SQLite

      case SQLITE_BUSY_RECOVERY:      zName = "SQLITE_BUSY_RECOVERY";     break;
      case SQLITE_BUSY_SNAPSHOT:      zName = "SQLITE_BUSY_SNAPSHOT";     break;
      case SQLITE_LOCKED:             zName = "SQLITE_LOCKED";            break;
      case SQLITE_LOCKED_SHAREDCACHE: zName = "SQLITE_LOCKED_SHAREDCACHE";break;
      case SQLITE_NOMEM:              zName = "SQLITE_NOMEM";             break;
      case SQLITE_READONLY:           zName = "SQLITE_READONLY";          break;
      case SQLITE_READONLY_RECOVERY:  zName = "SQLITE_READONLY_RECOVERY"; break;
      case SQLITE_READONLY_CANTLOCK:  zName = "SQLITE_READONLY_CANTLOCK"; break;
      case SQLITE_READONLY_CANTINIT:  zName = "SQLITE_READONLY_CANTINIT"; break;
      case SQLITE_READONLY_ROLLBACK:  zName = "SQLITE_READONLY_ROLLBACK"; break;
      case SQLITE_READONLY_DBMOVED:   zName = "SQLITE_READONLY_DBMOVED";  break;
      case SQLITE_INTERRUPT:          zName = "SQLITE_INTERRUPT";         break;
      case SQLITE_IOERR:              zName = "SQLITE_IOERR";             break;
      case SQLITE_IOERR_READ:         zName = "SQLITE_IOERR_READ";        break;
      case SQLITE_IOERR_SHORT_READ:   zName = "SQLITE_IOERR_SHORT_READ";  break;
      case SQLITE_IOERR_WRITE:        zName = "SQLITE_IOERR_WRITE";       break;

  unixInodeInfo *pInode;     /* unixInodeInfo that owns this SHM node */
  sqlite3_mutex *mutex;      /* Mutex to access this object */
  char *zFilename;           /* Name of the mmapped file */
  int h;                     /* Open file descriptor */
  int szRegion;              /* Size of shared-memory regions */
  u16 nRegion;               /* Size of array apRegion */
  u8 isReadonly;             /* True if read-only */
  u8 isUnlocked;             /* True if no DMS lock held */
  char **apRegion;           /* Array of mapped shared-memory regions */
  int nRef;                  /* Number of unixShm objects pointing to this */
  unixShm *pFirst;           /* All unixShm objects pointing to this */
#ifdef SQLITE_DEBUG
  u8 exclMask;               /* Mask of exclusive locks held */
  u8 sharedMask;             /* Mask of shared locks held */
  u8 nextShmId;              /* Next available unixShm.id value */

      robust_close(pFd, p->h, __LINE__);
      p->h = -1;
    }
    p->pInode->pShmNode = 0;
    sqlite3_free(p);
  }
}

/*
** The DMS lock has not yet been taken on shm file pShmNode. Attempt to
** take it now. Return SQLITE_OK if successful, or an SQLite error
** code otherwise.
**
** If the DMS cannot be locked because this is a readonly_shm=1 
** connection and no other process already holds a lock, return
** SQLITE_READONLY_CANTINIT and set pShmNode->isUnlocked=1.
*/
static int unixLockSharedMemory(unixFile *pDbFd, unixShmNode *pShmNode){
  struct flock lock;
  int rc = SQLITE_OK;

  /* Use F_GETLK to determine the locks other processes are holding
  ** on the DMS byte. If it indicates that another process is holding
  ** a SHARED lock, then this process may also take a SHARED lock
  ** and proceed with opening the *-shm file. 
  **
  ** Or, if no other process is holding any lock, then this process
  ** is the first to open it. In this case take an EXCLUSIVE lock on the
  ** DMS byte and truncate the *-shm file to zero bytes in size. Then
  ** downgrade to a SHARED lock on the DMS byte.
  **
  ** If another process is holding an EXCLUSIVE lock on the DMS byte,
  ** return SQLITE_BUSY to the caller (it will try again). An earlier
  ** version of this code attempted the SHARED lock at this point. But
  ** this introduced a subtle race condition: if the process holding
  ** EXCLUSIVE failed just before truncating the *-shm file, then this
  ** process might open and use the *-shm file without truncating it.
  ** And if the *-shm file has been corrupted by a power failure or
  ** system crash, the database itself may also become corrupt.  */
  lock.l_whence = SEEK_SET;
  lock.l_start = UNIX_SHM_DMS;
  lock.l_len = 1;
  lock.l_type = F_WRLCK;
  if( osFcntl(pShmNode->h, F_GETLK, &lock)!=0 ) {
    rc = SQLITE_IOERR_LOCK;
  }else if( lock.l_type==F_UNLCK ){
    if( pShmNode->isReadonly ){
      pShmNode->isUnlocked = 1;
      rc = SQLITE_READONLY_CANTINIT;
    }else{
      rc = unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1);
      if( rc==SQLITE_OK && robust_ftruncate(pShmNode->h, 0) ){
        rc = unixLogError(SQLITE_IOERR_SHMOPEN,"ftruncate",pShmNode->zFilename);
      }
    }
  }else if( lock.l_type==F_WRLCK ){
    rc = SQLITE_BUSY;
  }

  if( rc==SQLITE_OK ){
    assert( lock.l_type==F_UNLCK || lock.l_type==F_RDLCK );
    rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);
  }
  return rc;
}

#if defined(__APPLE__) && SQLITE_ENABLE_LOCKING_STYLE
static const char *proxySharedMemoryBasePath(unixFile *);
#endif

/*
** Open a shared-memory area associated with open database file pDbFd.  

** that no other processes are able to read or write the database.  In
** that case, we do not really need shared memory.  No shared memory
** file is created.  The shared memory will be simulated with heap memory.
*/
static int unixOpenSharedMemory(unixFile *pDbFd){
  struct unixShm *p = 0;          /* The connection to be opened */
  struct unixShmNode *pShmNode;   /* The underlying mmapped file */
  int rc;                         /* Result code */
  int rc = SQLITE_OK;             /* Result code */
  unixInodeInfo *pInode;          /* The inode of fd */
  char *zShmFilename;             /* Name of the file used for SHM */
  char *zShm;             /* Name of the file used for SHM */
  int nShmFilename;               /* Size of the SHM filename in bytes */

  /* Allocate space for the new unixShm object. */
  p = sqlite3_malloc64( sizeof(*p) );
  if( p==0 ) return SQLITE_NOMEM_BKPT;
  memset(p, 0, sizeof(*p));
  assert( pDbFd->pShm==0 );

#endif
    pShmNode = sqlite3_malloc64( sizeof(*pShmNode) + nShmFilename );
    if( pShmNode==0 ){
      rc = SQLITE_NOMEM_BKPT;
      goto shm_open_err;
    }
    memset(pShmNode, 0, sizeof(*pShmNode)+nShmFilename);
    zShmFilename = pShmNode->zFilename = (char*)&pShmNode[1];
    zShm = pShmNode->zFilename = (char*)&pShmNode[1];
#ifdef SQLITE_SHM_DIRECTORY
    sqlite3_snprintf(nShmFilename, zShmFilename, 
    sqlite3_snprintf(nShmFilename, zShm, 
                     SQLITE_SHM_DIRECTORY "/sqlite-shm-%x-%x",
                     (u32)sStat.st_ino, (u32)sStat.st_dev);
#else
    sqlite3_snprintf(nShmFilename, zShmFilename, "%s-shm", zBasePath);
    sqlite3FileSuffix3(pDbFd->zPath, zShmFilename);
    sqlite3_snprintf(nShmFilename, zShm, "%s-shm", zBasePath);
    sqlite3FileSuffix3(pDbFd->zPath, zShm);
#endif
    pShmNode->h = -1;
    pDbFd->pInode->pShmNode = pShmNode;
    pShmNode->pInode = pDbFd->pInode;
    if( sqlite3GlobalConfig.bCoreMutex ){
      pShmNode->mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_FAST);
      if( pShmNode->mutex==0 ){
        rc = SQLITE_NOMEM_BKPT;
        goto shm_open_err;
      }
    }

    if( pInode->bProcessLock==0 ){
      int openFlags = O_RDWR | O_CREAT;
      int fd;                     /* File descriptor for *-shm file */
      if( sqlite3_uri_boolean(pDbFd->zPath, "readonly_shm", 0)
#ifdef __APPLE__
       /* On MacOS and iOS, avoid even trying to open a read-only SHM file
       ** for writing, because doing so generates scary log messages */
       || (osAccess(zShmFilename, R_OK|W_OK)!=0
      if( osAccess(zShmFilename, R_OK|W_OK)!=0 && (errno==EPERM || errno==EACCES) ){
            && (errno==EPERM || errno==EACCES))
        pShmNode->h = -1;
      }else
#endif
      ){
        openFlags = O_RDONLY;
        pShmNode->isReadonly = 1;
      if( 0==sqlite3_uri_boolean(pDbFd->zPath, "readonly_shm", 0) ){
        pShmNode->h = robust_open(zShm, O_RDWR|O_CREAT, (sStat.st_mode&0777));
      }
      fd = robust_open(zShmFilename, openFlags, (sStat.st_mode&0777));

