SQLite

** Application data for the crash VFS
*/
struct crashAppData {
  sqlite3_vfs *pOrig;                   /* Wrapped vfs structure */
};

/*
** Open a crash-file file handle.

**
** The caller will have allocated pVfs->szOsFile bytes of space
** at pFile. This file uses this space for the CrashFile structure
** and allocates space for the "real" file structure using 
** sqlite3_malloc(). The assumption here is (pVfs->szOsFile) is
** equal or greater than sizeof(CrashFile).
*/

static int cfFullPathname(sqlite3_vfs *pCfVfs, const char *zPath, char *zPathOut){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;
  return pVfs->xFullPathname(pVfs, zPath, zPathOut);
}
static void *cfDlOpen(sqlite3_vfs *pCfVfs, const char *zPath){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;
  return pVfs->xDlOpen(pVfs, zPath);
}
static void cfDlError(sqlite3_vfs *pCfVfs, int nByte, char *zErrMsg){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;
  pVfs->xDlError(pVfs, nByte, zErrMsg);
}
static void *cfDlSym(sqlite3_vfs *pCfVfs, void *pHandle, const char *zSymbol){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;
  return pVfs->xDlSym(pVfs, pHandle, zSymbol);
}
static void cfDlClose(sqlite3_vfs *pCfVfs, void *pHandle){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;
  pVfs->xDlClose(pVfs, pHandle);
}
static int cfRandomness(sqlite3_vfs *pCfVfs, int nByte, char *zBufOut){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;
  return pVfs->xRandomness(pVfs, nByte, zBufOut);
}
static int cfSleep(sqlite3_vfs *pCfVfs, int nMicro){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pCfVfs->pAppData;

  
    cfOpen,               /* xOpen */
    cfDelete,             /* xDelete */
    cfAccess,             /* xAccess */
    cfGetTempName,        /* xGetTempName */
    cfFullPathname,       /* xFullPathname */
    cfDlOpen,             /* xDlOpen */
    cfDlError,            /* xDlError */
    cfDlSym,              /* xDlSym */
    cfDlClose,            /* xDlClose */
    cfRandomness,         /* xRandomness */
    cfSleep,              /* xSleep */
    cfCurrentTime         /* xCurrentTime */
  };


  if( crashVfs.pAppData==0 ){
    sqlite3_vfs *pOriginalVfs = sqlite3_vfs_find(0);



    crashVfs.mxPathname = pOriginalVfs->mxPathname;
    crashVfs.pAppData = (void *)pOriginalVfs;
    crashVfs.szOsFile = sizeof(CrashFile) + pOriginalVfs->szOsFile;
    /* sqlite3_vfs_unregister(pOriginalVfs); */
    sqlite3_vfs_register(&crashVfs, 1);
  }

*/

#include "sqliteInt.h"
#include <tcl.h>

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



*/
#if OS_UNIX && SQLITE_THREADSAFE

/*
** This demo uses pthreads.  If you do not have a pthreads implementation
** for your operating system, you will need to recode the threading 
** logic.
*/
#include <pthread.h>
#include <sched.h>

/* Useful macros used in several places */
#define MIN(x,y) ((x)<(y)?(x):(y))
#define MAX(x,y) ((x)>(y)?(x):(y))

/* Forward references */
typedef struct AsyncWrite AsyncWrite;
typedef struct AsyncFile AsyncFile;
typedef struct AsyncFileData AsyncFileData;

/* Enable for debugging */
static int sqlite3async_trace = 0;
# define ASYNC_TRACE(X) if( sqlite3async_trace ) asyncTrace X
static void asyncTrace(const char *zFormat, ...){
  char *z;
  va_list ap;

** on the value of AsyncWrite.op:
**
** ASYNC_WRITE:
**     iOffset -> Offset in file to write to.
**     nByte   -> Number of bytes of data to write (pointed to by zBuf).
**
** ASYNC_SYNC:

**     nByte   -> flags to pass to sqlite3OsSync().
**
** ASYNC_TRUNCATE:
**     iOffset -> Size to truncate file to.
**     nByte   -> Unused.
**
** ASYNC_CLOSE:
**     iOffset -> Unused.
**     nByte   -> Unused.
**
** ASYNC_OPENDIRECTORY:
**     iOffset -> Unused.
**     nByte   -> Number of bytes of zBuf points to (directory name).
**
** ASYNC_SETFULLSYNC:
**     iOffset -> Unused.
**     nByte   -> New value for the full-sync flag.
**
**
** ASYNC_DELETE:
**     iOffset -> Contains the "syncDir" flag.
**     nByte   -> Number of bytes of zBuf points to (file name).
**
** ASYNC_OPENEXCLUSIVE:
**     iOffset -> Value of "delflag".
**     nByte   -> Number of bytes of zBuf points to (file name).
**
**
** For an ASYNC_WRITE operation, zBuf points to the data to write to the file. 
** 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. */
  i64 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) */
};

