SQLite

/*
** Constants used for locking
*/
#define UNIX_SHM_BASE   ((18+SQLITE_SHM_NLOCK)*4)         /* first lock byte */
#define UNIX_SHM_DMS    (UNIX_SHM_BASE+SQLITE_SHM_NLOCK)  /* deadman switch */

#ifdef SQLITE_DEBUG
/*
** Return a pointer to a nul-terminated string in static memory that
** describes a locking mask.  The string is of the form "MSABCD" with
** each character representing a lock.  "M" for MUTEX, "S" for DMS, 
** and "A" through "D" for the region locks.  If a lock is held, the
** letter is shown.  If the lock is not held, the letter is converted
** to ".".
**
** This routine is for debugging purposes only and does not appear
** in a production build.
*/
static const char *unixShmLockString(u16 maskShared, u16 maskExclusive){
  static char zBuf[52];
  static int iBuf = 0;
  int i;
  u16 mask;
  char *z;

  z = &zBuf[iBuf];
  iBuf += 16;
  if( iBuf>=sizeof(zBuf) ) iBuf = 0;
  for(i=0, mask=1; i<SQLITE_SHM_NLOCK; i++, mask += mask){
    if( mask & maskShared ){
      z[i] = 's';
    }else if( mask & maskExclusive ){
      z[i] = 'E';
    }else{
      z[i] = '.';
    }
  }
  z[i] = 0;
  return z;
}
#endif /* SQLITE_DEBUG */

/*
** Apply posix advisory locks for all bytes from ofst through ofst+n-1.
**
** Locks block if the mask is exactly UNIX_SHM_C and are non-blocking
** otherwise.
*/
static int unixShmSystemLock(

    }else if( lockType==F_RDLCK ){
      OSTRACE(("read-lock failed"));
    }else{
      assert( lockType==F_WRLCK );
      OSTRACE(("write-lock %d failed", ofst));
    }
  }
  OSTRACE((" - afterwards %s\n",
           unixShmLockString(pShmNode->sharedMask, pShmNode->exclMask)));
  }
#endif

  return rc;        
}

      if( rc==SQLITE_OK ){
        p->sharedMask &= ~mask;
        p->exclMask |= mask;
      }
    }
  }
  sqlite3_mutex_leave(pShmNode->mutex);
  OSTRACE(("SHM-LOCK shmid-%d, pid-%d got %s\n",
           p->id, getpid(), unixShmLockString(p->sharedMask, p->exclMask)));
  return rc;
}

/*
** Implement a memory barrier or memory fence on shared memory.  
**
** All loads and stores begun before the barrier must complete before

  DWORD lastErrno;           /* The Windows errno from the last I/O error */
  int szMap;                 /* Size of the mapping of file into memory */
  char *pMMapBuf;            /* Where currently mmapped().  NULL if unmapped */
  int nRef;                  /* Number of winShm objects pointing to this */
  winShm *pFirst;            /* All winShm objects pointing to this */
  winShmNode *pNext;         /* Next in list of all winShmNode objects */
#ifdef SQLITE_DEBUG
  u8 exclMask;               /* Mask of exclusive locks held */
  u8 sharedMask;             /* Mask of shared locks held */
  u8 nextShmId;              /* Next available winShm.id value */
#endif
};

/*
** A global array of all winShmNode objects.
**

**
** The winShm.pFile element is initialized when the object is created
** and is read-only thereafter.
*/
struct winShm {
  winShmNode *pShmNode;      /* The underlying winShmNode object */
  winShm *pNext;             /* Next winShm with the same winShmNode */
  u8 lockState;              /* Current lock state */
  u8 hasMutex;               /* True if holding the winShmNode mutex */
  u8 hasMutexBuf;            /* True if holding pFile->mutexBuf */
  u8 sharedMask;             /* Mask of shared locks held */
  u8 exclMask;               /* Mask of exclusive locks held */
#ifdef SQLITE_DEBUG
  u8 id;                     /* Id of this connection with its winShmNode */
#endif
};