      if( pShmNode->h<0 ){
      /* If it was not possible to open the *-shm file in read/write mode,
      ** and the database file itself has been opened in read-only mode,
      ** try to open the *-shm file in read-only mode as well. Even if the
      ** database connection is read-only, it is still better to try opening
      ** the *-shm file in read/write mode first, as the same file descriptor
      ** may be also be used by a read/write database connection.  */
#if defined(SQLITE_ENABLE_PERSIST_WAL)&&(SQLITE_ENABLE_LOCKING_STYLE \
    || defined(__APPLE__))
      if( fd<0 && (errno==EPERM || errno==EACCES) && pShmNode->isReadonly==0 
       && (pDbFd->openFlags & O_RDWR)!=O_RDWR
      ){
        fd = robust_open(zShmFilename, O_RDONLY, (sStat.st_mode&0777));
        pShmNode->isReadonly = 1;
        pShmNode->h = robust_open(zShm, O_RDONLY, (sStat.st_mode&0777));
        if( pShmNode->h<0 ){
      }
#endif
      if( fd<0 ){
        rc = unixLogError(SQLITE_CANTOPEN_BKPT, "open", zShmFilename);
          rc = unixLogError(SQLITE_CANTOPEN_BKPT, "open", zShm);
        goto shm_open_err;
      }
      pShmNode->h = fd;
        pShmNode->isReadonly = 1;
      }

      /* If this process is running as root, make sure that the SHM file
      ** is owned by the same user that owns the original database.  Otherwise,
      ** the original owner will not be able to connect.
      */
      robustFchown(pShmNode->h, sStat.st_uid, sStat.st_gid);
  
      /* Check to see if another process is holding the dead-man switch.
      ** If not, truncate the file to zero length. 
      */
      rc = SQLITE_OK;
      if( unixShmSystemLock(pDbFd, F_WRLCK, UNIX_SHM_DMS, 1)==SQLITE_OK ){
        if( robust_ftruncate(pShmNode->h, 0) ){
          rc = unixLogError(SQLITE_IOERR_SHMOPEN, "ftruncate", zShmFilename);
      rc = unixLockSharedMemory(pDbFd, pShmNode);
        }else{
          /* If running as root set the uid/gid of the shm file to match
          ** the database */
          uid_t euid = geteuid();
          if( (!pShmNode->isReadonly) && euid==0 && (euid!=sStat.st_uid || getegid()!=sStat.st_gid) ){
            if( osFchown(pShmNode->h, sStat.st_uid, sStat.st_gid) ){
              rc = SQLITE_IOERR_SHMOPEN;
            }
          }
        }
      }
      if( rc==SQLITE_OK ){
        rc = unixShmSystemLock(pDbFd, F_RDLCK, UNIX_SHM_DMS, 1);
      }
      if( rc ) goto shm_open_err;
      if( rc!=SQLITE_OK && rc!=SQLITE_READONLY_CANTINIT ) goto shm_open_err;
    }
  }

  /* Make the new connection a child of the unixShmNode */
  p->pShmNode = pShmNode;
#ifdef SQLITE_DEBUG
  p->id = pShmNode->nextShmId++;

  ** at pShmNode->pFirst. This must be done while holding the pShmNode->mutex 
  ** mutex.
  */
  sqlite3_mutex_enter(pShmNode->mutex);
  p->pNext = pShmNode->pFirst;
  pShmNode->pFirst = p;
  sqlite3_mutex_leave(pShmNode->mutex);
  return SQLITE_OK;
  return rc;

  /* Jump here on any error */
shm_open_err:
  unixShmPurge(pDbFd);       /* This call frees pShmNode if required */
  sqlite3_free(p);
  unixLeaveMutex();
  return rc;

    rc = unixOpenSharedMemory(pDbFd);
    if( rc!=SQLITE_OK ) return rc;
  }

  p = pDbFd->pShm;
  pShmNode = p->pShmNode;
  sqlite3_mutex_enter(pShmNode->mutex);
  if( pShmNode->isUnlocked ){
    rc = unixLockSharedMemory(pDbFd, pShmNode);
    if( rc!=SQLITE_OK ) goto shmpage_out;
    pShmNode->isUnlocked = 0;
  }
  assert( szRegion==pShmNode->szRegion || pShmNode->nRegion==0 );
  assert( pShmNode->pInode==pDbFd->pInode );
  assert( pShmNode->h>=0 || pDbFd->pInode->bProcessLock==1 );
  assert( pShmNode->h<0 || pDbFd->pInode->bProcessLock==0 );

  /* Minimum number of regions required to be mapped. */
  nReqRegion = ((iRegion+nShmPerMap) / nShmPerMap) * nShmPerMap;

struct winShmNode {
  sqlite3_mutex *mutex;      /* Mutex to access this object */
  char *zFilename;           /* Name of the file */
  winFile hFile;             /* File handle from winOpen */

  int szRegion;              /* Size of shared-memory regions */
  int nRegion;               /* Size of array apRegion */
  u8 isReadonly;             /* True if read-only */
  u8 isUnlocked;             /* True if no DMS lock held */

  struct ShmRegion {
    HANDLE hMap;             /* File handle from CreateFileMapping */
    void *pMap;
  } *aRegion;
  DWORD lastErrno;           /* The Windows errno from the last I/O error */

  int nRef;                  /* Number of winShm objects pointing to this */

      sqlite3_free(p->aRegion);
      sqlite3_free(p);
    }else{
      pp = &p->pNext;
    }
  }
}

/*
** The DMS lock has not yet been taken on shm file pShmNode. Attempt to
** take it now. Return SQLITE_OK if successful, or an SQLite error
** code otherwise.
**
** If the DMS cannot be locked because this is a readonly_shm=1
** connection and no other process already holds a lock, return
** SQLITE_READONLY_CANTINIT and set pShmNode->isUnlocked=1.
*/
static int winLockSharedMemory(winShmNode *pShmNode){
  int rc = winShmSystemLock(pShmNode, WINSHM_WRLCK, WIN_SHM_DMS, 1);

  if( rc==SQLITE_OK ){
    if( pShmNode->isReadonly ){
      pShmNode->isUnlocked = 1;
      winShmSystemLock(pShmNode, WINSHM_UNLCK, WIN_SHM_DMS, 1);
      return SQLITE_READONLY_CANTINIT;
    }else if( winTruncate((sqlite3_file*)&pShmNode->hFile, 0) ){
      winShmSystemLock(pShmNode, WINSHM_UNLCK, WIN_SHM_DMS, 1);
      return winLogError(SQLITE_IOERR_SHMOPEN, osGetLastError(),
                         "winLockSharedMemory", pShmNode->zFilename);
    }
  }

  if( rc==SQLITE_OK ){
    winShmSystemLock(pShmNode, WINSHM_UNLCK, WIN_SHM_DMS, 1);
  }

  return winShmSystemLock(pShmNode, WINSHM_RDLCK, WIN_SHM_DMS, 1);
}

/*
** Open the shared-memory area associated with database file pDbFd.
**
** When opening a new shared-memory file, if no other instances of that
** file are currently open, in this process or in other processes, then
** the file must be truncated to zero length or have its header cleared.
*/
static int winOpenSharedMemory(winFile *pDbFd){
  struct winShm *p;                  /* The connection to be opened */
  struct winShmNode *pShmNode = 0;   /* The underlying mmapped file */
  int rc;                            /* Result code */
  struct winShmNode *pNew;           /* Newly allocated winShmNode */
  winShmNode *pShmNode = 0;          /* The underlying mmapped file */
  int rc = SQLITE_OK;                /* Result code */
  int rc2 = SQLITE_ERROR;            /* winOpen result code */
  winShmNode *pNew;                  /* Newly allocated winShmNode */
  int nName;                         /* Size of zName in bytes */

  assert( pDbFd->pShm==0 );    /* Not previously opened */

  /* Allocate space for the new sqlite3_shm object.  Also speculatively
  ** allocate space for a new winShmNode and filename.
  */

      pShmNode->mutex = sqlite3_mutex_alloc(SQLITE_MUTEX_FAST);
      if( pShmNode->mutex==0 ){
        rc = SQLITE_IOERR_NOMEM_BKPT;
        goto shm_open_err;
      }
    }

    if( 0==sqlite3_uri_boolean(pDbFd->zPath, "readonly_shm", 0) ){
    rc = winOpen(pDbFd->pVfs,
                 pShmNode->zFilename,             /* Name of the file (UTF-8) */
                 (sqlite3_file*)&pShmNode->hFile,  /* File handle here */
      rc2 = winOpen(pDbFd->pVfs,
                    pShmNode->zFilename,
                    (sqlite3_file*)&pShmNode->hFile,
                 SQLITE_OPEN_WAL | SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE,
                 0);
    if( SQLITE_OK!=rc ){
      goto shm_open_err;
    }

