SQLite

/*
** 2005 December 14
**
** 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.
**
*************************************************************************
**
** $Id: test_async.c,v 1.46 2008/09/15 14:08:04 danielk1977 Exp $
**
** This file contains an example implementation of an asynchronous IO 
** backend for SQLite.
**
** WHAT IS ASYNCHRONOUS I/O?
**
** With asynchronous I/O, write requests are handled by a separate thread

** Defining ENABLE_FILE_LOCKING when using an NFS or other remote 
** file-system may slow things down, as synchronous round-trips to the 
** server may be required to establish database file locks.
*/
#define ENABLE_FILE_LOCKING

#ifndef SQLITE_AMALGAMATION
# include "sqlite3.h"
# include <assert.h>
# include <string.h>
#endif
#include <tcl.h>

/*
** This test uses pthreads and hence only works on unix and with
** a threadsafe build of SQLite.
*/

  pthread_mutex_t lockMutex;   /* For access to aLock hash table */
  pthread_mutex_t queueMutex;  /* Mutex for access to write operation queue */
  pthread_mutex_t writerMutex; /* Prevents multiple writer threads */
  pthread_cond_t queueSignal;  /* For waking up sleeping writer thread */
  pthread_cond_t emptySignal;  /* Notify when the write queue is empty */
  AsyncWrite *pQueueFirst;     /* Next write operation to be processed */
  AsyncWrite *pQueueLast;      /* Last write operation on the list */
  AsyncLock *pLock;            /* Linked list of all AsyncLock structures */
  volatile int ioDelay;             /* Extra delay between write operations */
  volatile int writerHaltWhenIdle;  /* Writer thread halts when queue empty */
  volatile int writerHaltNow;       /* Writer thread halts after next op */
  int ioError;                 /* True if an IO error has occured */
  int nFile;                   /* Number of open files (from sqlite pov) */
} async = {
  PTHREAD_MUTEX_INITIALIZER,

** This space is sqlite3_malloc()d along with the AsyncWrite structure in a
** single blob, so is deleted when sqlite3_free() is called on the parent 
** structure.
*/
struct AsyncWrite {
  AsyncFileData *pFileData;    /* File to write data to or sync */
  int op;                      /* One of ASYNC_xxx etc. */
  sqlite_int64 iOffset;        /* See above */
  int nByte;          /* See above */
  char *zBuf;         /* Data to write to file (or NULL if op!=ASYNC_WRITE) */
  AsyncWrite *pNext;  /* Next write operation (to any file) */
};

/*
** An instance of this structure is created for each distinct open file 

** opened on the file in question and used to obtain the file-system 
** locks required by database connections within this process.
**
** See comments above the asyncLock() function for more details on 
** the implementation of database locking used by this backend.
*/
struct AsyncLock {
  char *zFile;
  int nFile;
  sqlite3_file *pFile;
  int eLock;
  AsyncFileLock *pList;
  AsyncLock *pNext;           /* Next in linked list headed by async.pLock */
};

/*
** An instance of the following structure is allocated along with each
** AsyncFileData structure (see AsyncFileData.lock), but is only used if the
** file was opened with the SQLITE_OPEN_MAIN_DB.
*/

  AsyncFileData *pData;
};
struct AsyncFileData {
  char *zName;               /* Underlying OS filename - used for debugging */
  int nName;                 /* Number of characters in zName */
  sqlite3_file *pBaseRead;   /* Read handle to the underlying Os file */
  sqlite3_file *pBaseWrite;  /* Write handle to the underlying Os file */
  AsyncFileLock lock;        /* Lock state for this handle */
  AsyncLock *pLock;          /* AsyncLock object for this file system entry */
  AsyncWrite close;
};

/*
** The following async_XXX functions are debugging wrappers around the
** corresponding pthread_XXX functions:
**

/*
** This is a utility function to allocate and populate a new AsyncWrite
** structure and insert it (via addAsyncWrite() ) into the global list.
*/
static int addNewAsyncWrite(
  AsyncFileData *pFileData, 
  int op, 
  sqlite3_int64 iOffset, 
  int nByte,
  const char *zByte
){
  AsyncWrite *p;
  if( op!=ASYNC_CLOSE && async.ioError ){
    return async.ioError;
  }