/*
** Size increment by which shared memory grows
*/
#define SQLITE_WIN_SHM_INCR  4096

/*
** Constants used for locking
*/
#define WIN_SHM_BASE      80        /* Byte offset of the first lock byte */
#define WIN_SHM_DMS       0x01      /* Mask for Dead-Man-Switch lock */
#define WIN_SHM_A         0x10      /* Mask for region locks... */
#define WIN_SHM_B         0x20
#define WIN_SHM_C         0x40
#define WIN_SHM_D         0x80

#ifdef SQLITE_DEBUG
/*
** Return a pointer to a nul-terminated string in static memory that
** describes a locking mask.  The string is of the form "MSABCD" with
** each character representing a lock.  "M" for MUTEX, "S" for DMS, 
** and "A" through "D" for the region locks.  If a lock is held, the
** letter is shown.  If the lock is not held, the letter is converted
** to ".".
**
** This routine is for debugging purposes only and does not appear
** in a production build.
*/
static const char *winShmLockString(u8 mask){
  static char zBuf[48];
  static int iBuf = 0;
  char *z;

  z = &zBuf[iBuf];
  iBuf += 8;
  if( iBuf>=sizeof(zBuf) ) iBuf = 0;

  z[0] = (mask & WIN_SHM_DMS)   ? 'S' : '.';
  z[1] = (mask & WIN_SHM_A)     ? 'A' : '.';
  z[2] = (mask & WIN_SHM_B)     ? 'B' : '.';
  z[3] = (mask & WIN_SHM_C)     ? 'C' : '.';
  z[4] = (mask & WIN_SHM_D)     ? 'D' : '.';
  z[5] = 0;
  return z;
}
#endif /* SQLITE_DEBUG */

/*
** Apply posix advisory locks for all bytes identified in lockMask.
**
** lockMask might contain multiple bits but all bits are guaranteed
** to be contiguous.
**
** Locks block if the mask is exactly WIN_SHM_C and are non-blocking
** otherwise.
*/
#define _SHM_UNLCK  1
#define _SHM_RDLCK  2
#define _SHM_WRLCK  3
static int winShmSystemLock(
  winShmNode *pFile,    /* Apply locks to this open shared-memory segment */
  int lockType,         /* _SHM_UNLCK, _SHM_RDLCK, or _SHM_WRLCK */

  u8 lockMask           /* Which bytes to lock or unlock */
){
  OVERLAPPED ovlp;
  DWORD dwFlags;
  int nBytes;           /* Number of bytes to lock */
  int i;                /* Offset into the locking byte range */
  int rc = 0;           /* Result code form Lock/UnlockFileEx() */
  u8 mask;              /* Mask of bits in lockMask */

  /* Access to the winShmNode object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) || pFile->nRef==0 );

  /* Initialize the locking parameters */
  if( lockMask==WIN_SHM_C && lockType!=_SHM_UNLCK ){
    dwFlags = 0;
    OSTRACE(("SHM-LOCK %d requesting blocking lock %s\n", 
             pFile->hFile.h,
             winShmLockString(lockMask)));
  }else{
    dwFlags = LOCKFILE_FAIL_IMMEDIATELY;
    OSTRACE(("SHM-LOCK %d requesting %s %s\n", 
             pFile->hFile.h,
             lockType!=_SHM_UNLCK ? "lock" : "unlock", 
             winShmLockString(lockMask)));
  }
  if( lockType == _SHM_WRLCK ) dwFlags |= LOCKFILE_EXCLUSIVE_LOCK;

  /* Find the first bit in lockMask that is set */
  for(i=0, mask=0x01; mask!=0 && (lockMask&mask)==0; mask <<= 1, i++){}
  assert( mask!=0 );
  memset(&ovlp, 0, sizeof(OVERLAPPED));
  ovlp.Offset = i+WIN_SHM_BASE;
  nBytes = 1;