    if( rc2!=SQLITE_OK ){
    /* Check to see if another process is holding the dead-man switch.
    ** If not, truncate the file to zero length.
    */
    if( winShmSystemLock(pShmNode, WINSHM_WRLCK, WIN_SHM_DMS, 1)==SQLITE_OK ){
      rc = winTruncate((sqlite3_file *)&pShmNode->hFile, 0);
      if( rc!=SQLITE_OK ){
        rc = winLogError(SQLITE_IOERR_SHMOPEN, osGetLastError(),
                         "winOpenShm", pDbFd->zPath);
      rc2 = winOpen(pDbFd->pVfs,
                    pShmNode->zFilename,
                    (sqlite3_file*)&pShmNode->hFile,
                    SQLITE_OPEN_WAL|SQLITE_OPEN_READONLY,
                    0);
      if( rc2!=SQLITE_OK ){
        rc = winLogError(rc2, osGetLastError(), "winOpenShm",
                         pShmNode->zFilename);
        goto shm_open_err;
      }
      pShmNode->isReadonly = 1;
    }
    if( rc==SQLITE_OK ){
      winShmSystemLock(pShmNode, WINSHM_UNLCK, WIN_SHM_DMS, 1);
      rc = winShmSystemLock(pShmNode, WINSHM_RDLCK, WIN_SHM_DMS, 1);
    }
    if( rc ) goto shm_open_err;

    rc = winLockSharedMemory(pShmNode);
    if( rc!=SQLITE_OK && rc!=SQLITE_READONLY_CANTINIT ) goto shm_open_err;
  }

  /* Make the new connection a child of the winShmNode */
  p->pShmNode = pShmNode;
#if defined(SQLITE_DEBUG) || defined(SQLITE_HAVE_OS_TRACE)
  p->id = pShmNode->nextShmId++;
#endif

  ** at pShmNode->pFirst. This must be done while holding the pShmNode->mutex
  ** mutex.
  */
  sqlite3_mutex_enter(pShmNode->mutex);
  p->pNext = pShmNode->pFirst;
  pShmNode->pFirst = p;
  sqlite3_mutex_leave(pShmNode->mutex);
  return SQLITE_OK;
  return rc;

  /* Jump here on any error */
shm_open_err:
  winShmSystemLock(pShmNode, WINSHM_UNLCK, WIN_SHM_DMS, 1);
  winShmPurge(pDbFd->pVfs, 0);      /* This call frees pShmNode if required */
  sqlite3_free(p);
  sqlite3_free(pNew);

  int szRegion,                   /* Size of regions */
  int isWrite,                    /* True to extend file if necessary */
  void volatile **pp              /* OUT: Mapped memory */
){
  winFile *pDbFd = (winFile*)fd;
  winShm *pShm = pDbFd->pShm;
  winShmNode *pShmNode;
  DWORD protect = PAGE_READWRITE;
  DWORD flags = FILE_MAP_WRITE | FILE_MAP_READ;
  int rc = SQLITE_OK;

  if( !pShm ){
    rc = winOpenSharedMemory(pDbFd);
    if( rc!=SQLITE_OK ) return rc;
    pShm = pDbFd->pShm;
  }
  pShmNode = pShm->pShmNode;

  sqlite3_mutex_enter(pShmNode->mutex);
  if( pShmNode->isUnlocked ){
    rc = winLockSharedMemory(pShmNode);
    if( rc!=SQLITE_OK ) goto shmpage_out;
    pShmNode->isUnlocked = 0;
  }
  assert( szRegion==pShmNode->szRegion || pShmNode->nRegion==0 );

  if( pShmNode->nRegion<=iRegion ){
    struct ShmRegion *apNew;           /* New aRegion[] array */
    int nByte = (iRegion+1)*szRegion;  /* Minimum required file size */
    sqlite3_int64 sz;                  /* Current size of wal-index file */

        pShmNode->aRegion, (iRegion+1)*sizeof(apNew[0])
    );
    if( !apNew ){
      rc = SQLITE_IOERR_NOMEM_BKPT;
      goto shmpage_out;
    }
    pShmNode->aRegion = apNew;

    if( pShmNode->isReadonly ){
      protect = PAGE_READONLY;
      flags = FILE_MAP_READ;
    }

    while( pShmNode->nRegion<=iRegion ){
      HANDLE hMap = NULL;         /* file-mapping handle */
      void *pMap = 0;             /* Mapped memory region */

#if SQLITE_OS_WINRT
      hMap = osCreateFileMappingFromApp(pShmNode->hFile.h,
          NULL, PAGE_READWRITE, nByte, NULL
          NULL, protect, nByte, NULL
      );
#elif defined(SQLITE_WIN32_HAS_WIDE)
      hMap = osCreateFileMappingW(pShmNode->hFile.h,
          NULL, PAGE_READWRITE, 0, nByte, NULL
          NULL, protect, 0, nByte, NULL
      );
#elif defined(SQLITE_WIN32_HAS_ANSI) && SQLITE_WIN32_CREATEFILEMAPPINGA
      hMap = osCreateFileMappingA(pShmNode->hFile.h,
          NULL, PAGE_READWRITE, 0, nByte, NULL
          NULL, protect, 0, nByte, NULL
      );
#endif
      OSTRACE(("SHM-MAP-CREATE pid=%lu, region=%d, size=%d, rc=%s\n",
               osGetCurrentProcessId(), pShmNode->nRegion, nByte,
               hMap ? "ok" : "failed"));
      if( hMap ){
        int iOffset = pShmNode->nRegion*szRegion;
        int iOffsetShift = iOffset % winSysInfo.dwAllocationGranularity;
#if SQLITE_OS_WINRT
        pMap = osMapViewOfFileFromApp(hMap, FILE_MAP_WRITE | FILE_MAP_READ,
        pMap = osMapViewOfFileFromApp(hMap, flags,
            iOffset - iOffsetShift, szRegion + iOffsetShift
        );
#else
        pMap = osMapViewOfFile(hMap, FILE_MAP_WRITE | FILE_MAP_READ,
        pMap = osMapViewOfFile(hMap, flags,
            0, iOffset - iOffsetShift, szRegion + iOffsetShift
        );
#endif
        OSTRACE(("SHM-MAP-MAP pid=%lu, region=%d, offset=%d, size=%d, rc=%s\n",
                 osGetCurrentProcessId(), pShmNode->nRegion, iOffset,
                 szRegion, pMap ? "ok" : "failed"));
      }

    int iOffset = iRegion*szRegion;
    int iOffsetShift = iOffset % winSysInfo.dwAllocationGranularity;
    char *p = (char *)pShmNode->aRegion[iRegion].pMap;
    *pp = (void *)&p[iOffsetShift];
  }else{
    *pp = 0;
  }
  if( pShmNode->isReadonly && rc==SQLITE_OK ) rc = SQLITE_READONLY;
  sqlite3_mutex_leave(pShmNode->mutex);
  return rc;
}

#else
# define winShmMap     0
# define winShmLock    0

  StrAccum acc;
  char zBuf[500];
  sqlite3StrAccumInit(&acc, 0, zBuf, sizeof(zBuf), 0);
  va_start(ap,zFormat);
  sqlite3VXPrintf(&acc, zFormat, ap);
  va_end(ap);
  sqlite3StrAccumFinish(&acc);
#ifdef SQLITE_OS_TRACE_PROC
  {
    extern void SQLITE_OS_TRACE_PROC(const char *zBuf, int nBuf);
    SQLITE_OS_TRACE_PROC(zBuf, sizeof(zBuf));
  }
#else
  fprintf(stdout,"%s", zBuf);
  fflush(stdout);
#endif
}
#endif


/*
** variable-argument wrapper around sqlite3VXPrintf().  The bFlags argument
** can contain the bit SQLITE_PRINTF_INTERNAL enable internal formats.

