)p;
  return rc;
}

static int btVfsSize(bt_file *pFile, sqlite4_int64 *piRes){
................................................................................
  return rc;
}

static int btVfsSectorSize(bt_file *pFile){
  BtFile *p = (BtFile*)pFile;
  return p->pBt->pVfs->xSectorSize(p->pFile);
}

static void btDeref(BtDb *p){
  p->nRef--;
  assert( p->nRef>=0 );
  if( p->nRef<=0 ) testFree(p);
}

static int btVfsClose(bt_file *pFile){
  BtFile *p = (BtFile*)pFile;
  int rc;

  btFlushSectors(p, 0);
  testFree(p->apSector);
  rc = p->pBt->pVfs->xClose(p->pFile);
  btDeref(p->pBt);
  testFree(p);
  return rc;
}

static int btVfsUnlink(sqlite4_env *pEnv, bt_env *pVfs, const char *zFile){
  BtDb *pBt = (BtDb*)pVfs->pVfsCtx;
  if( pBt->bCrash ) return SQLITE4_IOERR;
................................................................................
  BtFile *p = (BtFile*)pFile;
  if( p->pBt->bCrash ) return SQLITE4_IOERR;
  return p->pBt->pVfs->xShmUnmap(p->pFile, bDelete);
}

static int bt_close(TestDb *pTestDb){
  BtDb *p = (BtDb*)pTestDb;
  int rc = sqlite4BtClose(p->pBt);
  free(p->aBuffer);

  btDeref(p);
  return rc;
}

static int btMinTransaction(BtDb *p, int iMin, int *piLevel){
  int iLevel;
  int rc = SQLITE4_OK;

  iLevel = sqlite4BtTransactionLevel(p->pBt);
................................................................................
  if( bClear && zFilename && zFilename[0] ){
    char *zLog = sqlite3_mprintf("%s-wal", zFilename);
    unlink(zFilename);
    unlink(zLog);
    sqlite3_free(zLog);
  }
  
  rc = sqlite4BtNew(pEnv, 0, &pBt);
  if( rc==SQLITE4_OK ){
    p = (BtDb*)testMalloc(sizeof(BtDb));
    p->base.pMethods = &SqlMethods;
    p->pBt = pBt;
    p->pEnv = pEnv;
    p->nRef = 1;

    p->env.pVfsCtx = (void*)p;
    p->env.xFullpath = btVfsFullpath;
    p->env.xOpen = btVfsOpen;
    p->env.xSize = btVfsSize;
    p->env.xRead = btVfsRead;
    p->env.xWrite = btVfsWrite;

** BT_CONTROL_SAFETY:
**   The third argument is interpreted as a pointer to type (int). If
**   the value stored in the (int) location is 0, 1 or 2, then the current
**   b-tree safety level is set to 0, 1 or 2, respectively. Otherwise, the
**   integer value is set to the current safety level.
**
** BT_CONTROL_AUTOCKPT:












*/
#define BT_CONTROL_INFO     7706389
#define BT_CONTROL_SETVFS   7706390
#define BT_CONTROL_GETVFS   7706391
#define BT_CONTROL_SAFETY   7706392
#define BT_CONTROL_AUTOCKPT 7706393


int sqlite4BtControl(bt_db*, int op, void *pArg);

#define BT_SAFETY_OFF    0
#define BT_SAFETY_NORMAL 1
#define BT_SAFETY_FULL   2

................................................................................
  int (*xUnlink)(sqlite4_env*,bt_env*, const char *);
  int (*xLock)(bt_file*, int, int);
  int (*xTestLock)(bt_file*, int, int, int);
  int (*xShmMap)(bt_file*, int, int, void **);
  void (*xShmBarrier)(bt_file*);
  int (*xShmUnmap)(bt_file*, int);
};

** BT_CONTROL_SAFETY:
**   The third argument is interpreted as a pointer to type (int). If
**   the value stored in the (int) location is 0, 1 or 2, then the current
**   b-tree safety level is set to 0, 1 or 2, respectively. Otherwise, the
**   integer value is set to the current safety level.
**
** BT_CONTROL_AUTOCKPT:
**   The third argument is interpreted as a pointer to type (int). If
**   the indicated value is greater than or equal to zero, then the 
**   database connection auto-checkpoint value is set accordingly. If
**   the indicated value is less than zero, it is set to the current
**   auto-checkpoint value before returning.
**
** BT_CONTROL_LOGSIZE:
**   The third argument is interpreted as a pointer to type (int). The
**   value pointer to is set to the number of uncheckpointed frames
**   that stored in the log file according to the snapshot used by the
**   most recently completed transaction or checkpoint operation.
**   
*/
#define BT_CONTROL_INFO     7706389
#define BT_CONTROL_SETVFS   7706390
#define BT_CONTROL_GETVFS   7706391
#define BT_CONTROL_SAFETY   7706392
#define BT_CONTROL_AUTOCKPT 7706393
#define BT_CONTROL_LOGSIZE  7706394

int sqlite4BtControl(bt_db*, int op, void *pArg);