  /* Extend the locking range for each additional bit that is set */
  mask <<= 1;
  while( mask!=0 && (lockMask & mask)!=0 ){
    nBytes++;
    mask <<= 1;
  }

  /* Verify that all bits set in lockMask are contiguous */
  assert( mask==0 || (lockMask & ~(mask | (mask-1)))==0 );

  /* Release/Acquire the system-level lock */
  if( lockType==_SHM_UNLCK ){
    for(i=0; i<nBytes; i++, ovlp.Offset++){
      rc = UnlockFileEx(pFile->hFile.h, 0, 1, 0, &ovlp);
      if( !rc ) break;
    }
  }else{
    /* release old individual byte locks (if any)
    ** and set new individual byte locks */
    for(i=0; i<nBytes; i++, ovlp.Offset++){
      UnlockFileEx(pFile->hFile.h, 0, 1, 0, &ovlp);
      rc = LockFileEx(pFile->hFile.h, dwFlags, 0, 1, 0, &ovlp);
      if( !rc ) break;
    }
  }
  if( !rc ){
    OSTRACE(("SHM-LOCK %d %s ERROR 0x%08lx\n", 
             pFile->hFile.h,
             lockType==_SHM_UNLCK ? "UnlockFileEx" : "LockFileEx",
             GetLastError()));
    /* release individual byte locks (if any) */
    ovlp.Offset-=i;
    for(i=0; i<nBytes; i++, ovlp.Offset++){
      UnlockFileEx(pFile->hFile.h, 0, 1, 0, &ovlp);
    }
  }
  rc = (rc!=0) ? SQLITE_OK : SQLITE_BUSY;

  /* Update the global lock state and do debug tracing */
#ifdef SQLITE_DEBUG
  OSTRACE(("SHM-LOCK %d ", pFile->hFile.h));
  if( rc==SQLITE_OK ){
    if( lockType==_SHM_UNLCK ){
      OSTRACE(("unlock ok"));
      pFile->exclMask &= ~lockMask;
      pFile->sharedMask &= ~lockMask;
    }else if( lockType==_SHM_RDLCK ){
      OSTRACE(("read-lock ok"));
      pFile->exclMask &= ~lockMask;
      pFile->sharedMask |= lockMask;
    }else{
      assert( lockType==_SHM_WRLCK );
      OSTRACE(("write-lock ok"));
      pFile->exclMask |= lockMask;
      pFile->sharedMask &= ~lockMask;
    }
  }else{
    if( lockType==_SHM_UNLCK ){
      OSTRACE(("unlock failed"));
    }else if( lockType==_SHM_RDLCK ){
      OSTRACE(("read-lock failed"));
    }else{
      assert( lockType==_SHM_WRLCK );
      OSTRACE(("write-lock failed"));
    }
  }
  OSTRACE((" - change requested %s - afterwards %s:%s\n",
           winShmLockString(lockMask),
           winShmLockString(pFile->sharedMask),
           winShmLockString(pFile->exclMask)));
#endif

  return rc;
}

/*
** For connection p, unlock all of the locks identified by the unlockMask
** parameter.
*/
static int winShmUnlock(
  winShmNode *pFile,   /* The underlying shared-memory file */
  winShm *p,           /* The connection to be unlocked */
  u8 unlockMask         /* Mask of locks to be unlocked */
){
  int rc;      /* Result code */
  winShm *pX; /* For looping over all sibling connections */
  u8 allMask;  /* Union of locks held by connections other than "p" */

  /* Access to the winShmNode object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* don't attempt to unlock anything we don't have locks for */
  if( (unlockMask & (p->exclMask|p->sharedMask)) != unlockMask ){
    OSTRACE(("SHM-LOCK %d unlocking more than we have locked - requested %s - have %s\n",
             pFile->hFile.h,
             winShmLockString(unlockMask),
             winShmLockString(p->exclMask|p->sharedMask)));
    unlockMask &= (p->exclMask|p->sharedMask);
  }