#define SQLITE_LOCKED_SHAREDCACHE      (SQLITE_LOCKED |  (1<<8))
#define SQLITE_BUSY_RECOVERY           (SQLITE_BUSY   |  (1<<8))
#define SQLITE_BUSY_SNAPSHOT           (SQLITE_BUSY   |  (2<<8))
#define SQLITE_CANTOPEN_NOTEMPDIR      (SQLITE_CANTOPEN | (1<<8))
#define SQLITE_CANTOPEN_ISDIR          (SQLITE_CANTOPEN | (2<<8))
#define SQLITE_CANTOPEN_FULLPATH       (SQLITE_CANTOPEN | (3<<8))
#define SQLITE_CANTOPEN_CONVPATH       (SQLITE_CANTOPEN | (4<<8))
#define SQLITE_CANTOPEN_DIRTYWAL       (SQLITE_CANTOPEN | (5<<8))
#define SQLITE_CORRUPT_VTAB            (SQLITE_CORRUPT | (1<<8))
#define SQLITE_READONLY_RECOVERY       (SQLITE_READONLY | (1<<8))
#define SQLITE_READONLY_CANTLOCK       (SQLITE_READONLY | (2<<8))
#define SQLITE_READONLY_ROLLBACK       (SQLITE_READONLY | (3<<8))
#define SQLITE_READONLY_DBMOVED        (SQLITE_READONLY | (4<<8))
#define SQLITE_READONLY_CANTINIT       (SQLITE_READONLY | (5<<8))
#define SQLITE_ABORT_ROLLBACK          (SQLITE_ABORT | (2<<8))
#define SQLITE_CONSTRAINT_CHECK        (SQLITE_CONSTRAINT | (1<<8))
#define SQLITE_CONSTRAINT_COMMITHOOK   (SQLITE_CONSTRAINT | (2<<8))
#define SQLITE_CONSTRAINT_FOREIGNKEY   (SQLITE_CONSTRAINT | (3<<8))
#define SQLITE_CONSTRAINT_FUNCTION     (SQLITE_CONSTRAINT | (4<<8))
#define SQLITE_CONSTRAINT_NOTNULL      (SQLITE_CONSTRAINT | (5<<8))
#define SQLITE_CONSTRAINT_PRIMARYKEY   (SQLITE_CONSTRAINT | (6<<8))

  u8 exclusiveMode;          /* Non-zero if connection is in exclusive mode */
  u8 writeLock;              /* True if in a write transaction */
  u8 ckptLock;               /* True if holding a checkpoint lock */
  u8 readOnly;               /* WAL_RDWR, WAL_RDONLY, or WAL_SHM_RDONLY */
  u8 truncateOnCommit;       /* True to truncate WAL file on commit */
  u8 syncHeader;             /* Fsync the WAL header if true */
  u8 padToSectorBoundary;    /* Pad transactions out to the next sector */
  u8 bShmUnreliable;         /* SHM content is read-only and unreliable */
  WalIndexHdr hdr;           /* Wal-index header for current transaction */
  u32 minFrame;              /* Ignore wal frames before this one */
  u32 iReCksum;              /* On commit, recalculate checksums from here */
  const char *zWalName;      /* Name of WAL file */
  u32 nCkpt;                 /* Checkpoint sequence counter in the wal-header */
#ifdef SQLITE_DEBUG
  u8 lockError;              /* True if a locking error has occurred */

    sizeof(ht_slot)*HASHTABLE_NSLOT + HASHTABLE_NPAGE*sizeof(u32) \
)

/*
** Obtain a pointer to the iPage'th page of the wal-index. The wal-index
** is broken into pages of WALINDEX_PGSZ bytes. Wal-index pages are
** numbered from zero.
**
** If the wal-index is currently smaller the iPage pages then the size
** of the wal-index might be increased, but only if it is safe to do
** so.  It is safe to enlarge the wal-index if pWal->writeLock is true
** or pWal->exclusiveMode==WAL_HEAPMEMORY_MODE.
**
** If this call is successful, *ppPage is set to point to the wal-index
** page and SQLITE_OK is returned. If an error (an OOM or VFS error) occurs,
** then an SQLite error code is returned and *ppPage is set to 0.
*/
static int walIndexPage(Wal *pWal, int iPage, volatile u32 **ppPage){
  int rc = SQLITE_OK;

    if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
      pWal->apWiData[iPage] = (u32 volatile *)sqlite3MallocZero(WALINDEX_PGSZ);
      if( !pWal->apWiData[iPage] ) rc = SQLITE_NOMEM_BKPT;
    }else{
      rc = sqlite3OsShmMap(pWal->pDbFd, iPage, WALINDEX_PGSZ, 
          pWal->writeLock, (void volatile **)&pWal->apWiData[iPage]
      );
      assert( pWal->apWiData[iPage]!=0 || rc!=SQLITE_OK || pWal->writeLock==0 );
      testcase( pWal->apWiData[iPage]==0 && rc==SQLITE_OK );
      if( rc==SQLITE_READONLY ){
      if( (rc&0xff)==SQLITE_READONLY ){
        pWal->readOnly |= WAL_SHM_RDONLY;
        if( rc==SQLITE_READONLY ){
        rc = SQLITE_OK;
      }
    }
  }
  }

  *ppPage = pWal->apWiData[iPage];
  assert( iPage==0 || *ppPage || rc!=SQLITE_OK );
  return rc;
}

/*

** the necessary locks, this routine returns SQLITE_BUSY.
*/
static int walIndexRecover(Wal *pWal){
  int rc;                         /* Return Code */
  i64 nSize;                      /* Size of log file */
  u32 aFrameCksum[2] = {0, 0};
  int iLock;                      /* Lock offset to lock for checkpoint */
  int nLock;                      /* Number of locks to hold */

  /* Obtain an exclusive lock on all byte in the locking range not already
  ** locked by the caller. The caller is guaranteed to have locked the
  ** WAL_WRITE_LOCK byte, and may have also locked the WAL_CKPT_LOCK byte.
  ** If successful, the same bytes that are locked here are unlocked before
  ** this function returns.
  */
  assert( pWal->ckptLock==1 || pWal->ckptLock==0 );
  assert( WAL_ALL_BUT_WRITE==WAL_WRITE_LOCK+1 );
  assert( WAL_CKPT_LOCK==WAL_ALL_BUT_WRITE );
  assert( pWal->writeLock );
  iLock = WAL_ALL_BUT_WRITE + pWal->ckptLock;
  rc = walLockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
  nLock = SQLITE_SHM_NLOCK - iLock;
  rc = walLockExclusive(pWal, iLock, nLock);
  if( rc==SQLITE_OK ){
    rc = walLockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
    if( rc!=SQLITE_OK ){
      walUnlockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
    }
  }
  if( rc ){
    return rc;
  }

  WALTRACE(("WAL%p: recovery begin...\n", pWal));

  memset(&pWal->hdr, 0, sizeof(WalIndexHdr));

  rc = sqlite3OsFileSize(pWal->pWalFd, &nSize);
  if( rc!=SQLITE_OK ){
    goto recovery_error;

          pWal->hdr.mxFrame, pWal->zWalName
      );
    }
  }

recovery_error:
  WALTRACE(("WAL%p: recovery %s\n", pWal, rc ? "failed" : "ok"));
  walUnlockExclusive(pWal, iLock, nLock);
  walUnlockExclusive(pWal, iLock, WAL_READ_LOCK(0)-iLock);
  walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
  return rc;
}

/*
** Close an open wal-index.
*/
static void walIndexClose(Wal *pWal, int isDelete){
  if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE ){
  if( pWal->exclusiveMode==WAL_HEAPMEMORY_MODE || pWal->bShmUnreliable ){
    int i;
    for(i=0; i<pWal->nWiData; i++){
      sqlite3_free((void *)pWal->apWiData[i]);
      pWal->apWiData[i] = 0;
    }
  }else{
  }
  if( pWal->exclusiveMode!=WAL_HEAPMEMORY_MODE ){
    sqlite3OsShmUnmap(pWal->pDbFd, isDelete);
  }
}

/* 
** Open a connection to the WAL file zWalName. The database file must 
** already be opened on connection pDbFd. The buffer that zWalName points

    testcase( pWal->szPage>=65536 );
  }

  /* The header was successfully read. Return zero. */
  return 0;
}

/*
** This is the value that walTryBeginRead returns when it needs to
** be retried.
*/
#define WAL_RETRY  (-1)

/*
** Read the wal-index header from the wal-index and into pWal->hdr.
** If the wal-header appears to be corrupt, try to reconstruct the
** wal-index from the WAL before returning.
**
** Set *pChanged to 1 if the wal-index header value in pWal->hdr is
** changed by this operation.  If pWal->hdr is unchanged, set *pChanged

  /* Ensure that page 0 of the wal-index (the page that contains the 
  ** wal-index header) is mapped. Return early if an error occurs here.
  */
  assert( pChanged );
  rc = walIndexPage(pWal, 0, &page0);
  if( rc!=SQLITE_OK ){
    assert( rc!=SQLITE_READONLY ); /* READONLY changed to OK in walIndexPage */
    if( rc==SQLITE_READONLY_CANTINIT ){
      /* The SQLITE_READONLY_CANTINIT return means that the shared-memory
      ** was openable but is not writable, and this thread is unable to
      ** confirm that another write-capable connection has the shared-memory
      ** open, and hence the content of the shared-memory is unreliable,
      ** since the shared-memory might be inconsistent with the WAL file
      ** and there is no writer on hand to fix it. */
      assert( page0==0 );
      assert( pWal->writeLock==0 );
      assert( pWal->readOnly & WAL_SHM_RDONLY );
      pWal->bShmUnreliable = 1;
      pWal->exclusiveMode = WAL_HEAPMEMORY_MODE;
      *pChanged = 1;
    }else{
    return rc;
  };
  assert( page0 || pWal->writeLock==0 );
      return rc; /* Any other non-OK return is just an error */
    }
  }else{
    /* page0 can be NULL if the SHM is zero bytes in size and pWal->writeLock
    ** is zero, which prevents the SHM from growing */
    testcase( page0!=0 );
  }
  assert( page0!=0 || pWal->writeLock==0 );