/*
** Implementation of sqlite3OsWrite() for asynchronous files. Instead of 
** writing to the underlying file, this function adds an entry to the end of
** the global AsyncWrite list. Either SQLITE_OK or SQLITE_NOMEM may be
** returned.
*/
static int asyncWrite(
  sqlite3_file *pFile, 
  const void *pBuf, 
  int amt, 
  sqlite3_int64 iOff
){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  return addNewAsyncWrite(p, ASYNC_WRITE, iOff, amt, pBuf);
}

/*
** Read data from the file. First we read from the filesystem, then adjust 
** the contents of the buffer based on ASYNC_WRITE operations in the 
** write-op queue.
**
** This method holds the mutex from start to finish.
*/
static int asyncRead(
  sqlite3_file *pFile, 
  void *zOut, 
  int iAmt, 
  sqlite3_int64 iOffset
){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  int rc = SQLITE_OK;
  sqlite3_int64 filesize;
  int nRead;
  sqlite3_file *pBase = p->pBaseRead;

  /* Grab the write queue mutex for the duration of the call */
  pthread_mutex_lock(&async.queueMutex);

  /* If an I/O error has previously occurred in this virtual file 
  ** system, then all subsequent operations fail.
  */
  if( async.ioError!=SQLITE_OK ){
    rc = async.ioError;
    goto asyncread_out;
  }

  if( pBase->pMethods ){
    rc = pBase->pMethods->xFileSize(pBase, &filesize);
    if( rc!=SQLITE_OK ){
      goto asyncread_out;
    }
    nRead = MIN(filesize - iOffset, iAmt);
    if( nRead>0 ){
      rc = pBase->pMethods->xRead(pBase, zOut, nRead, iOffset);
      ASYNC_TRACE(("READ %s %d bytes at %d\n", p->zName, nRead, iOffset));
    }
  }

  if( rc==SQLITE_OK ){
    AsyncWrite *pWrite;
    char *zName = p->zName;

    for(pWrite=async.pQueueFirst; pWrite; pWrite = pWrite->pNext){
      if( pWrite->op==ASYNC_WRITE && (
        (pWrite->pFileData==p) ||
        (zName && pWrite->pFileData->zName==zName)
      )){
        int iBeginOut = (pWrite->iOffset-iOffset);
        int iBeginIn = -iBeginOut;
        int nCopy;

        if( iBeginIn<0 ) iBeginIn = 0;
        if( iBeginOut<0 ) iBeginOut = 0;
        nCopy = MIN(pWrite->nByte-iBeginIn, iAmt-iBeginOut);

  return rc;
}

/*
** Truncate the file to nByte bytes in length. This just adds an entry to 
** the write-op list, no IO actually takes place.
*/
static int asyncTruncate(sqlite3_file *pFile, sqlite3_int64 nByte){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  return addNewAsyncWrite(p, ASYNC_TRUNCATE, nByte, 0, 0);
}

/*
** Sync the file. This just adds an entry to the write-op list, the 
** sync() is done later by sqlite3_async_flush().
*/
static int asyncSync(sqlite3_file *pFile, int flags){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  return addNewAsyncWrite(p, ASYNC_SYNC, 0, flags, 0);
}

/*
** Read the size of the file. First we read the size of the file system 
** entry, then adjust for any ASYNC_WRITE or ASYNC_TRUNCATE operations 
** currently in the write-op list. 
**
** This method holds the mutex from start to finish.
*/
int asyncFileSize(sqlite3_file *pFile, sqlite3_int64 *piSize){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  int rc = SQLITE_OK;
  sqlite3_int64 s = 0;
  sqlite3_file *pBase;

  pthread_mutex_lock(&async.queueMutex);