#define BT_SAFETY_OFF    0
#define BT_SAFETY_NORMAL 1
#define BT_SAFETY_FULL   2

................................................................................
  int (*xUnlink)(sqlite4_env*,bt_env*, const char *);
  int (*xLock)(bt_file*, int, int);
  int (*xTestLock)(bt_file*, int, int, int);
  int (*xShmMap)(bt_file*, int, int, void **);
  void (*xShmBarrier)(bt_file*);
  int (*xShmUnmap)(bt_file*, int);
};

/*
** Flags for xOpen
*/
#define BT_OPEN_DATABASE   0x0001
#define BT_OPEN_LOG        0x0002
#define BT_OPEN_READONLY   0x0004

bt_env *sqlite4BtPagerGetEnv(BtPager*);
void sqlite4BtPagerSetEnv(BtPager*, bt_env*);

void sqlite4BtPagerSetSafety(BtPager*, int*);
void sqlite4BtPagerSetAutockpt(BtPager*, int*);



/*
** End of bt_pager.c interface.
*************************************************************************/

/*************************************************************************
** File-system interface.
*/

/* Flags for the 3rd argument to xOpen */
#define BT_OPEN_READONLY 0x0001

/* Candidate values for the 3rd argument to bt_env.xLock() */
#define BT_LOCK_UNLOCK 0
#define BT_LOCK_SHARED 1
#define BT_LOCK_EXCL   2

/* Size of shared-memory chunks - 48KB. */
#define BT_SHM_CHUNK_SIZE (48*1024)
................................................................................

/*************************************************************************
** Interface to bt_lock.c functionality.
*/
typedef struct BtShared BtShared;
typedef struct BtLock BtLock;
typedef struct BtReadSlot BtReadSlot;


struct BtLock {
  /* These three are set by the bt_pager module and thereafter used by 
  ** the bt_lock, bt_pager and bt_log modules. */
  sqlite4_env *pEnv;              /* SQLite environment */
  bt_env *pVfs;                   /* Bt environment */
  bt_file *pFd;                   /* Database file descriptor */
  int iDebugId;                   /* Sometimes useful when debugging */
................................................................................
  int nAutoCkpt;                  /* Auto-checkpoint when log is this large */

  /* These are used only by the bt_lock module. */
  BtShared *pShared;              /* Shared by all handles on this file */
  BtLock *pNext;                  /* Next connection using pShared */
  u32 mExclLock;                  /* Mask of exclusive locks held */
  u32 mSharedLock;                /* Mask of shared locks held */

};

struct BtReadSlot {
  u32 iFirst;
  u32 iLast;
};

bt_env *sqlite4BtPagerGetEnv(BtPager*);
void sqlite4BtPagerSetEnv(BtPager*, bt_env*);

void sqlite4BtPagerSetSafety(BtPager*, int*);
void sqlite4BtPagerSetAutockpt(BtPager*, int*);

void sqlite4BtPagerLogsize(BtPager*, int*);

/*
** End of bt_pager.c interface.
*************************************************************************/

/*************************************************************************
** File-system interface.
*/




/* Candidate values for the 3rd argument to bt_env.xLock() */
#define BT_LOCK_UNLOCK 0
#define BT_LOCK_SHARED 1
#define BT_LOCK_EXCL   2

/* Size of shared-memory chunks - 48KB. */
#define BT_SHM_CHUNK_SIZE (48*1024)
................................................................................

/*************************************************************************
** Interface to bt_lock.c functionality.
*/
typedef struct BtShared BtShared;
typedef struct BtLock BtLock;
typedef struct BtReadSlot BtReadSlot;
typedef struct BtFile BtFile;

struct BtLock {
  /* These three are set by the bt_pager module and thereafter used by 
  ** the bt_lock, bt_pager and bt_log modules. */
  sqlite4_env *pEnv;              /* SQLite environment */
  bt_env *pVfs;                   /* Bt environment */
  bt_file *pFd;                   /* Database file descriptor */
  int iDebugId;                   /* Sometimes useful when debugging */
................................................................................
  int nAutoCkpt;                  /* Auto-checkpoint when log is this large */

  /* These are used only by the bt_lock module. */
  BtShared *pShared;              /* Shared by all handles on this file */
  BtLock *pNext;                  /* Next connection using pShared */
  u32 mExclLock;                  /* Mask of exclusive locks held */
  u32 mSharedLock;                /* Mask of shared locks held */
  BtFile *pBtFile;                /* Used to defer close if necessary */
};

struct BtReadSlot {
  u32 iFirst;
  u32 iLast;
};

#define BT_LOCK_READER_DBONLY 5   /* Reading the db file only */
#define BT_LOCK_READER0       6   /* Array of BT_NREADER locks */

#define BT_LOCK_UNLOCK     0
#define BT_LOCK_SHARED     1
#define BT_LOCK_EXCL       2

typedef struct BtFile BtFile;

/*
** Global data. All global variables used by code in this file are grouped
** into the following structure instance.
**
** pDatabase:
**   Linked list of all Database objects allocated within this process.
**   This list may not be traversed without holding the global mutex (see
................................................................................