  /* If the first page of the wal-index has been mapped, try to read the
  ** wal-index header immediately, without holding any lock. This usually
  ** works, but may fail if the wal-index header is corrupt or currently 
  ** being modified by another thread or process.
  */
  badHdr = (page0 ? walIndexTryHdr(pWal, pChanged) : 1);

  /* If the first attempt failed, it might have been due to a race
  ** with a writer.  So get a WRITE lock and try again.
  */
  assert( badHdr==0 || pWal->writeLock==0 );
  if( badHdr ){
    if( pWal->readOnly & WAL_SHM_RDONLY ){
    if( pWal->bShmUnreliable==0 && (pWal->readOnly & WAL_SHM_RDONLY) ){
      if( SQLITE_OK==(rc = walLockShared(pWal, WAL_WRITE_LOCK)) ){
        walUnlockShared(pWal, WAL_WRITE_LOCK);
        rc = SQLITE_READONLY_RECOVERY;
      }
    }else if( SQLITE_OK==(rc = walLockExclusive(pWal, WAL_WRITE_LOCK, 1)) ){
      pWal->writeLock = 1;
      if( SQLITE_OK==(rc = walIndexPage(pWal, 0, &page0)) ){

  /* If the header is read successfully, check the version number to make
  ** sure the wal-index was not constructed with some future format that
  ** this version of SQLite cannot understand.
  */
  if( badHdr==0 && pWal->hdr.iVersion!=WALINDEX_MAX_VERSION ){
    rc = SQLITE_CANTOPEN_BKPT;
  }
  if( pWal->bShmUnreliable ){
    if( rc!=SQLITE_OK ){
      walIndexClose(pWal, 0);
      pWal->bShmUnreliable = 0;
      assert( pWal->nWiData>0 && pWal->apWiData[0]==0 );
      /* walIndexRecover() might have returned SHORT_READ if a concurrent
      ** writer truncated the WAL out from under it.  If that happens, it
      ** indicates that a writer has fixed the SHM file for us, so retry */
      if( rc==SQLITE_IOERR_SHORT_READ ) rc = WAL_RETRY;

    }
    pWal->exclusiveMode = WAL_NORMAL_MODE;
  }

  return rc;
}

/*
** Open a transaction in a connection where the shared-memory is read-only
** and where we cannot verify that there is a separate write-capable connection
** on hand to keep the shared-memory up-to-date with the WAL file.
**
** This can happen, for example, when the shared-memory is implemented by
** memory-mapping a *-shm file, where a prior writer has shut down and
** left the *-shm file on disk, and now the present connection is trying
** to use that database but lacks write permission on the *-shm file.
** Other scenarios are also possible, depending on the VFS implementation.
**
** Precondition:
**
**    The *-wal file has been read and an appropriate wal-index has been
**    constructed in pWal->apWiData[] using heap memory instead of shared
**    memory. 
**
** If this function returns SQLITE_OK, then the read transaction has
** been successfully opened. In this case output variable (*pChanged) 
** is set to true before returning if the caller should discard the
** contents of the page cache before proceeding. Or, if it returns 
** WAL_RETRY, then the heap memory wal-index has been discarded and 
** the caller should retry opening the read transaction from the 
** beginning (including attempting to map the *-shm file). 
**
** If an error occurs, an SQLite error code is returned.
*/
static int walBeginShmUnreliable(Wal *pWal, int *pChanged){
  i64 szWal;                      /* Size of wal file on disk in bytes */
  i64 iOffset;                    /* Current offset when reading wal file */
  u8 aBuf[WAL_HDRSIZE];           /* Buffer to load WAL header into */
  u8 *aFrame = 0;                 /* Malloc'd buffer to load entire frame */
  int szFrame;                    /* Number of bytes in buffer aFrame[] */
  u8 *aData;                      /* Pointer to data part of aFrame buffer */
  volatile void *pDummy;          /* Dummy argument for xShmMap */
  int rc;                         /* Return code */
  u32 aSaveCksum[2];              /* Saved copy of pWal->hdr.aFrameCksum */
** This is the value that walTryBeginRead returns when it needs to
** be retried.

  assert( pWal->bShmUnreliable );
  assert( pWal->readOnly & WAL_SHM_RDONLY );
  assert( pWal->nWiData>0 && pWal->apWiData[0] );

  /* Take WAL_READ_LOCK(0). This has the effect of preventing any
  ** writers from running a checkpoint, but does not stop them
  ** from running recovery.  */
  rc = walLockShared(pWal, WAL_READ_LOCK(0));
  if( rc!=SQLITE_OK ){
    if( rc==SQLITE_BUSY ) rc = WAL_RETRY;
    goto begin_unreliable_shm_out;
  }
  pWal->readLock = 0;

  /* Check to see if a separate writer has attached to the shared-memory area,
  ** thus making the shared-memory "reliable" again.  Do this by invoking
  ** the xShmMap() routine of the VFS and looking to see if the return
  ** is SQLITE_READONLY instead of SQLITE_READONLY_CANTINIT.
  **
  ** If the shared-memory is now "reliable" return WAL_RETRY, which will
  ** cause the heap-memory WAL-index to be discarded and the actual
  ** shared memory to be used in its place.
  **
  ** This step is important because, even though this connection is holding
  ** the WAL_READ_LOCK(0) which prevents a checkpoint, a writer might
  ** have already checkpointed the WAL file and, while the current
  ** is active, wrap the WAL and start overwriting frames that this
  ** process wants to use.
  **
  ** Once sqlite3OsShmMap() has been called for an sqlite3_file and has
  ** returned any SQLITE_READONLY value, it must return only SQLITE_READONLY
  ** or SQLITE_READONLY_CANTINIT or some error for all subsequent invocations,
  ** even if some external agent does a "chmod" to make the shared-memory
  ** writable by us, until sqlite3OsShmUnmap() has been called.
  ** This is a requirement on the VFS implementation.
   */
  rc = sqlite3OsShmMap(pWal->pDbFd, 0, WALINDEX_PGSZ, 0, &pDummy);
  assert( rc!=SQLITE_OK ); /* SQLITE_OK not possible for read-only connection */
  if( rc!=SQLITE_READONLY_CANTINIT ){
    rc = (rc==SQLITE_READONLY ? WAL_RETRY : rc);
    goto begin_unreliable_shm_out;
  }

  /* We reach this point only if the real shared-memory is still unreliable.
  ** Assume the in-memory WAL-index substitute is correct and load it
  ** into pWal->hdr.
  */
  memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));

  /* Make sure some writer hasn't come in and changed the WAL file out
  ** from under us, then disconnected, while we were not looking.
*/
  rc = sqlite3OsFileSize(pWal->pWalFd, &szWal);
  if( rc!=SQLITE_OK ){
    goto begin_unreliable_shm_out;
  }
  if( szWal<WAL_HDRSIZE ){
    /* If the wal file is too small to contain a wal-header and the
    ** wal-index header has mxFrame==0, then it must be safe to proceed
    ** reading the database file only. However, the page cache cannot
    ** be trusted, as a read/write connection may have connected, written
    ** the db, run a checkpoint, truncated the wal file and disconnected
    ** since this client's last read transaction.  */
    *pChanged = 1;
    rc = (pWal->hdr.mxFrame==0 ? SQLITE_OK : WAL_RETRY);
    goto begin_unreliable_shm_out;
  }

  /* Check the salt keys at the start of the wal file still match. */
  rc = sqlite3OsRead(pWal->pWalFd, aBuf, WAL_HDRSIZE, 0);
  if( rc!=SQLITE_OK ){
    goto begin_unreliable_shm_out;
  }
  if( memcmp(&pWal->hdr.aSalt, &aBuf[16], 8) ){
    /* Some writer has wrapped the WAL file while we were not looking.
    ** Return WAL_RETRY which will cause the in-memory WAL-index to be
    ** rebuilt. */
#define WAL_RETRY  (-1)
    rc = WAL_RETRY;
    goto begin_unreliable_shm_out;
  }

  /* Allocate a buffer to read frames into */
  szFrame = pWal->hdr.szPage + WAL_FRAME_HDRSIZE;
  aFrame = (u8 *)sqlite3_malloc64(szFrame);
  if( aFrame==0 ){
    rc = SQLITE_NOMEM_BKPT;
    goto begin_unreliable_shm_out;
  }
  aData = &aFrame[WAL_FRAME_HDRSIZE];

  /* Check to see if a complete transaction has been appended to the
  ** wal file since the heap-memory wal-index was created. If so, the
  ** heap-memory wal-index is discarded and WAL_RETRY returned to
  ** the caller.  */
  aSaveCksum[0] = pWal->hdr.aFrameCksum[0];
  aSaveCksum[1] = pWal->hdr.aFrameCksum[1];
  for(iOffset=walFrameOffset(pWal->hdr.mxFrame+1, pWal->hdr.szPage); 
      iOffset+szFrame<=szWal; 
      iOffset+=szFrame
  ){
    u32 pgno;                   /* Database page number for frame */
    u32 nTruncate;              /* dbsize field from frame header */