  /* Read the filesystem size from the base file. If pBaseRead is NULL, this
  ** means the file hasn't been opened yet. In this case all relevant data 
  ** must be in the write-op queue anyway, so we can omit reading from the
  ** file-system.
  */
  pBase = p->pBaseRead;
  if( pBase->pMethods ){
    rc = pBase->pMethods->xFileSize(pBase, &s);
  }

  if( rc==SQLITE_OK ){
    AsyncWrite *pWrite;
    for(pWrite=async.pQueueFirst; pWrite; pWrite = pWrite->pNext){
      if( pWrite->op==ASYNC_DELETE 
       && p->zName 
       && strcmp(p->zName, pWrite->zBuf)==0 
      ){
        s = 0;
      }else if( pWrite->pFileData && (
          (pWrite->pFileData==p) 
       || (p->zName && pWrite->pFileData->zName==p->zName) 
      )){
        switch( pWrite->op ){
          case ASYNC_WRITE:
            s = MAX(pWrite->iOffset + (sqlite3_int64)(pWrite->nByte), s);
            break;
          case ASYNC_TRUNCATE:
            s = MIN(s, pWrite->iOffset);
            break;
        }
      }
    }

      if( pIter->eAsyncLock>eRequired ){
        eRequired = pIter->eAsyncLock;
        assert(eRequired>=0 && eRequired<=SQLITE_LOCK_EXCLUSIVE);
      }
    }

    if( eRequired>pLock->eLock ){
      rc = pLock->pFile->pMethods->xLock(pLock->pFile, eRequired);
      if( rc==SQLITE_OK ){
        pLock->eLock = eRequired;
      }
    }
    else if( eRequired<pLock->eLock && eRequired<=SQLITE_LOCK_SHARED ){
      rc = pLock->pFile->pMethods->xUnlock(pLock->pFile, eRequired);
      if( rc==SQLITE_OK ){
        pLock->eLock = eRequired;
      }
    }
  }

  return rc;
}

/*
** Return the AsyncLock structure from the global async.pLock list 
** associated with the file-system entry identified by path zName 
** (a string of nName bytes). If no such structure exists, return 0.
*/
static AsyncLock *findLock(const char *zName, int nName){
  AsyncLock *p = async.pLock;
  while( p && (p->nFile!=nName || memcmp(p->zFile, zName, nName)) ){
    p = p->pNext;
  }
  return p;
}

/*
** The following two methods - asyncLock() and asyncUnlock() - are used
** to obtain and release locks on database files opened with the
** asynchronous backend.
*/
static int asyncLock(sqlite3_file *pFile, int eLock){
  int rc = SQLITE_OK;
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;

  if( p->zName ){
    pthread_mutex_lock(&async.lockMutex);
    if( p->lock.eLock<eLock ){
      AsyncLock *pLock = p->pLock;
      AsyncFileLock *pIter;

      assert(pLock && pLock->pList);
      for(pIter=pLock->pList; pIter; pIter=pIter->pNext){
        if( pIter!=&p->lock && (
          (eLock==SQLITE_LOCK_EXCLUSIVE && pIter->eLock>=SQLITE_LOCK_SHARED) ||
          (eLock==SQLITE_LOCK_PENDING && pIter->eLock>=SQLITE_LOCK_RESERVED) ||
          (eLock==SQLITE_LOCK_RESERVED && pIter->eLock>=SQLITE_LOCK_RESERVED) ||
          (eLock==SQLITE_LOCK_SHARED && pIter->eLock>=SQLITE_LOCK_PENDING)
        )){
          rc = SQLITE_BUSY;
        }
      }
      if( rc==SQLITE_OK ){
        p->lock.eLock = eLock;
        p->lock.eAsyncLock = MAX(p->lock.eAsyncLock, eLock);
      }
      assert(p->lock.eAsyncLock>=p->lock.eLock);
      if( rc==SQLITE_OK ){
        rc = getFileLock(pLock);
      }
    }
    pthread_mutex_unlock(&async.lockMutex);
  }

  ASYNC_TRACE(("LOCK %d (%s) rc=%d\n", eLock, p->zName, rc));
  return rc;
}
static int asyncUnlock(sqlite3_file *pFile, int eLock){
  int rc = SQLITE_OK;
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  if( p->zName ){
    AsyncFileLock *pLock = &p->lock;
    pthread_mutex_lock(&async.lockMutex);
    pLock->eLock = MIN(pLock->eLock, eLock);
    pthread_mutex_unlock(&async.lockMutex);
    rc = addNewAsyncWrite(p, ASYNC_UNLOCK, 0, eLock, 0);
  }
  return rc;
}