/* 
** The AsyncFile structure is a subclass of sqlite3_file used for 
** asynchronous IO. 
**
** All of the actual data for the structure is stored in the structure
** pointed to by AsyncFile.pData, which is allocated as part of the
** sqlite3OsOpen() using sqlite3_malloc(). The reason for this is that the
** lifetime of the AsyncFile structure is ended by the caller after OsClose()
** is called, but the data in AsyncFileData may be required by the
** writer thread after that point.
*/
struct AsyncFile {
  sqlite3_io_methods *pMethod;

  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 */
};

/*
** Add an entry to the end of the global write-op list. pWrite should point 
** to an AsyncWrite structure allocated using sqlite3_malloc().  The writer
** thread will call sqlite3_free() to free the structure after the specified
** operation has been completed.

    assert( async.pQueueFirst );
    async.pQueueLast->pNext = pWrite;
  }else{
    async.pQueueFirst = pWrite;
  }
  async.pQueueLast = pWrite;
  ASYNC_TRACE(("PUSH %p (%s %s %d)\n", pWrite, azOpcodeName[pWrite->op],
         pWrite->pFileData ? pWrite->pFileData->zName : "-", pWrite->iOffset));

  if( pWrite->op==ASYNC_CLOSE ){
    async.nFile--;
    if( async.nFile==0 ){
      async.ioError = SQLITE_OK;
    }
  }

}

/*
** 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, 
  i64 iOffset, 
  int nByte,
  const char *zByte
){
  AsyncWrite *p;
  if( op!=ASYNC_CLOSE && async.ioError ){
    return async.ioError;
  }
  p = sqlite3_malloc(sizeof(AsyncWrite) + (zByte?nByte:0));
  if( !p ){
    return SQLITE_NOMEM;
  }
  p->op = op;
  p->iOffset = iOffset;
  p->nByte = nByte;
  p->pFileData = pFileData;
  p->pNext = 0;
  if( zByte ){
    p->zBuf = (char *)&p[1];
    memcpy(p->zBuf, zByte, nByte);
  }else{
    p->zBuf = 0;
  }
  addAsyncWrite(p);
  return SQLITE_OK;
}

/*
** Close the file. This just adds an entry to the write-op list, the file is
** not actually closed.
*/
static int asyncClose(sqlite3_file *pFile){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  return addNewAsyncWrite(p, ASYNC_CLOSE, 0, 0, 0);
}

/*
** 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, i64 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, i64 iOffset){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  int rc = SQLITE_OK;
  i64 filesize;
  int nRead;

  sqlite3_file *pBase = p->pBaseRead;

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

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

  if( pBase->pMethods ){
    rc = sqlite3OsFileSize(pBase, &filesize);
    if( rc!=SQLITE_OK ){
      goto asyncread_out;
    }




    nRead = MIN(filesize - iOffset, iAmt);
    if( nRead>0 ){
      rc = sqlite3OsRead(pBase, zOut, nRead, iOffset);
      ASYNC_TRACE(("READ %s %d bytes at %d\n", p->zName, nRead, iOffset));
    }
  }

  if( rc==SQLITE_OK ){
    AsyncWrite *pWrite;


    for(pWrite=async.pQueueFirst; pWrite; pWrite = pWrite->pNext){
      if( pWrite->pFileData==p && pWrite->op==ASYNC_WRITE ){
        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);

        if( nCopy>0 ){
          memcpy(&((char *)zOut)[iBeginOut], &pWrite->zBuf[iBeginIn], nCopy);
          ASYNC_TRACE(("OVERREAD %d bytes at %d\n", nCopy, iBeginOut+iOffset));
        }
      }
    }


  }

asyncread_out:
  pthread_mutex_unlock(&async.queueMutex);
  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, i64 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, i64 *piSize){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  int rc = SQLITE_OK;
  i64 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 = sqlite3OsFileSize(pBase, &s);
  }

  if( rc==SQLITE_OK ){
    AsyncWrite *pWrite;
    for(pWrite=async.pQueueFirst; pWrite; pWrite = pWrite->pNext){
      if( pWrite->pFileData==p ){
        switch( pWrite->op ){
          case ASYNC_WRITE:
            s = MAX(pWrite->iOffset + (i64)(pWrite->nByte), s);
            break;
          case ASYNC_TRUNCATE:
            s = MIN(s, pWrite->iOffset);
            break;
        }
      }
    }
    *piSize = s;
  }
  pthread_mutex_unlock(&async.queueMutex);
  return rc;