    /* Read and decode the next log frame. */
    rc = sqlite3OsRead(pWal->pWalFd, aFrame, szFrame, iOffset);
    if( rc!=SQLITE_OK ) break;
    if( !walDecodeFrame(pWal, &pgno, &nTruncate, aData, aFrame) ) break;

    /* If nTruncate is non-zero, then a complete transaction has been
    ** appended to this wal file. Set rc to WAL_RETRY and break out of
    ** the loop.  */
    if( nTruncate ){
      rc = WAL_RETRY;
      break;
    }
  }
  pWal->hdr.aFrameCksum[0] = aSaveCksum[0];
  pWal->hdr.aFrameCksum[1] = aSaveCksum[1];

 begin_unreliable_shm_out:
  sqlite3_free(aFrame);
  if( rc!=SQLITE_OK ){
    int i;
    for(i=0; i<pWal->nWiData; i++){
      sqlite3_free((void*)pWal->apWiData[i]);
      pWal->apWiData[i] = 0;
    }
    pWal->bShmUnreliable = 0;
    sqlite3WalEndReadTransaction(pWal);
    *pChanged = 1;
  }
  return rc;
}

/*
** Attempt to start a read transaction.  This might fail due to a race or
** other transient condition.  When that happens, it returns WAL_RETRY to
** indicate to the caller that it is safe to retry immediately.
**
** On success return SQLITE_OK.  On a permanent failure (such an

  int mxI;                        /* Index of largest aReadMark[] value */
  int i;                          /* Loop counter */
  int rc = SQLITE_OK;             /* Return code  */
  u32 mxFrame;                    /* Wal frame to lock to */

  assert( pWal->readLock<0 );     /* Not currently locked */

  /* useWal may only be set for read/write connections */
  assert( (pWal->readOnly & WAL_SHM_RDONLY)==0 || useWal==0 );

  /* Take steps to avoid spinning forever if there is a protocol error.
  **
  ** Circumstances that cause a RETRY should only last for the briefest
  ** instances of time.  No I/O or other system calls are done while the
  ** locks are held, so the locks should not be held for very long. But 
  ** if we are unlucky, another process that is holding a lock might get
  ** paged out or take a page-fault that is time-consuming to resolve, 

      return SQLITE_PROTOCOL;
    }
    if( cnt>=10 ) nDelay = (cnt-9)*(cnt-9)*39;
    sqlite3OsSleep(pWal->pVfs, nDelay);
  }

  if( !useWal ){
    assert( rc==SQLITE_OK );
    if( pWal->bShmUnreliable==0 ){
    rc = walIndexReadHdr(pWal, pChanged);
    }
    if( rc==SQLITE_BUSY ){
      /* If there is not a recovery running in another thread or process
      ** then convert BUSY errors to WAL_RETRY.  If recovery is known to
      ** be running, convert BUSY to BUSY_RECOVERY.  There is a race here
      ** which might cause WAL_RETRY to be returned even if BUSY_RECOVERY
      ** would be technically correct.  But the race is benign since with
      ** WAL_RETRY this routine will be called again and will probably be

      }else if( rc==SQLITE_BUSY ){
        rc = SQLITE_BUSY_RECOVERY;
      }
    }
    if( rc!=SQLITE_OK ){
      return rc;
    }
    else if( pWal->bShmUnreliable ){
      return walBeginShmUnreliable(pWal, pChanged);
  }

  }

  assert( pWal->nWiData>0 );
  assert( pWal->apWiData[0]!=0 );
  pInfo = walCkptInfo(pWal);
  if( !useWal && pInfo->nBackfill==pWal->hdr.mxFrame 
#ifdef SQLITE_ENABLE_SNAPSHOT
   && (pWal->pSnapshot==0 || pWal->hdr.mxFrame==0
     || 0==memcmp(&pWal->hdr, pWal->pSnapshot, sizeof(WalIndexHdr)))
#endif
  ){

      }else if( rc!=SQLITE_BUSY ){
        return rc;
      }
    }
  }
  if( mxI==0 ){
    assert( rc==SQLITE_BUSY || (pWal->readOnly & WAL_SHM_RDONLY)!=0 );
    return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTLOCK;
    return rc==SQLITE_BUSY ? WAL_RETRY : SQLITE_READONLY_CANTINIT;
  }

  rc = walLockShared(pWal, WAL_READ_LOCK(mxI));
  if( rc ){
    return rc==SQLITE_BUSY ? WAL_RETRY : rc;
  }
  /* Now that the read-lock has been obtained, check that neither the

  /* If the "last page" field of the wal-index header snapshot is 0, then
  ** no data will be read from the wal under any circumstances. Return early
  ** in this case as an optimization.  Likewise, if pWal->readLock==0, 
  ** then the WAL is ignored by the reader so return early, as if the 
  ** WAL were empty.
  */
  if( iLast==0 || pWal->readLock==0 ){
  if( iLast==0 || (pWal->readLock==0 && pWal->bShmUnreliable==0) ){
    *piRead = 0;
    return SQLITE_OK;
  }

  /* Search the hash table or tables for an entry matching page number
  ** pgno. Each iteration of the following for() loop searches one
  ** hash table (each hash table indexes up to HASHTABLE_NPAGE frames).

#ifdef SQLITE_ENABLE_EXPENSIVE_ASSERT
  /* If expensive assert() statements are available, do a linear search
  ** of the wal-index file content. Make sure the results agree with the
  ** result obtained using the hash indexes above.  */
  {
    u32 iRead2 = 0;
    u32 iTest;
    assert( pWal->minFrame>0 );
    for(iTest=iLast; iTest>=pWal->minFrame; iTest--){
    assert( pWal->bShmUnreliable || pWal->minFrame>0 );
    for(iTest=iLast; iTest>=pWal->minFrame && iTest>0; iTest--){
      if( walFramePgno(pWal, iTest)==pgno ){
        iRead2 = iTest;
        break;
      }
    }
    assert( iRead==iRead2 );
  }

  ** locks are taken in this case). Nor should the pager attempt to
  ** upgrade to exclusive-mode following such an error.
  */
  assert( pWal->readLock>=0 || pWal->lockError );
  assert( pWal->readLock>=0 || (op<=0 && pWal->exclusiveMode==0) );

  if( op==0 ){
    if( pWal->exclusiveMode ){
      pWal->exclusiveMode = 0;
    if( pWal->exclusiveMode!=WAL_NORMAL_MODE ){
      pWal->exclusiveMode = WAL_NORMAL_MODE;
      if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){
        pWal->exclusiveMode = 1;
        pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
      }
      rc = pWal->exclusiveMode==0;
      rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
    }else{
      /* Already in locking_mode=NORMAL */
      rc = 0;
    }
  }else if( op>0 ){
    assert( pWal->exclusiveMode==0 );
    assert( pWal->exclusiveMode==WAL_NORMAL_MODE );
    assert( pWal->readLock>=0 );
    walUnlockShared(pWal, WAL_READ_LOCK(pWal->readLock));
    pWal->exclusiveMode = 1;
    pWal->exclusiveMode = WAL_EXCLUSIVE_MODE;
    rc = 1;
  }else{
    rc = pWal->exclusiveMode==0;
    rc = pWal->exclusiveMode==WAL_NORMAL_MODE;
  }
  return rc;
}

/* 
** Return true if the argument is non-NULL and the WAL module is using
** heap-memory for the wal-index. Otherwise, if the argument is NULL or the

  }
} {4 10}
do_test wal2-1.1 {
  execsql { SELECT count(a), sum(a) FROM t1 } db2
} {4 10}

set RECOVER [list                                      \
  {0 1 lock exclusive}   {1 7 lock exclusive}          \
  {1 7 unlock exclusive} {0 1 unlock exclusive}        \
  {0 1 lock exclusive}   {1 2 lock exclusive} {4 4 lock exclusive} \
  {1 2 unlock exclusive} {4 4 unlock exclusive} {0 1 unlock exclusive}  \
]
set READ [list                                         \
  {4 1 lock shared}    {4 1 unlock shared}             \
]
set INITSLOT [list                                     \
  {4 1 lock exclusive} {4 1 unlock exclusive}          \
]