/*
** This function is called when the pager layer first opens a database file
** and is checking for a hot-journal.
*/
static int asyncCheckReservedLock(sqlite3_file *pFile, int *pResOut){
  int ret = 0;
  AsyncFileLock *pIter;

  AsyncFileData *p = ((AsyncFile *)pFile)->pData;

  pthread_mutex_lock(&async.lockMutex);

  for(pIter=p->pLock->pList; pIter; pIter=pIter->pNext){
    if( pIter->eLock>=SQLITE_LOCK_RESERVED ){
      ret = 1;
    }
  }
  pthread_mutex_unlock(&async.lockMutex);

  ASYNC_TRACE(("CHECK-LOCK %d (%s)\n", ret, p->zName));
  *pResOut = ret;
  return SQLITE_OK;
}

/* 
** sqlite3_file_control() implementation.
*/
static int asyncFileControl(sqlite3_file *id, int op, void *pArg){
  switch( op ){
    case SQLITE_FCNTL_LOCKSTATE: {
      pthread_mutex_lock(&async.lockMutex);
      *(int*)pArg = ((AsyncFile*)id)->pData->lock.eLock;
      pthread_mutex_unlock(&async.lockMutex);

  return 512;
}
static int asyncDeviceCharacteristics(sqlite3_file *pFile){
  return 0;
}

static int unlinkAsyncFile(AsyncFileData *pData){

  AsyncFileLock **ppIter;
  int rc = SQLITE_OK;

  if( pData->zName ){
    AsyncLock *pLock = pData->pLock;
    for(ppIter=&pLock->pList; *ppIter; ppIter=&((*ppIter)->pNext)){
      if( (*ppIter)==&pData->lock ){
        *ppIter = pData->lock.pNext;
        break;
      }
    }
    if( !pLock->pList ){
      AsyncLock **pp;
      if( pLock->pFile ){
        pLock->pFile->pMethods->xClose(pLock->pFile);
      }
      for(pp=&async.pLock; *pp!=pLock; pp=&((*pp)->pNext));
      *pp = pLock->pNext;
      sqlite3_free(pLock);




    }else{
      rc = getFileLock(pLock);
    }
  }

  return rc;
}

/*
** Open a file.

    asyncFileControl,                /* xFileControl */
    asyncSectorSize,                 /* xSectorSize */
    asyncDeviceCharacteristics       /* xDeviceCharacteristics */
  };

  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  AsyncFile *p = (AsyncFile *)pFile;
  int nName = 0;
  int rc = SQLITE_OK;
  int nByte;
  AsyncFileData *pData;
  AsyncLock *pLock = 0;
  char *z;
  int isExclusive = (flags&SQLITE_OPEN_EXCLUSIVE);

  /* If zName is NULL, then the upper layer is requesting an anonymous file */
  if( zName ){
    nName = strlen(zName)+1;
  }

  nByte = (
    sizeof(AsyncFileData) +        /* AsyncFileData structure */
    2 * pVfs->szOsFile +           /* AsyncFileData.pBaseRead and pBaseWrite */
    nName                          /* AsyncFileData.zName */
  ); 
  z = sqlite3_malloc(nByte);
  if( !z ){
    return SQLITE_NOMEM;
  }
  memset(z, 0, nByte);
  pData = (AsyncFileData*)z;
  z += sizeof(pData[0]);
  pData->pBaseRead = (sqlite3_file*)z;
  z += pVfs->szOsFile;
  pData->pBaseWrite = (sqlite3_file*)z;
  pData->close.pFileData = pData;
  pData->close.op = ASYNC_CLOSE;

  if( zName ){
    z += pVfs->szOsFile;
    pData->zName = z;
    pData->nName = nName;


    memcpy(pData->zName, zName, nName);
  }

  if( !isExclusive ){
    rc = pVfs->xOpen(pVfs, zName, pData->pBaseRead, flags, pOutFlags);
    if( rc==SQLITE_OK && ((*pOutFlags)&SQLITE_OPEN_READWRITE) ){
      rc = pVfs->xOpen(pVfs, zName, pData->pBaseWrite, flags, 0);
    }
  }

  pthread_mutex_lock(&async.lockMutex);