/*
** Relinquish the mutex obtained by calling btLockMutexEnter().
*/
static void btLockMutexLeave(sqlite4_env *pEnv){
  sqlite4_mutex_leave(sqlite4_mutex_alloc(pEnv, SQLITE4_MUTEX_STATIC_KV));
}

















static int btLockLockopNonblocking(
  BtLock *p,                      /* BtLock handle */
  int iLock,                      /* Slot to lock */
  int eOp                         /* One of BT_LOCK_UNLOCK, SHARED or EXCL */
){
  const u32 mask = ((u32)1 << iLock);
................................................................................
    }
    assert( nExcl==0 || nExcl==1 );
    assert( nExcl==0 || nShared==0 );

    switch( eOp ){
      case BT_LOCK_UNLOCK:
        if( nShared==0 ){
#if 0
          lockSharedFile(db->pEnv, p, iLock, LSM_LOCK_UNLOCK);
#endif
        }
        p->mExclLock &= ~mask;
        p->mSharedLock &= ~mask;
        break;

      case BT_LOCK_SHARED:
        if( nExcl ){
          rc = SQLITE4_BUSY;
        }else{
          if( nShared==0 ){
#if 0
            rc = lockSharedFile(db->pEnv, p, iLock, LSM_LOCK_SHARED);
#endif
          }
          if( rc==SQLITE4_OK ){
            p->mSharedLock |= mask;
            p->mExclLock &= ~mask;
          }
        }
        break;

      default:
        assert( eOp==BT_LOCK_EXCL );
        if( nExcl || nShared ){
          rc = SQLITE4_BUSY;
        }else{
#if 0
          rc = lockSharedFile(db->pEnv, p, iLock, LSM_LOCK_EXCL);
#endif
          if( rc==SQLITE4_OK ){
            p->mSharedLock &= ~mask;
            p->mExclLock |= mask;
          }
        }
        break;
    }
................................................................................
    if( rc!=SQLITE4_BUSY || bBlock==0 ) break;
    /* todo: Fix blocking locks */
    btLockDelay();
  }
  return rc;
}



































/*
** Connect to the database as a read/write connection. If recovery
** is required (i.e. if this is the first connection to the db), invoke 
** the xRecover() method.
**
** Return SQLITE4_OK if successful, or an SQLite4 error code if an
** error occurs.
*/
int sqlite4BtLockConnect(BtLock *p, int (*xRecover)(BtLock*)){
  sqlite4_env *pEnv = p->pEnv;

  int rc = SQLITE4_OK;
  const char *zName;
  int nName;
  BtShared *pShared;

  zName = sqlite4BtPagerFilename((BtPager*)p, BT_PAGERFILE_DATABASE);
  nName = strlen(zName);
................................................................................
      sqlite4_free(pEnv, pShared);
      sqlite4_mutex_free(pMutex);
      pShared = 0;
      pMutex = 0;
      rc = btErrorBkpt(SQLITE4_NOMEM);
    }else{
      memset(pShared, 0, sizeof(BtShared));

      pShared->nName = nName;
      pShared->zName = (char *)&pShared[1];
      memcpy(pShared->zName, zName, nName+1);
      pShared->pNext = gBtShared.pDatabase;
      pShared->pClientMutex = pMutex;
      gBtShared.pDatabase = pShared;
    }
  }

  if( rc==SQLITE4_OK ){
    pShared->nRef++;
  }
  btLockMutexLeave(p->pEnv);

  /* Add this connection to the linked list at BtShared.pLock */
  if( rc==SQLITE4_OK ){
    sqlite4_mutex_enter(pShared->pClientMutex);




























    p->pNext = pShared->pLock;
    pShared->pLock = p;
    sqlite4_mutex_leave(pShared->pClientMutex);
    p->pShared = pShared;





  }

  if( rc==SQLITE4_OK ){
    rc = btLockLockop(p, BT_LOCK_DMS1, BT_LOCK_EXCL, 1);
    if( rc==SQLITE4_OK ){
      rc = btLockLockop(p, BT_LOCK_DMS2_RW, BT_LOCK_EXCL, 0);
      if( rc==SQLITE4_OK ){
................................................................................
    }
    if( rc==SQLITE4_BUSY ) rc = SQLITE4_OK;
    btLockLockop(p, BT_LOCK_DMS2_RW, BT_LOCK_UNLOCK, 0);
    btLockLockop(p, BT_LOCK_DMS2_RO, BT_LOCK_UNLOCK, 0);
    btLockLockop(p, BT_LOCK_DMS1, BT_LOCK_UNLOCK, 0);
  }

  btLockMutexEnter(p->pEnv);
  pShared->nRef--;
  if( pShared->nRef==0 ){
    int i;
    BtShared **ppS;
    for(ppS=&gBtShared.pDatabase; *ppS!=pShared; ppS=&(*ppS)->pNext);
    *ppS = (*ppS)->pNext;

    sqlite4_mutex_free(pShared->pClientMutex);
    for(i=0; i<pShared->nShmChunk; i++){
      sqlite4_free(p->pEnv, pShared->apShmChunk[i]);
    }
    sqlite4_free(p->pEnv, pShared->apShmChunk);
    sqlite4_free(p->pEnv, pShared);
  }
  btLockMutexLeave(p->pEnv);

  return rc;
}

#ifndef NDEBUG
static void assertNoLockedSlots(BtLock *pLock){
  u32 mask = (1 << (BT_LOCK_READER0+BT_NREADER+1)) - (1 << BT_LOCK_READER0);
  assert( (pLock->mExclLock & mask)==0 );
................................................................................
  if( (iFirst<aLog[0] || iFirst>aLog[1])
   && (iFirst<aLog[2] || iFirst>aLog[3])
   && (iFirst<aLog[4] || iFirst>aLog[5])
  ){
    iLast = 0;
  }