  /* Compute locks held by sibling connections */
  allMask = 0;
  for(pX=pFile->pFirst; pX; pX=pX->pNext){
    if( pX==p ) continue;
    assert( (pX->exclMask & (p->exclMask|p->sharedMask))==0 );
    allMask |= pX->sharedMask;
  }

  /* Unlock the system-level locks */
  if( (unlockMask & allMask)!=unlockMask ){
    rc = winShmSystemLock(pFile, _SHM_UNLCK, unlockMask & ~allMask);
  }else{
    rc = SQLITE_OK;
  }

  /* Undo the local locks */
  if( rc==SQLITE_OK ){
    p->exclMask &= ~unlockMask;
    p->sharedMask &= ~unlockMask;
  } 
  return rc;
}

/*
** Get reader locks for connection p on all locks in the readMask parameter.
*/
static int winShmSharedLock(
  winShmNode *pFile,   /* The underlying shared-memory file */
  winShm *p,           /* The connection to get the shared locks */
  u8 readMask           /* Mask of shared locks to be acquired */
){
  int rc;        /* Result code */
  winShm *pX;   /* For looping over all sibling connections */
  u8 allShared;  /* Union of locks held by connections other than "p" */

  /* Access to the winShmNode object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* Find out which shared locks are already held by sibling connections.
  ** If any sibling already holds an exclusive lock, go ahead and return
  ** SQLITE_BUSY.
  */
  allShared = 0;
  for(pX=pFile->pFirst; pX; pX=pX->pNext){
    if( pX==p ) continue;
    if( (pX->exclMask & readMask)!=0 ) return SQLITE_BUSY;
    allShared |= pX->sharedMask;
  }

  /* Get shared locks at the system level, if necessary */
  if( (~allShared) & readMask ){
    rc = winShmSystemLock(pFile, _SHM_RDLCK, readMask);
  }else{
    rc = SQLITE_OK;
  }

  /* Get the local shared locks */
  if( rc==SQLITE_OK ){
    p->sharedMask |= readMask;
  }
  return rc;
}

/*
** For connection p, get an exclusive lock on all locks identified in
** the writeMask parameter.
*/
static int winShmExclusiveLock(
  winShmNode *pFile,    /* The underlying shared-memory file */
  winShm *p,            /* The connection to get the exclusive locks */
  u8 writeMask           /* Mask of exclusive locks to be acquired */
){
  int rc;        /* Result code */
  winShm *pX;   /* For looping over all sibling connections */

  /* Access to the winShmNode object is serialized by the caller */
  assert( sqlite3_mutex_held(pFile->mutex) );

  /* Make sure no sibling connections hold locks that will block this
  ** lock.  If any do, return SQLITE_BUSY right away.
  */
  for(pX=pFile->pFirst; pX; pX=pX->pNext){
    if( pX==p ) continue;
    if( (pX->exclMask & writeMask)!=0 ) return SQLITE_BUSY;
    if( (pX->sharedMask & writeMask)!=0 ) return SQLITE_BUSY;
  }

  /* Get the exclusive locks at the system level.  Then if successful
  ** also mark the local connection as being locked.
  */
  rc = winShmSystemLock(pFile, _SHM_WRLCK, writeMask);
  if( rc==SQLITE_OK ){
    p->sharedMask &= ~writeMask;
    p->exclMask |= writeMask;
  }
  return rc;
}

/*
** Purge the winShmNodeList list of all entries with winShmNode.nRef==0.
**
** This is not a VFS shared-memory method; it is a utility function called

      rc = SQLITE_CANTOPEN_BKPT;
      goto shm_open_err;
    }

    /* Check to see if another process is holding the dead-man switch.
    ** If not, truncate the file to zero length. 
    */
    if( winShmSystemLock(pShmNode, _SHM_WRLCK, WIN_SHM_DMS)==SQLITE_OK ){
      rc = winTruncate((sqlite3_file *)&pShmNode->hFile, 0);
    }
    if( rc==SQLITE_OK ){

      rc = winShmSystemLock(pShmNode, _SHM_RDLCK, WIN_SHM_DMS);
    }
    if( rc ) goto shm_open_err;
  }