}

/*
** No disk locking is performed.  We keep track of locks locally in
** the async.aLock hash table.  Locking should appear to work the same
** as with standard (unmodified) SQLite as long as all connections 
** come from this one process.  Connections from external processes
** cannot see our internal hash table (obviously) and will thus not
** honor our locks.
*/
static int asyncLock(sqlite3_file *pFile, int lockType){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  ASYNC_TRACE(("LOCK %d (%s)\n", lockType, p->zName));
  pthread_mutex_lock(&async.lockMutex);
  sqlite3HashInsert(&async.aLock, p->zName, p->nName, (void*)lockType);
  pthread_mutex_unlock(&async.lockMutex);
  return SQLITE_OK;
}
static int asyncUnlock(sqlite3_file *pFile, int lockType){
  return asyncLock(pFile, lockType);
}

/*
** 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){
  AsyncFileData *p = ((AsyncFile *)pFile)->pData;
  int rc;
  pthread_mutex_lock(&async.lockMutex);
  rc = (int)sqlite3HashFind(&async.aLock, p->zName, p->nName);
  pthread_mutex_unlock(&async.lockMutex);
  ASYNC_TRACE(("CHECK-LOCK %d (%s)\n", rc, p->zName));
  return rc>SHARED_LOCK;
}


/* 
** This is a no-op, as the asynchronous backend does not support locking.

*/
static int asyncBreakLock(sqlite3_file *id){
  return SQLITE_OK;
}

/* 
** This is broken. But sqlite3OsLockState() is only used for testing anyway.
*/
static int asyncLockState(sqlite3_file *id){
  return SQLITE_OK;
}

/* 
** Return the device characteristics and sector-size of the device. It

** is not tricky to implement these correctly, as this backend might 
** not have an open file handle at this point.
*/
static int asyncSectorSize(sqlite3_file *pFile){
  return 512;

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

}

/*

** Open a file.
*/
static int asyncOpen(
  sqlite3_vfs *pAsyncVfs,
  const char *zName,
  sqlite3_file *pFile,
  int flags,
  int *pOutFlags
){

  static sqlite3_io_methods async_methods = {

    1,                               /* iVersion */

    asyncClose,                      /* xClose */

    asyncRead,                       /* xRead */
    asyncWrite,                      /* xWrite */

    asyncTruncate,                   /* xTruncate */
    asyncSync,                       /* xSync */


    asyncFileSize,                   /* xFileSize */
    asyncLock,                       /* xLock */
    asyncUnlock,                     /* xUnlock */

    asyncCheckReservedLock,          /* xCheckReservedLock */
    asyncBreakLock,                  /* xBreakLock */
    asyncLockState,                  /* xLockState */
    asyncSectorSize,                 /* xSectorSize */

    asyncDeviceCharacteristics       /* xDeviceCharacteristics */





  };









  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  AsyncFile *p = (AsyncFile *)pFile;
  int nName = strlen(zName);;
  int rc;
  int nByte;

  AsyncFileData *pData;


  nByte = (
    sizeof(AsyncFileData) +        /* AsyncFileData structure */
    2 * pVfs->szOsFile +           /* AsyncFileData.zName */
    nName + 1                      /* AsyncFileData.pBaseRead and pBaseWrite */

  );
  pData = sqlite3_malloc(nByte);


  if( !pData ){
    return SQLITE_NOMEM;
  }
  memset(pData, 0, nByte);








  pData->zName = (char *)&pData[1];
  pData->nName = nName;
  pData->pBaseRead = (sqlite3_file *)&pData->zName[nName+1];
  pData->pBaseWrite = (sqlite3_file *)&pData->zName[nName+1+pVfs->szOsFile];
  memcpy(pData->zName, zName, nName+1);





  if( flags&SQLITE_OPEN_EXCLUSIVE ){
    rc = addNewAsyncWrite(pData, ASYNC_OPENEXCLUSIVE, (i64)flags, 0, 0);
    if( pOutFlags ) *pOutFlags = flags;
  }else{
    rc = sqlite3OsOpen(pVfs, zName, pData->pBaseRead, flags, pOutFlags);
    if( rc==SQLITE_OK && ((*pOutFlags)&SQLITE_OPEN_READWRITE) ){
      rc = sqlite3OsOpen(pVfs, zName, pData->pBaseWrite, flags, 0);
    }