BtReadSlot ss[BT_NREADER];
memcpy(ss, aSlot, BT_NREADER*sizeof(BtReadSlot));
int iOk = -1;

  if( iLast==0 ){
    rc = btLockLockop(pLock, BT_LOCK_READER_DBONLY, BT_LOCK_SHARED, 0);
  }else{
    const int nMaxRetry = 100;
    int nAttempt = 100;           /* Remaining lock attempts */

    for(nAttempt=0; rc==SQLITE4_BUSY && nAttempt<nMaxRetry; nAttempt++){
................................................................................
      int iIdxLast = sqlite4BtLogFrameToIdx(aLog, iLast);

      assert( iIdxFirst>=0 && iIdxLast>=0 );

      /* Try to find a slot populated with the values required. */
      for(i=0; i<BT_NREADER; i++){
        if( aSlot[i].iFirst==iFirst && aSlot[i].iLast==iLast ){
          iOk = i;
          break;
        }
      }

      /* Or, if there is no slot with the required values - try to create one */
      if( i==BT_NREADER ){
        for(i=0; i<BT_NREADER; i++){
................................................................................
}

int sqlite4BtLockShmMap(BtLock *pLock, int iChunk, int nByte, u8 **ppOut){
  int rc = SQLITE4_OK;
  BtShared *pShared = pLock->pShared;
  u8 *pOut = 0;

  assert( pShared->bMultiProc==0 && pShared->bReadonly==0 );

  sqlite4_mutex_enter(pShared->pClientMutex);
  if( pShared->nShmChunk<=iChunk ){
    u8 **apNew;
    int nNew = iChunk+1;
    int nByte = sizeof(u8*)*nNew;

................................................................................
      memset(&apNew[pShared->nShmChunk],0,nByte-sizeof(u8*)*pShared->nShmChunk);
      pShared->nShmChunk = nNew;
      pShared->apShmChunk = apNew;
    }
  }

  if( rc==SQLITE4_OK ){




    if( pShared->apShmChunk[iChunk]==0 ){
      u8 *p = (u8*)sqlite4_malloc(pLock->pEnv, nByte);

      if( p ) memset(p, 0, nByte);
      pShared->apShmChunk[iChunk] = p;


    }





    pOut = pShared->apShmChunk[iChunk];
    if( pOut==0 ){
      rc = btErrorBkpt(SQLITE4_NOMEM);
    }



  }
  sqlite4_mutex_leave(pShared->pClientMutex);
  
  *ppOut = pOut;
  return rc;
}

#define BT_LOCK_READER_DBONLY 5   /* Reading the db file only */
#define BT_LOCK_READER0       6   /* Array of BT_NREADER locks */

#define BT_LOCK_UNLOCK     0
#define BT_LOCK_SHARED     1
#define BT_LOCK_EXCL       2



/*
** Global data. All global variables used by code in this file are grouped
** into the following structure instance.
**
** pDatabase:
**   Linked list of all Database objects allocated within this process.
**   This list may not be traversed without holding the global mutex (see
................................................................................

/*
** Relinquish the mutex obtained by calling btLockMutexEnter().
*/
static void btLockMutexLeave(sqlite4_env *pEnv){
  sqlite4_mutex_leave(sqlite4_mutex_alloc(pEnv, SQLITE4_MUTEX_STATIC_KV));
}

/*
** Take the specified lock on the shared file-handle associated with
** the connection passed as the first argument.
*/
static int btLockSharedFile(BtLock *p, int iLock, int eOp){
  int rc = SQLITE4_OK;
  BtShared *pShared = p->pShared;

  /* This is a no-op in single process mode */
  assert( (pShared->bMultiProc==0)==(pShared->pFile==0) );
  if( pShared->pFile ){
    rc = p->pVfs->xLock(pShared->pFile, iLock, eOp);
  }
  return rc;
}

static int btLockLockopNonblocking(
  BtLock *p,                      /* BtLock handle */
  int iLock,                      /* Slot to lock */
  int eOp                         /* One of BT_LOCK_UNLOCK, SHARED or EXCL */
){
  const u32 mask = ((u32)1 << iLock);
................................................................................
    }
    assert( nExcl==0 || nExcl==1 );
    assert( nExcl==0 || nShared==0 );

    switch( eOp ){
      case BT_LOCK_UNLOCK:
        if( nShared==0 ){
          btLockSharedFile(p, iLock, BT_LOCK_UNLOCK);


        }
        p->mExclLock &= ~mask;
        p->mSharedLock &= ~mask;
        break;

      case BT_LOCK_SHARED:
        if( nExcl ){
          rc = SQLITE4_BUSY;
        }else{
          if( nShared==0 ){
            rc = btLockSharedFile(p, iLock, BT_LOCK_SHARED);