  /* Make the new connection a child of the winShmNode */
  p->pShmNode = pShmNode;
  p->pNext = pShmNode->pFirst;
#ifdef SQLITE_DEBUG
  p->id = pShmNode->nextShmId++;
#endif
  pShmNode->pFirst = p;
  pShmNode->nRef++;
  pDbFd->pShm = p;
  winShmLeaveMutex();
  return SQLITE_OK;

  /* Jump here on any error */
shm_open_err:
  winShmSystemLock(pShmNode, _SHM_UNLCK, WIN_SHM_DMS);
  winShmPurge();                 /* This call frees pShmNode if required */
  sqlite3_free(p);
  sqlite3_free(pNew);
  winShmLeaveMutex();
  return rc;
}

  winShmNode *pShmNode; /* The underlying shared-memory file */
  winShm **pp;          /* For looping over sibling connections */

  pDbFd = (winFile*)fd;
  p = pDbFd->pShm;
  pShmNode = p->pShmNode;

  /* Verify that the connection being closed holds no locks */
  assert( p->exclMask==0 );
  assert( p->sharedMask==0 );

  /* Remove connection p from the set of connections associated
  ** with pShmNode */
  sqlite3_mutex_enter(pShmNode->mutex);
  for(pp=&pShmNode->pFirst; (*pp)!=p; pp = &(*pp)->pNext){}
  *pp = p->pNext;

  /* Free the connection p */

  winShmNode *pShmNode = p->pShmNode;
  int rc = SQLITE_OK;

  *pNewSize = 0;
  if( reqSize>=0 ){
    sqlite3_int64 sz;
    rc = winFileSize((sqlite3_file *)&pShmNode->hFile, &sz);
    if( SQLITE_OK==rc ){
      reqSize = (reqSize + SQLITE_WIN_SHM_INCR - 1)/SQLITE_WIN_SHM_INCR;
      reqSize *= SQLITE_WIN_SHM_INCR;
      if( reqSize>sz ){
        rc = winTruncate((sqlite3_file *)&pShmNode->hFile, reqSize);
      }
    }
  }
  if( SQLITE_OK==rc ){
    sqlite3_int64 sz;
    rc = winFileSize((sqlite3_file *)&pShmNode->hFile, &sz);
    if( SQLITE_OK==rc ){
      *pNewSize = (int)sz;

** *ppBuf and *pNewMapSize might be NULL and zero if no space has
** yet been allocated to the underlying storage.
*/
static int winShmGet(
  sqlite3_file *fd,        /* The database file holding the shared memory */
  int reqMapSize,          /* Requested size of mapping. -1 means don't care */
  int *pNewMapSize,        /* Write new size of mapping here */
  void **ppBuf             /* Write mapping buffer origin here */
){
  winFile *pDbFd = (winFile*)fd;
  winShm *p = pDbFd->pShm;
  winShmNode *pShmNode = p->pShmNode;
  int rc = SQLITE_OK;

  if( p->lockState!=SQLITE_SHM_CHECKPOINT && p->hasMutexBuf==0 ){
    assert( sqlite3_mutex_notheld(pShmNode->mutex) );
    sqlite3_mutex_enter(pShmNode->mutexBuf);
    p->hasMutexBuf = 1;
  }
  sqlite3_mutex_enter(pShmNode->mutex);
  if( pShmNode->szMap==0 || reqMapSize>pShmNode->szMap ){
    int actualSize;