# UPDATE: This has now changed. When running a checkpoint, if recovery is
# required the client grabs all exclusive locks (just as it would for a
# recovery performed as a pre-cursor to a normal database transaction).
#
set expected_locks [list]
lappend expected_locks {1 1 lock exclusive}   ;# Lock checkpoint
lappend expected_locks {0 1 lock exclusive}   ;# Lock writer
lappend expected_locks {2 6 lock exclusive}   ;# Lock recovery & all aReadMark[]
lappend expected_locks {2 6 unlock exclusive} ;# Unlock recovery & aReadMark[]
lappend expected_locks {2 1 lock exclusive}   ;# Lock recovery
lappend expected_locks {4 4 lock exclusive}   ;# Lock all aReadMark[]
lappend expected_locks {2 1 unlock exclusive} ;# Unlock recovery 
lappend expected_locks {4 4 unlock exclusive} ;# Unlock all aReadMark[] 
lappend expected_locks {0 1 unlock exclusive} ;# Unlock writer
lappend expected_locks {3 1 lock exclusive}   ;# Lock aReadMark[0]
lappend expected_locks {3 1 unlock exclusive} ;# Unlock aReadMark[0]
lappend expected_locks {1 1 unlock exclusive} ;# Unlock checkpoint
do_test wal2-5.1 {
  proc tvfs_cb {method args} {
    set ::shm_file [lindex $args 0]

  testvfs tvfs
  tvfs script tvfs_cb
  sqlite3 db test.db -vfs tvfs
  set {} {}
} {}

set RECOVERY {
  {0 1 lock exclusive} {1 7 lock exclusive} 
  {1 7 unlock exclusive} {0 1 unlock exclusive}
  {0 1 lock exclusive} {1 2 lock exclusive} {4 4 lock exclusive}
  {1 2 unlock exclusive} {4 4 unlock exclusive} {0 1 unlock exclusive}
}
set READMARK0_READ {
  {3 1 lock shared} {3 1 unlock shared}
}
set READMARK0_WRITE {
  {3 1 lock shared} 
  {0 1 lock exclusive} {3 1 unlock shared} 

    forcecopy $shmpath proxysv_test.db-shm 
  }
  faultsim_save_and_close

  foreach {tn db_perm wal_perm shm_perm can_open can_read can_write} {
    2   00644   00644   00644   1   1   1
    3   00644   00400   00644   1   1   0
    4   00644   00644   00400   1   0   0
    4   00644   00644   00400   1   1   0
    5   00400   00644   00644   1   1   0

    7   00644   00000   00644   1   0   0
    8   00644   00644   00000   1   0   0
    9   00000   00644   00644   0   0   0
  } {
    faultsim_restore

  do_test 1.1.13  { sql2 "INSERT INTO t1 VALUES('i', 'j')" } {}

  do_test 1.2.1 {
    code2 { db2 close }
    code1 { db close }
    list [file exists test.db-wal] [file exists $shmpath]
  } {1 1}

  do_test 1.2.2 {
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
    sql1 { SELECT * FROM t1 }
  } {a b c d e f g h i j}
    list [catch { sql1 { SELECT * FROM t1 } } msg] $msg
  } {0 {a b c d e f g h i j}}

  do_test 1.2.3 {
    code1 { db close }
    file attributes $shmpath -permissions rw-r--r--
    hexio_write $shmpath 0 01020304
    file attributes $shmpath -permissions r--r--r--
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
    csql1 { SELECT * FROM t1 }
  } {1 {attempt to write a readonly database}}
  } {0 {a b c d e f g h i j}}
  do_test 1.2.4 {
    code1 { sqlite3_extended_errcode db } 
  } {SQLITE_READONLY_RECOVERY}
  } {SQLITE_OK}

  do_test 1.2.5 {
    file attributes $shmpath -permissions rw-r--r--
    code2 { sqlite3 db2 test.db }
    sql2 "SELECT * FROM t1" 
  } {a b c d e f g h i j}
  file attributes $shmpath -permissions r--r--r--

    set {} {}
  } {}
  do_test 1.2.8 { sql1 "SELECT * FROM t1" } {a b c d e f g h i j k l}

  # Now check that if the readonly_shm option is not supplied, or if it
  # is set to zero, it is not possible to connect to the database without
  # read-write access to the shm.
  # 
  # UPDATE: os_unix.c now opens the *-shm file in readonly mode 
  # automatically.
  #
  do_test 1.3.1 {
    code1 { db close }
    code1 { sqlite3 db test.db }
    csql1 { SELECT * FROM t1 }
  } {1 {unable to open database file}}
  } {0 {a b c d e f g h i j k l}}

  # Also test that if the -shm file can be opened for read/write access,
  # it is not if readonly_shm=1 is present in the URI.
  do_test 1.3.2.1 {
    ifcapable enable_persist_wal {
      code1 { file_control_persist_wal db 0 }
      code2 { file_control_persist_wal db2 0 }

  } {1 {unable to open database file}}
  do_test 1.3.2.3 {
    code1 { db close }
    close [open $shmpath w]
    file attributes $shmpath -permissions r--r--r--
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
    csql1 { SELECT * FROM t1 }
  } {1 {attempt to write a readonly database}}
  } {0 {a b c d e f g h i j k l}}
  do_test 1.3.2.4 {
    code1 { sqlite3_extended_errcode db } 
  } {SQLITE_READONLY_RECOVERY}
  } {SQLITE_OK}

  #-----------------------------------------------------------------------
  # Test cases 1.4.* check that checkpoints and log wraps don't prevent
  # read-only connections from reading the database.
  do_test 1.4.1 {
    code1 { db close }
    forcedelete test.db-shm

# 2011 May 09
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
#
# This file contains tests for using WAL databases in read-only mode.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/lock_common.tcl
source $testdir/wal_common.tcl
set ::testprefix walro2

# And only if the build is WAL-capable.
#
ifcapable !wal {
  finish_test
  return
}

proc copy_to_test2 {bZeroShm} {
  forcecopy test.db test.db2
  forcecopy test.db-wal test.db2-wal
  if {$bZeroShm} {
    forcedelete test.db2-shm
    set fd [open test.db2-shm w]
    seek $fd [expr [file size test.db-shm]-1]
    puts -nonewline $fd "\0"
    close $fd
  } else {
    forcecopy test.db-shm test.db2-shm
  }
}

foreach bZeroShm {0 1} {
set TN [expr $bZeroShm+1]
do_multiclient_test tn {
  
  # Close all connections and delete the database.
  #
  code1 { db close  }
  code2 { db2 close }
  code3 { db3 close }
  forcedelete test.db
  
  # Do not run tests with the connections in the same process.
  #
  if {$tn==2} continue

  foreach c {code1 code2 code3} {
    $c {
      sqlite3_shutdown
      sqlite3_config_uri 1
    }
  }

  do_test $TN.1.1 {
    code2 { sqlite3 db2 test.db }
    sql2 { 
      CREATE TABLE t1(x, y);
      PRAGMA journal_mode = WAL;
      INSERT INTO t1 VALUES('a', 'b');
      INSERT INTO t1 VALUES('c', 'd');
    }
    file exists test.db-shm
  } {1}

  do_test $TN.1.2.1 {
    copy_to_test2 $bZeroShm
    code1 {
      sqlite3 db file:test.db2?readonly_shm=1
    }

    sql1 { SELECT * FROM t1 }
  } {a b c d}
  do_test $TN.1.2.2 {
    sql1 { SELECT * FROM t1 }
  } {a b c d}

  do_test $TN.1.3.1 {
    code3 { sqlite3 db3 test.db2 }
    sql3 { SELECT * FROM t1 }
  } {a b c d}

  do_test $TN.1.3.2 {
    sql1 { SELECT * FROM t1 }
  } {a b c d}

  code1 { db close  }
  code2 { db2 close }
  code3 { db3 close }

  do_test $TN.2.1 {
    code2 { sqlite3 db2 test.db }
    sql2 { 
      INSERT INTO t1 VALUES('e', 'f');
      INSERT INTO t1 VALUES('g', 'h');
    }
    file exists test.db-shm
  } {1}

  do_test $TN.2.2 {
    copy_to_test2 $bZeroShm
    code1 {
      sqlite3 db file:test.db2?readonly_shm=1
    }
    sql1 { 
      BEGIN;
      SELECT * FROM t1;
    }
  } {a b c d e f g h}

  do_test $TN.2.3.1 {
    code3 { sqlite3 db3 test.db2 }
    sql3 { SELECT * FROM t1 }
  } {a b c d e f g h}
  do_test $TN.2.3.2 {
    sql3 { INSERT INTO t1 VALUES('i', 'j') }
    code3 { db3 close }
    sql1 { COMMIT } 
  } {}
  do_test $TN.2.3.3 {
    sql1 { SELECT * FROM t1 }
  } {a b c d e f g h i j}