          }
          if( rc==SQLITE4_OK ){
            p->mSharedLock |= mask;
            p->mExclLock &= ~mask;
          }
        }
        break;

      default:
        assert( eOp==BT_LOCK_EXCL );
        if( nExcl || nShared ){
          rc = SQLITE4_BUSY;
        }else{
          rc = btLockSharedFile(p, iLock, BT_LOCK_EXCL);


          if( rc==SQLITE4_OK ){
            p->mSharedLock &= ~mask;
            p->mExclLock |= mask;
          }
        }
        break;
    }
................................................................................
    if( rc!=SQLITE4_BUSY || bBlock==0 ) break;
    /* todo: Fix blocking locks */
    btLockDelay();
  }
  return rc;
}

static void btLockSharedDeref(
  sqlite4_env *pEnv, 
  bt_env *pVfs, 
  BtShared *pShared
){
  btLockMutexEnter(pEnv);
  pShared->nRef--;
  if( pShared->nRef==0 ){
    BtShared **ppS;
    for(ppS=&gBtShared.pDatabase; *ppS!=pShared; ppS=&(*ppS)->pNext);
    *ppS = (*ppS)->pNext;
    while( pShared->pBtFile ){
      BtFile *p = pShared->pBtFile;
      pShared->pBtFile = p->pNext;
      pVfs->xClose(p->pFd);
      sqlite4_free(pEnv, p);
    }
    sqlite4_mutex_free(pShared->pClientMutex);

    /* If they were allocated in heap space, free all "shared" memory chunks */
    if( pShared->pFile==0 ){
      int i;
      for(i=0; i<pShared->nShmChunk; i++){
        sqlite4_free(pEnv, pShared->apShmChunk[i]);
      }
    }else{
      pVfs->xClose(pShared->pFile);
    }
    sqlite4_free(pEnv, pShared->apShmChunk);
    sqlite4_free(pEnv, pShared);
  }
  btLockMutexLeave(pEnv);
}

/*
** Connect to the database as a read/write connection. If recovery
** is required (i.e. if this is the first connection to the db), invoke 
** the xRecover() method.
**
** Return SQLITE4_OK if successful, or an SQLite4 error code if an
** error occurs.
*/
int sqlite4BtLockConnect(BtLock *p, int (*xRecover)(BtLock*)){
  sqlite4_env *pEnv = p->pEnv;
  bt_env *pVfs = p->pVfs;
  int rc = SQLITE4_OK;
  const char *zName;
  int nName;
  BtShared *pShared;

  zName = sqlite4BtPagerFilename((BtPager*)p, BT_PAGERFILE_DATABASE);
  nName = strlen(zName);
................................................................................
      sqlite4_free(pEnv, pShared);
      sqlite4_mutex_free(pMutex);
      pShared = 0;
      pMutex = 0;
      rc = btErrorBkpt(SQLITE4_NOMEM);
    }else{
      memset(pShared, 0, sizeof(BtShared));
      pShared->bMultiProc = 1;
      pShared->nName = nName;
      pShared->zName = (char *)&pShared[1];
      memcpy(pShared->zName, zName, nName+1);
      pShared->pNext = gBtShared.pDatabase;
      pShared->pClientMutex = pMutex;
      gBtShared.pDatabase = pShared;
    }
  }

  if( rc==SQLITE4_OK ){
    pShared->nRef++;
  }
  btLockMutexLeave(p->pEnv);

  /* Add this connection to the linked list at BtShared.pLock */
  if( rc==SQLITE4_OK ){
    sqlite4_mutex_enter(pShared->pClientMutex);

    /* If this is a multi-process connection and the shared file-handle
    ** has not yet been opened, open it now. Under the cover of the
    ** client-mutex.  */
    if( pShared->pFile==0 && pShared->bMultiProc ){
      rc = pVfs->xOpen(pEnv, pVfs, pShared->zName, 0, &pShared->pFile);
    }

    if( rc==SQLITE4_OK ){
      if( pShared->pBtFile ){
        p->pBtFile = pShared->pBtFile;
        pShared->pBtFile = p->pBtFile->pNext;
        p->pBtFile->pNext = 0;
        p->pFd = p->pBtFile->pFd;
      }else{
        p->pBtFile = (BtFile*)sqlite4_malloc(pEnv, sizeof(BtFile));
        if( p->pBtFile ){
          int flags = BT_OPEN_DATABASE;
          p->pBtFile->pNext = 0;
          rc = pVfs->xOpen(pEnv, pVfs, pShared->zName, flags, &p->pFd);
          p->pBtFile->pFd = p->pFd;
        }else{
          rc = btErrorBkpt(SQLITE4_NOMEM);
        }
      }
    }

    if( rc==SQLITE4_OK ){
      p->pNext = pShared->pLock;
      pShared->pLock = p;

      p->pShared = pShared;
    }
    sqlite4_mutex_leave(pShared->pClientMutex);
    if( rc!=SQLITE4_OK ){
      btLockSharedDeref(pEnv, pVfs, pShared);
    }
  }