** If the shared-memory object is in lock state RECOVER, then we do not
** really want to release the lock, so in that case too, this routine
** is a no-op.
*/
static int winShmRelease(sqlite3_file *fd){
  winFile *pDbFd = (winFile*)fd;
  winShm *p = pDbFd->pShm;
  if( p->hasMutexBuf && p->lockState!=SQLITE_SHM_RECOVER ){
    winShmNode *pShmNode = p->pShmNode;
    assert( sqlite3_mutex_notheld(pShmNode->mutex) );
    sqlite3_mutex_leave(pShmNode->mutexBuf);
    p->hasMutexBuf = 0;
  }
  return SQLITE_OK;
}

/*
** Symbolic names for LOCK states used for debugging.
*/
#ifdef SQLITE_DEBUG
static const char *azLkName[] = {
  "UNLOCK",
  "READ",
  "READ_FULL",
  "WRITE",
  "PENDING",
  "CHECKPOINT",
  "RECOVER"
};
#endif


/*
** Change the lock state for a shared-memory segment.
*/
static int winShmLock(
  sqlite3_file *fd,          /* Database holding the shared memory */
  int desiredLock,           /* One of SQLITE_SHM_xxxxx locking states */

  int *pGotLock              /* The lock you actually got */
){
  winFile *pDbFd = (winFile*)fd;
  winShm *p = pDbFd->pShm;
  winShmNode *pShmNode = p->pShmNode;
  int rc = SQLITE_PROTOCOL;

  /* Note that SQLITE_SHM_READ_FULL and SQLITE_SHM_PENDING are never
  ** directly requested; they are side effects from requesting
  ** SQLITE_SHM_READ and SQLITE_SHM_CHECKPOINT, respectively.
  */
  assert( desiredLock==SQLITE_SHM_UNLOCK
       || desiredLock==SQLITE_SHM_READ
       || desiredLock==SQLITE_SHM_WRITE
       || desiredLock==SQLITE_SHM_CHECKPOINT
       || desiredLock==SQLITE_SHM_RECOVER );

  /* Return directly if this is just a lock state query, or if
  ** the connection is already in the desired locking state.
  */
  if( desiredLock==p->lockState
   || (desiredLock==SQLITE_SHM_READ && p->lockState==SQLITE_SHM_READ_FULL)
  ){
    OSTRACE(("SHM-LOCK %d shmid-%d, pid-%d request %s and got %s\n",
             pShmNode->hFile.h,
             p->id, (int)GetCurrentProcessId(), azLkName[desiredLock],
             azLkName[p->lockState]));
    if( pGotLock ) *pGotLock = p->lockState;
    return SQLITE_OK;
  }

  OSTRACE(("SHM-LOCK %d shmid-%d, pid-%d request %s->%s\n",
           pShmNode->hFile.h,
           p->id, (int)GetCurrentProcessId(), azLkName[p->lockState], 
           azLkName[desiredLock]));
  