  if( zName && rc==SQLITE_OK ){
    pLock = findLock(pData->zName, pData->nName);
    if( !pLock ){
      int nByte = pVfs->szOsFile + sizeof(AsyncLock) + pData->nName + 1; 
      pLock = (AsyncLock *)sqlite3_malloc(nByte);
      if( pLock ){
        memset(pLock, 0, nByte);
#ifdef ENABLE_FILE_LOCKING
        if( flags&SQLITE_OPEN_MAIN_DB ){
          pLock->pFile = (sqlite3_file *)&pLock[1];
          rc = pVfs->xOpen(pVfs, zName, pLock->pFile, flags, 0);
          if( rc!=SQLITE_OK ){
            sqlite3_free(pLock);
            pLock = 0;
          }
        }
#endif
        if( pLock ){
          pLock->nFile = pData->nName;
          pLock->zFile = &((char *)(&pLock[1]))[pVfs->szOsFile];
          memcpy(pLock->zFile, pData->zName, pLock->nFile);


          pLock->pNext = async.pLock;
          async.pLock = pLock;
        }
      }else{
        rc = SQLITE_NOMEM;
      }
    }
  }

  if( rc==SQLITE_OK ){

    p->pMethod = &async_methods;
    p->pData = pData;

    /* Link AsyncFileData.lock into the linked list of 
    ** AsyncFileLock structures for this file.
    */
    if( zName ){
      pData->lock.pNext = pLock->pList;
      pLock->pList = &pData->lock;
      pData->zName = pLock->zFile;
    }


  }else{
    if( pData->pBaseRead->pMethods ){
      pData->pBaseRead->pMethods->xClose(pData->pBaseRead);
    }
    if( pData->pBaseWrite->pMethods ){
      pData->pBaseWrite->pMethods->xClose(pData->pBaseWrite);
    }
    sqlite3_free(pData);
  }

  pthread_mutex_unlock(&async.lockMutex);

  if( rc==SQLITE_OK ){
    incrOpenFileCount();
    pData->pLock = pLock;
  }

  if( rc==SQLITE_OK && isExclusive ){
    rc = addNewAsyncWrite(pData, ASYNC_OPENEXCLUSIVE, (sqlite3_int64)flags,0,0);
    if( rc==SQLITE_OK ){
      if( pOutFlags ) *pOutFlags = flags;
    }else{
      pthread_mutex_lock(&async.lockMutex);
      unlinkAsyncFile(pData);
      pthread_mutex_unlock(&async.lockMutex);
      sqlite3_free(pData);

  assert(flags==SQLITE_ACCESS_READWRITE 
      || flags==SQLITE_ACCESS_READ 
      || flags==SQLITE_ACCESS_EXISTS 
  );

  pthread_mutex_lock(&async.queueMutex);
  rc = pVfs->xAccess(pVfs, zName, flags, &ret);
  if( rc==SQLITE_OK && flags==SQLITE_ACCESS_EXISTS ){
    for(p=async.pQueueFirst; p; p = p->pNext){
      if( p->op==ASYNC_DELETE && 0==strcmp(p->zBuf, zName) ){
        ret = 0;
      }else if( p->op==ASYNC_OPENEXCLUSIVE 
             && p->pFileData->zName
             && 0==strcmp(p->pFileData->zName, zName) 
      ){
        ret = 1;
      }
    }
  }
  ASYNC_TRACE(("ACCESS(%s): %s = %d\n", 

  sqlite3_vfs *pAsyncVfs, 
  const char *zPath, 
  int nPathOut,
  char *zPathOut
){
  int rc;
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  rc = pVfs->xFullPathname(pVfs, zPath, nPathOut, zPathOut);

  /* Because of the way intra-process file locking works, this backend
  ** needs to return a canonical path. The following block assumes the
  ** file-system uses unix style paths. 
  */
  if( rc==SQLITE_OK ){
    int iIn;

**
** This routine is not even remotely threadsafe.  Do not call
** this routine while any SQLite database connections are open.
*/
static void asyncEnable(int enable){
  if( enable ){
    if( !async_vfs.pAppData ){

      async_vfs.pAppData = (void *)sqlite3_vfs_find(0);
      async_vfs.mxPathname = ((sqlite3_vfs *)async_vfs.pAppData)->mxPathname;
      sqlite3_vfs_register(&async_vfs, 1);