  #-----------------------------------------------------------------------
  # 3.1.*: That a readonly_shm connection can read a database file if both
  #        the *-wal and *-shm files are zero bytes in size.
  #
  # 3.2.*: That it flushes the cache if, between transactions on a db with a
  #        zero byte *-wal file, some other connection modifies the db, then
  #        does "PRAGMA wal_checkpoint=truncate" to truncate the wal file
  #        back to zero bytes in size.
  #
  # 3.3.*: That, if between transactions some other process wraps the wal
  #        file, the readonly_shm client reruns recovery.
  #
  catch { code1 { db close } }
  catch { code2 { db2 close } }
  catch { code3 { db3 close } }
  do_test $TN.3.1.0 {
    list [file exists test.db-wal] [file exists test.db-shm]
  } {0 0}
  do_test $TN.3.1.1 {
    close [open test.db-wal w]
    close [open test.db-shm w]
    code1 {
      sqlite3 db file:test.db?readonly_shm=1
    }
    sql1 { SELECT * FROM t1 }
  } {a b c d e f g h}

  do_test $TN.3.2.0 {
    list [file size test.db-wal] [file size test.db-shm]
  } {0 0}
  do_test $TN.3.2.1 {
    code2 { sqlite3 db2 test.db }
    sql2 { INSERT INTO t1 VALUES(1, 2) ; PRAGMA wal_checkpoint=truncate }
    code2 { db2 close }
    sql1 { SELECT * FROM t1 }
  } {a b c d e f g h 1 2}
  do_test $TN.3.2.2 {
    list [file size test.db-wal] [file size test.db-shm]
  } {0 32768}

  do_test $TN.3.3.0 {
    code2 { sqlite3 db2 test.db }
    sql2 { 
      INSERT INTO t1 VALUES(3, 4);
      INSERT INTO t1 VALUES(5, 6);
      INSERT INTO t1 VALUES(7, 8);
      INSERT INTO t1 VALUES(9, 10);
    }
    code2 { db2 close }
    code1 { db close }
    list [file size test.db-wal] [file size test.db-shm]
  } [list [wal_file_size 4 1024] 32768]
  do_test $TN.3.3.1 {
    code1 { sqlite3 db file:test.db?readonly_shm=1 }
    sql1 { SELECT * FROM t1 }
  } {a b c d e f g h 1 2 3 4 5 6 7 8 9 10}
  do_test $TN.3.3.2 {
    code2 { sqlite3 db2 test.db }
    sql2 { 
      PRAGMA wal_checkpoint; 
      DELETE FROM t1;
      INSERT INTO t1 VALUES('i', 'ii');
    }
    code2 { db2 close }
    list [file size test.db-wal] [file size test.db-shm]
  } [list [wal_file_size 4 1024] 32768]
  do_test $TN.3.3.3 {
    sql1 { SELECT * FROM t1 }
  } {i ii}

  #-----------------------------------------------------------------------
  #
  #
  catch { code1 { db close } }
  catch { code2 { db2 close } }
  catch { code3 { db3 close } }

  do_test $TN.4.0 {
    code1 { forcedelete test.db }
    code1 { sqlite3 db test.db }
    sql1 {
      PRAGMA journal_mode = wal;
      CREATE TABLE t1(x);
      INSERT INTO t1 VALUES('hello');
      INSERT INTO t1 VALUES('world');
    }

    copy_to_test2 $bZeroShm

    code1 { db close }
  } {}

  do_test $TN.4.1.1 {
    code2 { sqlite3 db2 file:test.db2?readonly_shm=1 }
    sql2 { SELECT * FROM t1 }
  } {hello world}

  do_test $TN.4.1.2 {
    code3 { sqlite3 db3 test.db2 }
    sql3 {
      INSERT INTO t1 VALUES('!');
      PRAGMA wal_checkpoint = truncate;
    }
    code3 { db3 close }
  } {}
  do_test $TN.4.1.3 {
    sql2 { SELECT * FROM t1 }
  } {hello world !}

  catch { code1 { db close } }
  catch { code2 { db2 close } }
  catch { code3 { db3 close } }

  do_test $TN.4.2.1 {
    code1 { sqlite3 db test.db }
    sql1 {
      INSERT INTO t1 VALUES('!');
      INSERT INTO t1 VALUES('!');

      PRAGMA cache_size = 10;
      CREATE TABLE t2(x);

      BEGIN;
        WITH s(i) AS (
          SELECT 1 UNION ALL SELECT i+1 FROM s WHERE i<500
          )
        INSERT INTO t2 SELECT randomblob(500) FROM s;
        SELECT count(*) FROM t2;
    } 
  } {500}
  do_test $TN.4.2.2 {
    file size test.db-wal
  } {461152}
  do_test $TN.4.2.4 {
    file_control_persist_wal db 1; db close

    copy_to_test2 $bZeroShm
    code2 { sqlite3 db2 file:test.db2?readonly_shm=1 }
    sql2 {
      SELECT * FROM t1;
      SELECT count(*) FROM t2;
    }
  } {hello world ! ! 0}

  #-----------------------------------------------------------------------
  #
  #
  catch { code1 { db close } }
  catch { code2 { db2 close } }
  catch { code3 { db3 close } }

  do_test $TN.5.0 {
    code1 { forcedelete test.db }
    code1 { sqlite3 db test.db }
    sql1 {
      PRAGMA journal_mode = wal;
      CREATE TABLE t1(x);
      INSERT INTO t1 VALUES('hello');
      INSERT INTO t1 VALUES('world');
      INSERT INTO t1 VALUES('!');
      INSERT INTO t1 VALUES('world');
      INSERT INTO t1 VALUES('hello');
    }

    copy_to_test2 $bZeroShm
    
    code1 { db close }
  } {}

  do_test $TN.5.1 {
    code2 { sqlite3 db2 file:test.db2?readonly_shm=1 }
    sql2 {
      SELECT * FROM t1;
    }
  } {hello world ! world hello}

  do_test $TN.5.2 {
    code1 {
      proc handle_read {op args} {
        if {$op=="xRead" && [file tail [lindex $args 0]]=="test.db2-wal"} {
          set ::res2 [sql2 { SELECT * FROM t1 }]
        }
        puts "$msg xRead $args"
        return "SQLITE_OK"
      }
      testvfs tvfs -fullshm 1

      sqlite3 db file:test.db2?vfs=tvfs
      db eval { SELECT * FROM sqlite_master }

      tvfs filter xRead
      tvfs script handle_read
    }
    sql1 {
      PRAGMA wal_checkpoint = truncate;
    }
    code1 { set ::res2 }
  } {hello world ! world hello}

  do_test $TN.5.3 {
    code1 { db close }
    code1 { tvfs delete }
  } {}

  #-----------------------------------------------------------------------
  #
  #
  catch { code1 { db close } }
  catch { code2 { db2 close } }
  catch { code3 { db3 close } }

  do_test $TN.6.1 {
    code1 { forcedelete test.db }
    code1 { sqlite3 db test.db }
    sql1 {
      PRAGMA journal_mode = wal;
      CREATE TABLE t1(x);
      INSERT INTO t1 VALUES('hello');
      INSERT INTO t1 VALUES('world');
      INSERT INTO t1 VALUES('!');
      INSERT INTO t1 VALUES('world');
      INSERT INTO t1 VALUES('hello');
    }

    copy_to_test2 $bZeroShm
    
    code1 { db close }
  } {}

  do_test $TN.6.2 {
    code1 {
      set ::nRem 5
      proc handle_read {op args} {
        if {$op=="xRead" && [file tail [lindex $args 0]]=="test.db2-wal"} {
          incr ::nRem -1
          if {$::nRem==0} {
            code2 { sqlite3 db2 test.db2 }
            sql2  { PRAGMA wal_checkpoint = truncate }
          }
        }
        return "SQLITE_OK"
      }
      testvfs tvfs -fullshm 1

      tvfs filter xRead
      tvfs script handle_read

      sqlite3 db file:test.db2?readonly_shm=1&vfs=tvfs
      db eval { SELECT * FROM t1 }
    }
  } {hello world ! world hello}

  do_test $TN.6.3 {
    code1 { db close }
    code1 { tvfs delete }
  } {}
}
} ;# foreach bZeroShm

finish_test

# 2011 May 09
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
#
# This file contains tests for using WAL databases in read-only mode.
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/malloc_common.tcl
set ::testprefix walro2

# And only if the build is WAL-capable.
#
ifcapable !wal {
  finish_test
  return
}

db close
sqlite3_shutdown
sqlite3_config_uri 1
sqlite3 db test.db

do_execsql_test 1.0 {
  CREATE TABLE t1(b);
  PRAGMA journal_mode = wal;
  INSERT INTO t1 VALUES('hello');
  INSERT INTO t1 VALUES('world');
  INSERT INTO t1 VALUES('!');
  INSERT INTO t1 VALUES('world');
  INSERT INTO t1 VALUES('hello');
  PRAGMA cache_size = 10;
  BEGIN;
    WITH s(i) AS ( SELECT 1 UNION ALL SELECT i+1 FROM s WHERE i<30 ) 
    INSERT INTO t1(b) SELECT randomblob(800) FROM s;
} {wal}
file_control_persist_wal db 1; db close
faultsim_save_and_close

do_faultsim_test 1 -faults oom* -prep {
  catch { db close }
  faultsim_restore
  sqlite3 db file:test.db?readonly_shm=1
} -body {
  execsql { SELECT * FROM t1 }
} -test {
  faultsim_test_result {0 {hello world ! world hello}}
}



finish_test