  if( desiredLock==SQLITE_SHM_RECOVER && !p->hasMutexBuf ){
    assert( sqlite3_mutex_notheld(pShmNode->mutex) );
    sqlite3_mutex_enter(pShmNode->mutexBuf);
    p->hasMutexBuf = 1;
  }
  sqlite3_mutex_enter(pShmNode->mutex);
  switch( desiredLock ){
    case SQLITE_SHM_UNLOCK: {
      assert( p->lockState!=SQLITE_SHM_RECOVER );
      winShmUnlock(pShmNode, p, WIN_SHM_A|WIN_SHM_B|WIN_SHM_C|WIN_SHM_D);
      rc = SQLITE_OK;
      p->lockState = SQLITE_SHM_UNLOCK;
      break;
    }
    case SQLITE_SHM_READ: {
      if( p->lockState==SQLITE_SHM_UNLOCK ){
        int nAttempt;
        rc = SQLITE_BUSY;
        assert( p->lockState==SQLITE_SHM_UNLOCK );
        for(nAttempt=0; nAttempt<5 && rc==SQLITE_BUSY; nAttempt++){
          rc = winShmSharedLock(pShmNode, p, WIN_SHM_A|WIN_SHM_B);
          if( rc==SQLITE_BUSY ){
            rc = winShmSharedLock(pShmNode, p, WIN_SHM_D);
            if( rc==SQLITE_OK ){
              p->lockState = SQLITE_SHM_READ_FULL;
            }
          }else{
            winShmUnlock(pShmNode, p, WIN_SHM_B);
            p->lockState = SQLITE_SHM_READ;
          }
        }
      }else{
       assert( p->lockState==SQLITE_SHM_WRITE
               || p->lockState==SQLITE_SHM_RECOVER );
        rc = winShmSharedLock(pShmNode, p, WIN_SHM_A);
        winShmUnlock(pShmNode, p, WIN_SHM_C|WIN_SHM_D);
        p->lockState = SQLITE_SHM_READ;
      }
      break;
    }
    case SQLITE_SHM_WRITE: {
      assert( p->lockState==SQLITE_SHM_READ 
              || p->lockState==SQLITE_SHM_READ_FULL );
      rc = winShmExclusiveLock(pShmNode, p, WIN_SHM_C|WIN_SHM_D);
      if( rc==SQLITE_OK ){
        p->lockState = SQLITE_SHM_WRITE;
      }
      break;
    }
    case SQLITE_SHM_CHECKPOINT: {
      assert( p->lockState==SQLITE_SHM_UNLOCK
           || p->lockState==SQLITE_SHM_PENDING
      );
      if( p->lockState==SQLITE_SHM_UNLOCK ){
        rc = winShmExclusiveLock(pShmNode, p, WIN_SHM_B|WIN_SHM_C);
        if( rc==SQLITE_OK ){
          p->lockState = SQLITE_SHM_PENDING;
        }
      }
      if( p->lockState==SQLITE_SHM_PENDING ){
        rc = winShmExclusiveLock(pShmNode, p, WIN_SHM_A);
        if( rc==SQLITE_OK ){
          p->lockState = SQLITE_SHM_CHECKPOINT;
        }
      }
      break;
    }
    default: {
      assert( desiredLock==SQLITE_SHM_RECOVER );
      assert( p->lockState==SQLITE_SHM_READ
           || p->lockState==SQLITE_SHM_READ_FULL
      );
      assert( sqlite3_mutex_held(pShmNode->mutexBuf) );
      rc = winShmExclusiveLock(pShmNode, p, WIN_SHM_C);
      if( rc==SQLITE_OK ){
        p->lockState = SQLITE_SHM_RECOVER;
      }
      break;
    }
  }
  sqlite3_mutex_leave(pShmNode->mutex);
  OSTRACE(("SHM-LOCK %d shmid-%d, pid-%d got %s\n",
           pShmNode->hFile.h, 
           p->id, (int)GetCurrentProcessId(), azLkName[p->lockState]));
  if( pGotLock ) *pGotLock = p->lockState;
  return rc;
}

/*
** Implement a memory barrier or memory fence on shared memory.  
**
** All loads and stores begun before the barrier must complete before

        pWal->nCkpt++;
        pWal->hdr.mxFrame = 0;
        sqlite3Put4byte((u8*)pWal->hdr.aSalt,
                         1 + sqlite3Get4byte((u8*)pWal->hdr.aSalt));
        sqlite3_randomness(4, &pWal->hdr.aSalt[1]);
        walIndexWriteHdr(pWal);
        pInfo->nBackfill = 0;

        memset(&pInfo->aReadMark[1], 0, sizeof(pInfo->aReadMark)-sizeof(u32));
        rc = sqlite3OsTruncate(pWal->pDbFd, 
                               ((i64)pWal->hdr.nPage*(i64)pWal->szPage));
        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }
    }
    walUnlockShared(pWal, WAL_READ_LOCK(0));
    pWal->readLock = -1;