  }






  if( rc==SQLITE_OK ){
    p->pMethod = &async_methods;
    p->pData = pData;
    incrOpenFileCount();
  }else{



    sqlite3OsClose(pData->pBaseRead);
    sqlite3OsClose(pData->pBaseWrite);


    sqlite3_free(pData);
  }



  return rc;
}

/*
** Implementation of sqlite3OsDelete. Add an entry to the end of the 
** write-op queue to perform the delete.
*/
static int asyncDelete(sqlite3_vfs *pAsyncVfs, const char *z, int syncDir){
  return addNewAsyncWrite(0, ASYNC_DELETE, syncDir, strlen(z)+1, z);
}

/*








** Implementation of sqlite3OsAccess. This method holds the mutex from


** start to finish.
*/
static int asyncAccess(sqlite3_vfs *pAsyncVfs, const char *zName, int flags){
  int ret;
  AsyncWrite *p;
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;

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

  pthread_mutex_lock(&async.queueMutex);
  ret = sqlite3OsAccess(pVfs, zName, flags);


  if( 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 
             && 0==strcmp(p->pFileData->zName, zName) 
      ){
        ret = 1;
      }
    }
  }
  ASYNC_TRACE(("ACCESS(%s): %s = %d\n", 
    flags==SQLITE_ACCESS_READWRITE?"read-write":
    flags==SQLITE_ACCESS_READONLY?"read-only":"exists"
    , zName, ret)
  );
  pthread_mutex_unlock(&async.queueMutex);
  return ret;
}

static int asyncGetTempName(sqlite3_vfs *pAsyncVfs, char *zBufOut){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return pVfs->xGetTempName(pVfs, zBufOut);
}
static int asyncFullPathname(
  sqlite3_vfs *pAsyncVfs, 
  const char *zPath, 
  char *zPathOut
){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return sqlite3OsFullPathname(pVfs, zPath, zPathOut);
}
static void *asyncDlOpen(sqlite3_vfs *pAsyncVfs, const char *zPath){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return pVfs->xDlOpen(pVfs, zPath);
}
static void asyncDlError(sqlite3_vfs *pAsyncVfs, int nByte, char *zErrMsg){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  pVfs->xDlError(pVfs, nByte, zErrMsg);
}
static void *asyncDlSym(
  sqlite3_vfs *pAsyncVfs, 
  void *pHandle, 
  const char *zSymbol
){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return pVfs->xDlSym(pVfs, pHandle, zSymbol);
}
static void asyncDlClose(sqlite3_vfs *pAsyncVfs, void *pHandle){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  pVfs->xDlClose(pVfs, pHandle);
}
static int asyncRandomness(sqlite3_vfs *pAsyncVfs, int nByte, char *zBufOut){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return pVfs->xRandomness(pVfs, nByte, zBufOut);
}
static int asyncSleep(sqlite3_vfs *pAsyncVfs, int nMicro){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return pVfs->xSleep(pVfs, nMicro);
}
static int asyncCurrentTime(sqlite3_vfs *pAsyncVfs, double *pTimeOut){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  return pVfs->xCurrentTime(pVfs, pTimeOut);
}

static sqlite3_vfs async_vfs = {
  1,                    /* iVersion */
  sizeof(AsyncFile),    /* szOsFile */
  0,                    /* mxPathname */
  0,                    /* pNext */
  "async",              /* zName */
  0,                    /* pAppData */
  asyncOpen,            /* xOpen */
  asyncDelete,          /* xDelete */
  asyncAccess,          /* xAccess */
  asyncGetTempName,     /* xGetTempName */
  asyncFullPathname,    /* xFullPathname */
  asyncDlOpen,          /* xDlOpen */
  asyncDlError,         /* xDlError */
  asyncDlSym,           /* xDlSym */
  asyncDlClose,         /* xDlClose */
  asyncRandomness,      /* xDlError */
  asyncSleep,           /* xDlSym */
  asyncCurrentTime      /* xDlClose */
};

/*
** Call this routine to enable or disable the
** asynchronous IO features implemented in this file. 
**
** 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);
      sqlite3HashInit(&async.aLock, SQLITE_HASH_BINARY, 1);






    }







  }else{
    if( async_vfs.pAppData ){
      sqlite3_vfs_unregister(&async_vfs);
      async_vfs.pAppData = 0;
      sqlite3HashClear(&async.aLock);






    }







  }
}

/* 
** This procedure runs in a separate thread, reading messages off of the
** write queue and processing them one by one.  
**