  if( rc==SQLITE4_OK ){
    rc = btLockLockop(p, BT_LOCK_DMS1, BT_LOCK_EXCL, 1);
    if( rc==SQLITE4_OK ){
      rc = btLockLockop(p, BT_LOCK_DMS2_RW, BT_LOCK_EXCL, 0);
      if( rc==SQLITE4_OK ){
................................................................................
    }
    if( rc==SQLITE4_BUSY ) rc = SQLITE4_OK;
    btLockLockop(p, BT_LOCK_DMS2_RW, BT_LOCK_UNLOCK, 0);
    btLockLockop(p, BT_LOCK_DMS2_RO, BT_LOCK_UNLOCK, 0);
    btLockLockop(p, BT_LOCK_DMS1, BT_LOCK_UNLOCK, 0);
  }

  sqlite4_mutex_enter(pShared->pClientMutex);
  assert( p->pBtFile->pNext==0 );
  p->pBtFile->pNext = pShared->pBtFile;
  pShared->pBtFile = p->pBtFile;




  sqlite4_mutex_leave(pShared->pClientMutex);







  btLockSharedDeref(p->pEnv, p->pVfs, pShared);
  return rc;
}

#ifndef NDEBUG
static void assertNoLockedSlots(BtLock *pLock){
  u32 mask = (1 << (BT_LOCK_READER0+BT_NREADER+1)) - (1 << BT_LOCK_READER0);
  assert( (pLock->mExclLock & mask)==0 );
................................................................................
  if( (iFirst<aLog[0] || iFirst>aLog[1])
   && (iFirst<aLog[2] || iFirst>aLog[3])
   && (iFirst<aLog[4] || iFirst>aLog[5])
  ){
    iLast = 0;
  }





  if( iLast==0 ){
    rc = btLockLockop(pLock, BT_LOCK_READER_DBONLY, BT_LOCK_SHARED, 0);
  }else{
    const int nMaxRetry = 100;
    int nAttempt = 100;           /* Remaining lock attempts */

    for(nAttempt=0; rc==SQLITE4_BUSY && nAttempt<nMaxRetry; nAttempt++){
................................................................................
      int iIdxLast = sqlite4BtLogFrameToIdx(aLog, iLast);

      assert( iIdxFirst>=0 && iIdxLast>=0 );

      /* Try to find a slot populated with the values required. */
      for(i=0; i<BT_NREADER; i++){
        if( aSlot[i].iFirst==iFirst && aSlot[i].iLast==iLast ){

          break;
        }
      }

      /* Or, if there is no slot with the required values - try to create one */
      if( i==BT_NREADER ){
        for(i=0; i<BT_NREADER; i++){
................................................................................
}

int sqlite4BtLockShmMap(BtLock *pLock, int iChunk, int nByte, u8 **ppOut){
  int rc = SQLITE4_OK;
  BtShared *pShared = pLock->pShared;
  u8 *pOut = 0;

  assert( pShared->bReadonly==0 );

  sqlite4_mutex_enter(pShared->pClientMutex);
  if( pShared->nShmChunk<=iChunk ){
    u8 **apNew;
    int nNew = iChunk+1;
    int nByte = sizeof(u8*)*nNew;

................................................................................
      memset(&apNew[pShared->nShmChunk],0,nByte-sizeof(u8*)*pShared->nShmChunk);
      pShared->nShmChunk = nNew;
      pShared->apShmChunk = apNew;
    }
  }

  if( rc==SQLITE4_OK ){

    assert( (pShared->bMultiProc==0)==(pShared->pFile==0) );
    if( pShared->pFile==0 ){
      /* Single process mode. Allocate memory from the heap. */
      if( pShared->apShmChunk[iChunk]==0 ){
        u8 *p = (u8*)sqlite4_malloc(pLock->pEnv, nByte);
        if( p ){
          memset(p, 0, nByte);
          pShared->apShmChunk[iChunk] = p;
        }else{
          rc = btErrorBkpt(SQLITE4_NOMEM);
        }
      }
    }else{
      /* Multi-process mode. Request shared memory from VFS */
      void *pShm = 0;
      rc = pLock->pVfs->xShmMap(pShared->pFile, iChunk, nByte, &pShm);
      pShared->apShmChunk[iChunk] = (u8*)pShm;


    }

    pOut = pShared->apShmChunk[iChunk];
    assert( pOut || rc!=SQLITE4_OK );
  }
  sqlite4_mutex_leave(pShared->pClientMutex);
  
  *ppOut = pOut;
  return rc;
}

int sqlite4BtLogOpen(BtPager *pPager, int bRecover, BtLog **ppLog){
  BtLock *pLock = (BtLock*)pPager;
  bt_env *pVfs = pLock->pVfs;
  sqlite4_env *pEnv = pLock->pEnv;
  int rc;                         /* Return code */
  const char *zWal;               /* Name of log file to open */
  BtLog *pLog;                    /* Log handle to return */


  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 ){
      BtShm *pShm = btLogShm(pLog);
      memset(pShm, 0, sizeof(BtShm));
      pShm->ckpt.iFirstRead = 1;

int sqlite4BtLogOpen(BtPager *pPager, int bRecover, BtLog **ppLog){
  BtLock *pLock = (BtLock*)pPager;
  bt_env *pVfs = pLock->pVfs;
  sqlite4_env *pEnv = pLock->pEnv;
  int rc;                         /* Return code */
  const char *zWal;               /* Name of log file to open */
  BtLog *pLog;                    /* Log handle to return */
  int flags = BT_OPEN_LOG;