    }
  }else{
    if( async_vfs.pAppData ){
      sqlite3_vfs_unregister(&async_vfs);
      async_vfs.pAppData = 0;
    }
  }

      case ASYNC_NOOP:
        break;

      case ASYNC_WRITE:
        assert( pBase );
        ASYNC_TRACE(("WRITE %s %d bytes at %d\n",
                p->pFileData->zName, p->nByte, p->iOffset));
        rc = pBase->pMethods->xWrite(pBase, (void *)(p->zBuf), p->nByte, p->iOffset);
        break;

      case ASYNC_SYNC:
        assert( pBase );
        ASYNC_TRACE(("SYNC %s\n", p->pFileData->zName));
        rc = pBase->pMethods->xSync(pBase, p->nByte);
        break;

      case ASYNC_TRUNCATE:
        assert( pBase );
        ASYNC_TRACE(("TRUNCATE %s to %d bytes\n", 
                p->pFileData->zName, p->iOffset));
        rc = pBase->pMethods->xTruncate(pBase, p->iOffset);
        break;

      case ASYNC_CLOSE: {
        AsyncFileData *pData = p->pFileData;
        ASYNC_TRACE(("CLOSE %s\n", p->pFileData->zName));
        if( pData->pBaseWrite->pMethods ){
          pData->pBaseWrite->pMethods->xClose(pData->pBaseWrite);
        }
        if( pData->pBaseRead->pMethods ){
          pData->pBaseRead->pMethods->xClose(pData->pBaseRead);
        }

        /* Unlink AsyncFileData.lock from the linked list of AsyncFileLock 
        ** structures for this file. Obtain the async.lockMutex mutex 
        ** before doing so.
        */
        pthread_mutex_lock(&async.lockMutex);
        rc = unlinkAsyncFile(pData);
        pthread_mutex_unlock(&async.lockMutex);

        async.pQueueFirst = p->pNext;
        sqlite3_free(pData);
        doNotFree = 1;
        break;
      }

      case ASYNC_UNLOCK: {

        AsyncFileData *pData = p->pFileData;
        int eLock = p->nByte;
        pthread_mutex_lock(&async.lockMutex);
        pData->lock.eAsyncLock = MIN(
            pData->lock.eAsyncLock, MAX(pData->lock.eLock, eLock)
        );
        assert(pData->lock.eAsyncLock>=pData->lock.eLock);

        rc = getFileLock(pData->pLock);
        pthread_mutex_unlock(&async.lockMutex);
        break;
      }

      case ASYNC_DELETE:
        ASYNC_TRACE(("DELETE %s\n", p->zBuf));
        rc = pVfs->xDelete(pVfs, p->zBuf, (int)p->iOffset);
        break;

      case ASYNC_OPENEXCLUSIVE: {
        int flags = (int)p->iOffset;
        AsyncFileData *pData = p->pFileData;
        ASYNC_TRACE(("OPEN %s flags=%d\n", p->zBuf, (int)p->iOffset));
        assert(pData->pBaseRead->pMethods==0 && pData->pBaseWrite->pMethods==0);
        rc = pVfs->xOpen(pVfs, pData->zName, pData->pBaseRead, flags, 0);
        assert( holdingMutex==0 );
        pthread_mutex_lock(&async.queueMutex);
        holdingMutex = 1;
        break;
      }

      default: assert(!"Illegal value for AsyncWrite.op");

    */
    if( rc!=SQLITE_OK ){
      async.ioError = rc;
    }

    if( async.ioError && !async.pQueueFirst ){
      pthread_mutex_lock(&async.lockMutex);
      if( 0==async.pLock ){
        async.ioError = SQLITE_OK;
      }
      pthread_mutex_unlock(&async.lockMutex);
    }

    /* Drop the queue mutex before continuing to the next write operation
    ** in order to give other threads a chance to work with the write queue.
    */
    if( !async.pQueueFirst || !async.ioError ){
      pthread_mutex_unlock(&async.queueMutex);
      holdingMutex = 0;
      if( async.ioDelay>0 ){
        pVfs->xSleep(pVfs, async.ioDelay);
      }else{
        sched_yield();
      }
    }
  }
  
  pthread_mutex_unlock(&async.writerMutex);