**
** An artifical delay of async.ioDelay milliseconds is inserted before
** each write operation in order to simulate the effect of a slow disk.
**
** Only one instance of this procedure may be running at a time.
*/
static void *asyncWriterThread(void *NotUsed){
  sqlite3_vfs *pVfs = (sqlite3_vfs *)(async_vfs.pAppData);
  AsyncWrite *p = 0;
  int rc = SQLITE_OK;
  int holdingMutex = 0;

  if( pthread_mutex_trylock(&async.writerMutex) ){
    return 0;
  }
  while( async.writerHaltNow==0 ){
    sqlite3_file *pBase = 0;

    if( !holdingMutex ){
      pthread_mutex_lock(&async.queueMutex);
    }
    while( (p = async.pQueueFirst)==0 ){
      pthread_cond_broadcast(&async.emptySignal);
      if( async.writerHaltWhenIdle ){

    **     * ASYNC_SYNC and ASYNC_WRITE operations, if 
    **       SQLITE_ASYNC_TWO_FILEHANDLES was set at compile time and two
    **       file-handles are open for the particular file being "synced".
    */
    if( async.ioError!=SQLITE_OK && p->op!=ASYNC_CLOSE ){
      p->op = ASYNC_NOOP;
    }
    if( p->pFileData ){
      pBase = p->pFileData->pBaseWrite;
      if( 
        p->op==ASYNC_CLOSE || 
        p->op==ASYNC_OPENEXCLUSIVE ||
        (pBase->pMethods && (p->op==ASYNC_SYNC || p->op==ASYNC_WRITE) ) 
      ){
        pthread_mutex_unlock(&async.queueMutex);
        holdingMutex = 0;
      }
      if( !pBase->pMethods ){
        pBase = p->pFileData->pBaseRead;
      }
    }

    switch( p->op ){
      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 = sqlite3OsWrite(pBase, (void *)(p->zBuf), p->nByte, p->iOffset);

        break;

      case ASYNC_SYNC:
        assert( pBase );
        ASYNC_TRACE(("SYNC %s\n", p->pFileData->zName));
        rc = sqlite3OsSync(pBase, p->nByte);
        break;

      case ASYNC_TRUNCATE:
        assert( pBase );
        ASYNC_TRACE(("TRUNCATE %s to %d bytes\n", 
                p->pFileData->zName, p->iOffset));
        rc = sqlite3OsTruncate(pBase, p->iOffset);
        break;

      case ASYNC_CLOSE:
        ASYNC_TRACE(("CLOSE %s\n", p->pFileData->zName));
        sqlite3OsClose(p->pFileData->pBaseWrite);
        sqlite3OsClose(p->pFileData->pBaseRead);
        sqlite3_free(p->pFileData);












        break;

      case ASYNC_DELETE:
        ASYNC_TRACE(("DELETE %s\n", p->zBuf));


        rc = sqlite3OsDelete(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 = sqlite3OsOpen(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 we didn't hang on to the mutex during the IO op, obtain it now

    ** in order to give other threads a chance to work with the write queue.
    */
    if( !async.pQueueFirst || !async.ioError ){
      sqlite3ApiExit(0, 0);
      pthread_mutex_unlock(&async.queueMutex);
      holdingMutex = 0;
      if( async.ioDelay>0 ){
        sqlite3OsSleep(pVfs, async.ioDelay);
      }else{
        sched_yield();
      }
    }
  }
  
  pthread_mutex_unlock(&async.writerMutex);

  Tcl_Obj *CONST objv[]
){
  if( objc!=1 && objc!=2 ){
    Tcl_WrongNumArgs(interp, 1, objv, "?YES/NO?");
    return TCL_ERROR;
  }
  if( objc==1 ){
    Tcl_SetObjResult(interp, Tcl_NewBooleanObj(async_vfs.pAppData!=0));
  }else{
    int en;
    if( Tcl_GetBooleanFromObj(interp, objv[1], &en) ) return TCL_ERROR;
    asyncEnable(en);
  }
  return TCL_OK;
}

#
#    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: async2.test,v 1.4 2007/08/25 12:29:30 danielk1977 Exp $


set testdir [file dirname $argv0]
source $testdir/tester.tcl

if {
  [info commands sqlite3async_enable]=="" ||
  [info command sqlite_malloc_stat]==""
} {
  # The async logic is not built into this system
  puts "Skipping async2 tests: not compiled with required features"
  finish_test
  return
}




# Enable asynchronous IO.

set setup_script {
  CREATE TABLE counter(c);
  INSERT INTO counter(c) VALUES (1);
}