  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, flags, &pLog->pFd);

  if( rc==SQLITE4_OK && bRecover ){
    rc = btLogMapShm(pLog, 0);
    if( rc==SQLITE4_OK ){
      BtShm *pShm = btLogShm(pLog);
      memset(pShm, 0, sizeof(BtShm));
      pShm->ckpt.iFirstRead = 1;

    }

    case BT_CONTROL_AUTOCKPT: {
      int *pInt = (int*)pArg;
      sqlite4BtPagerSetAutockpt(db->pPager, pInt);
      break;
    }






  }

  return rc;
}

    }

    case BT_CONTROL_AUTOCKPT: {
      int *pInt = (int*)pArg;
      sqlite4BtPagerSetAutockpt(db->pPager, pInt);
      break;
    }

    case BT_CONTROL_LOGSIZE: {
      int *pInt = (int*)pArg;
      sqlite4BtPagerLogsize(db->pPager, pInt);
      break;
    }
  }

  return rc;
}

** Close a pager database handle.
*/
int sqlite4BtPagerClose(BtPager *p){
  int rc;

  rc = sqlite4BtLockDisconnect((BtLock*)p, btCheckpoint, btCleanup);

  if( p->btl.pFd ){
    p->btl.pVfs->xClose(p->btl.pFd);
  }

  p->iTransactionLevel = 0;
  btCloseSavepoints(p, 0);
  btPurgeCache(p);
  sqlite4BtLogClose(p->pLog, 0);
  sqlite4_free(p->btl.pEnv, p->zFile);
  sqlite4_free(p->btl.pEnv, p->aSavepoint);
  sqlite4_free(p->btl.pEnv, p);
................................................................................
** fails, the BtPager is rendered unusable (and must be closed by the
** caller using BtPagerClose()).
**
** If successful, SQLITE4_OK is returned. Otherwise, an SQLite error code.
*/
int sqlite4BtPagerOpen(BtPager *p, const char *zFilename){
  int rc;                         /* Return code */
  int flags = 0;                  /* Flags to pass to xOpen() */
  sqlite4_env *pEnv = p->btl.pEnv;
  bt_env *pVfs = p->btl.pVfs;

  assert( p->btl.pFd==0 && p->zFile==0 );

  rc = pVfs->xFullpath(pEnv, pVfs, zFilename, &p->zFile);
  if( rc==SQLITE4_OK ){
    p->nFile = strlen(p->zFile);
    rc = pVfs->xOpen(pEnv, pVfs, zFilename, flags, &p->btl.pFd);
  }

  /* If the database file was successfully opened, connect to the
  ** database and open the database logger. */
  if( rc==SQLITE4_OK ){
    rc = sqlite4BtLockConnect((BtLock*)p, btRecover);
    if( rc==SQLITE4_OK && p->pLog==0 ){
      rc = sqlite4BtLogOpen(p, 0, &p->pLog);
    }
  }

  assert( (rc==SQLITE4_OK)==(p->btl.pFd!=0 && p->pLog!=0) );
................................................................................
void sqlite4BtPagerSetAutockpt(BtPager *pPager, int *piVal){
  int iVal = *piVal;
  if( iVal>=0 ){
    pPager->btl.nAutoCkpt = iVal;
  }
  *piVal = pPager->btl.nAutoCkpt;
}





#ifndef NDEBUG
int sqlite4BtPagerRefcount(BtPager *p){
  return p->nTotalRef;
}
#endif

** Close a pager database handle.
*/
int sqlite4BtPagerClose(BtPager *p){
  int rc;

  rc = sqlite4BtLockDisconnect((BtLock*)p, btCheckpoint, btCleanup);





  p->iTransactionLevel = 0;
  btCloseSavepoints(p, 0);
  btPurgeCache(p);
  sqlite4BtLogClose(p->pLog, 0);
  sqlite4_free(p->btl.pEnv, p->zFile);
  sqlite4_free(p->btl.pEnv, p->aSavepoint);
  sqlite4_free(p->btl.pEnv, p);
................................................................................
** fails, the BtPager is rendered unusable (and must be closed by the
** caller using BtPagerClose()).
**
** If successful, SQLITE4_OK is returned. Otherwise, an SQLite error code.
*/
int sqlite4BtPagerOpen(BtPager *p, const char *zFilename){
  int rc;                         /* Return code */

  sqlite4_env *pEnv = p->btl.pEnv;
  bt_env *pVfs = p->btl.pVfs;

  assert( p->btl.pFd==0 && p->zFile==0 );

  rc = pVfs->xFullpath(pEnv, pVfs, zFilename, &p->zFile);
  if( rc==SQLITE4_OK ){
    p->nFile = strlen(p->zFile);






    rc = sqlite4BtLockConnect((BtLock*)p, btRecover);
    if( rc==SQLITE4_OK && p->pLog==0 ){
      rc = sqlite4BtLogOpen(p, 0, &p->pLog);
    }
  }

  assert( (rc==SQLITE4_OK)==(p->btl.pFd!=0 && p->pLog!=0) );
................................................................................
void sqlite4BtPagerSetAutockpt(BtPager *pPager, int *piVal){
  int iVal = *piVal;
  if( iVal>=0 ){
    pPager->btl.nAutoCkpt = iVal;
  }
  *piVal = pPager->btl.nAutoCkpt;
}

void sqlite4BtPagerLogsize(BtPager *pPager, int *pnFrame){
  *pnFrame = sqlite4BtLogSize(pPager->pLog);
}

#ifndef NDEBUG
int sqlite4BtPagerRefcount(BtPager *p){
  return p->nTotalRef;
}
#endif

  return rc;
}

static int btPosixOsUnlink(sqlite4_env *pEnv, bt_env *pVfs, const char *zFile){
  int prc = unlink(zFile);
  return prc ? btErrorBkpt(SQLITE4_IOERR) : SQLITE4_OK;
}



int btPosixOsLock(bt_file *pFile, int iLock, int eType){
  int rc = SQLITE4_OK;
  PosixFile *p = (PosixFile *)pFile;
  static const short aType[3] = { F_UNLCK, F_RDLCK, F_WRLCK };
  struct flock lock;

  assert( aType[BT_LOCK_UNLOCK]==F_UNLCK );
  assert( aType[BT_LOCK_SHARED]==F_RDLCK );
  assert( aType[BT_LOCK_EXCL]==F_WRLCK );
  assert( eType>=0 && eType<(sizeof(aType)/sizeof(aType[0])) );
  assert( iLock>0 && iLock<=32 );

  memset(&lock, 0, sizeof(lock));
  lock.l_whence = SEEK_SET;
  lock.l_len = 1;
  lock.l_type = aType[eType];
  lock.l_start = (4096-iLock);

  if( fcntl(p->fd, F_SETLK, &lock) ){
    int e = errno;
    if( e==EACCES || e==EAGAIN ){
      rc = SQLITE4_BUSY;
    }else{
      rc = btErrorBkpt(SQLITE4_IOERR);
................................................................................
  static const short aType[3] = { 0, F_RDLCK, F_WRLCK };
  struct flock lock;

  assert( eType==BT_LOCK_SHARED || eType==BT_LOCK_EXCL );
  assert( aType[BT_LOCK_SHARED]==F_RDLCK );
  assert( aType[BT_LOCK_EXCL]==F_WRLCK );
  assert( eType>=0 && eType<(sizeof(aType)/sizeof(aType[0])) );
  assert( iLock>0 && iLock<=32 );

  memset(&lock, 0, sizeof(lock));
  lock.l_whence = SEEK_SET;
  lock.l_len = nLock;
  lock.l_type = aType[eType];
  lock.l_start = (4096-iLock);

  if( fcntl(p->fd, F_GETLK, &lock) ){
    rc = btErrorBkpt(SQLITE4_IOERR);
  }else if( lock.l_type!=F_UNLCK ){
    rc = SQLITE4_BUSY;
  }

  return rc;
}

static int btPosixOsUnlink(sqlite4_env *pEnv, bt_env *pVfs, const char *zFile){
  int prc = unlink(zFile);
  return prc ? btErrorBkpt(SQLITE4_IOERR) : SQLITE4_OK;
}

#define btPosixLockToByte(iLock) (100 + (iLock))

int btPosixOsLock(bt_file *pFile, int iLock, int eType){
  int rc = SQLITE4_OK;
  PosixFile *p = (PosixFile *)pFile;
  static const short aType[3] = { F_UNLCK, F_RDLCK, F_WRLCK };
  struct flock lock;

  assert( aType[BT_LOCK_UNLOCK]==F_UNLCK );
  assert( aType[BT_LOCK_SHARED]==F_RDLCK );
  assert( aType[BT_LOCK_EXCL]==F_WRLCK );
  assert( eType>=0 && eType<(sizeof(aType)/sizeof(aType[0])) );
  assert( iLock>=0 && iLock<=32 );

  memset(&lock, 0, sizeof(lock));
  lock.l_whence = SEEK_SET;
  lock.l_len = 1;
  lock.l_type = aType[eType];
  lock.l_start = btPosixLockToByte(iLock);

  if( fcntl(p->fd, F_SETLK, &lock) ){
    int e = errno;
    if( e==EACCES || e==EAGAIN ){
      rc = SQLITE4_BUSY;
    }else{
      rc = btErrorBkpt(SQLITE4_IOERR);
................................................................................
  static const short aType[3] = { 0, F_RDLCK, F_WRLCK };
  struct flock lock;

  assert( eType==BT_LOCK_SHARED || eType==BT_LOCK_EXCL );
  assert( aType[BT_LOCK_SHARED]==F_RDLCK );
  assert( aType[BT_LOCK_EXCL]==F_WRLCK );
  assert( eType>=0 && eType<(sizeof(aType)/sizeof(aType[0])) );
  assert( iLock>=0 && iLock<=32 );

  memset(&lock, 0, sizeof(lock));
  lock.l_whence = SEEK_SET;
  lock.l_len = nLock;
  lock.l_type = aType[eType];
  lock.l_start = btPosixLockToByte(iLock);

  if( fcntl(p->fd, F_GETLK, &lock) ){
    rc = btErrorBkpt(SQLITE4_IOERR);
  }else if( lock.l_type!=F_UNLCK ){
    rc = SQLITE4_BUSY;
  }