text: /*
  ** 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: sqlite3async.c,v 1.7 2009/07/18 11:52:04 danielk1977 Exp $
  **
  ** This file contains the implementation of an asynchronous IO backend
  ** for SQLite.
  */
  
  #if !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_ASYNCIO)
  
  #include "sqlite3async.h"
  #include "sqlite3.h"
  #include <stdarg.h>
  #include <string.h>
  #include <assert.h>
  
  /* Useful macros used in several places */
  #define MIN(x,y) ((x)<(y)?(x):(y))
  #define MAX(x,y) ((x)>(y)?(x):(y))
  
  #ifndef SQLITE_AMALGAMATION
  /* Macro to mark parameters as unused and silence compiler warnings. */
  #define UNUSED_PARAMETER(x) (void)(x)
  #endif
  
  /* Forward references */
  typedef struct AsyncWrite AsyncWrite;
  typedef struct AsyncFile AsyncFile;
  typedef struct AsyncFileData AsyncFileData;
  typedef struct AsyncFileLock AsyncFileLock;
  typedef struct AsyncLock AsyncLock;
  
  /* Enable for debugging */
  #ifndef NDEBUG
  #include <stdio.h>
  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;
    va_start(ap, zFormat);
    z = sqlite3_vmprintf(zFormat, ap);
    va_end(ap);
    fprintf(stderr, "[%d] %s", 0 /* (int)pthread_self() */, z);
    sqlite3_free(z);
  }
  #else
  # define ASYNC_TRACE(X)
  #endif
  
  /*
  ** THREAD SAFETY NOTES
  **
  ** Basic rules:
  **
  **     * Both read and write access to the global write-op queue must be
  **       protected by the async.queueMutex. As are the async.ioError and
  **       async.nFile variables.
  **
  **     * The async.pLock list and all AsyncLock and AsyncFileLock
  **       structures must be protected by the async.lockMutex mutex.
  **
  **     * The file handles from the underlying system are not assumed to
  **       be thread safe.
  **
  **     * See the last two paragraphs under "The Writer Thread" for
  **       an assumption to do with file-handle synchronization by the Os.
  **
  ** Deadlock prevention:
  **
  **     There are three mutex used by the system: the "writer" mutex,
  **     the "queue" mutex and the "lock" mutex. Rules are:
  **
  **     * It is illegal to block on the writer mutex when any other mutex
  **       are held, and
  **
  **     * It is illegal to block on the queue mutex when the lock mutex
  **       is held.
  **
  **     i.e. mutex's must be grabbed in the order "writer", "queue", "lock".
  **
  ** File system operations (invoked by SQLite thread):
  **
  **     xOpen
  **     xDelete
  **     xFileExists
  **
  ** File handle operations (invoked by SQLite thread):
  **
  **         asyncWrite, asyncClose, asyncTruncate, asyncSync
  **
  **     The operations above add an entry to the global write-op list. They
  **     prepare the entry, acquire the async.queueMutex momentarily while
  **     list pointers are  manipulated to insert the new entry, then release
  **     the mutex and signal the writer thread to wake up in case it happens
  **     to be asleep.
  **
  **
  **         asyncRead, asyncFileSize.
  **
  **     Read operations. Both of these read from both the underlying file
  **     first then adjust their result based on pending writes in the
  **     write-op queue.   So async.queueMutex is held for the duration
  **     of these operations to prevent other threads from changing the
  **     queue in mid operation.
  **
  **
  **         asyncLock, asyncUnlock, asyncCheckReservedLock
  **
  **     These primitives implement in-process locking using a hash table
  **     on the file name.  Files are locked correctly for connections coming
  **     from the same process.  But other processes cannot see these locks
  **     and will therefore not honor them.
  **
  **
  ** The writer thread:
  **
  **     The async.writerMutex is used to make sure only there is only
  **     a single writer thread running at a time.
  **
  **     Inside the writer thread is a loop that works like this:
  **
  **         WHILE (write-op list is not empty)
  **             Do IO operation at head of write-op list
  **             Remove entry from head of write-op list
  **         END WHILE
  **
  **     The async.queueMutex is always held during the <write-op list is
  **     not empty> test, and when the entry is removed from the head
  **     of the write-op list. Sometimes it is held for the interim
  **     period (while the IO is performed), and sometimes it is
  **     relinquished. It is relinquished if (a) the IO op is an
  **     ASYNC_CLOSE or (b) when the file handle was opened, two of
  **     the underlying systems handles were opened on the same
  **     file-system entry.
  **
  **     If condition (b) above is true, then one file-handle
  **     (AsyncFile.pBaseRead) is used exclusively by sqlite threads to read the
  **     file, the other (AsyncFile.pBaseWrite) by sqlite3_async_flush()
  **     threads to perform write() operations. This means that read
  **     operations are not blocked by asynchronous writes (although
  **     asynchronous writes may still be blocked by reads).
  **
  **     This assumes that the OS keeps two handles open on the same file
  **     properly in sync. That is, any read operation that starts after a
  **     write operation on the same file system entry has completed returns
  **     data consistent with the write. We also assume that if one thread
  **     reads a file while another is writing it all bytes other than the
  **     ones actually being written contain valid data.
  **
  **     If the above assumptions are not true, set the preprocessor symbol
  **     SQLITE_ASYNC_TWO_FILEHANDLES to 0.
  */
  
  
  #ifndef NDEBUG
  # define TESTONLY( X ) X
  #else
  # define TESTONLY( X )
  #endif
  
  /*
  ** PORTING FUNCTIONS
  **
  ** There are two definitions of the following functions. One for pthreads
  ** compatible systems and one for Win32. These functions isolate the OS
  ** specific code required by each platform.
  **
  ** The system uses three mutexes and a single condition variable. To
  ** block on a mutex, async_mutex_enter() is called. The parameter passed
  ** to async_mutex_enter(), which must be one of ASYNC_MUTEX_LOCK,
  ** ASYNC_MUTEX_QUEUE or ASYNC_MUTEX_WRITER, identifies which of the three
  ** mutexes to lock. Similarly, to unlock a mutex, async_mutex_leave() is
  ** called with a parameter identifying the mutex being unlocked. Mutexes
  ** are not recursive - it is an error to call async_mutex_enter() to
  ** lock a mutex that is already locked, or to call async_mutex_leave()
  ** to unlock a mutex that is not currently locked.
  **
  ** The async_cond_wait() and async_cond_signal() functions are modelled
  ** on the pthreads functions with similar names. The first parameter to
  ** both functions is always ASYNC_COND_QUEUE. When async_cond_wait()
  ** is called the mutex identified by the second parameter must be held.
  ** The mutex is unlocked, and the calling thread simultaneously begins
  ** waiting for the condition variable to be signalled by another thread.
  ** After another thread signals the condition variable, the calling
  ** thread stops waiting, locks mutex eMutex and returns. The
  ** async_cond_signal() function is used to signal the condition variable.
  ** It is assumed that the mutex used by the thread calling async_cond_wait()
  ** is held by the caller of async_cond_signal() (otherwise there would be
  ** a race condition).
  **
  ** It is guaranteed that no other thread will call async_cond_wait() when
  ** there is already a thread waiting on the condition variable.
  **
  ** The async_sched_yield() function is called to suggest to the operating
  ** system that it would be a good time to shift the current thread off the
  ** CPU. The system will still work if this function is not implemented
  ** (it is not currently implemented for win32), but it might be marginally
  ** more efficient if it is.
  */
  static void async_mutex_enter(int eMutex);
  static void async_mutex_leave(int eMutex);
  static void async_cond_wait(int eCond, int eMutex);
  static void async_cond_signal(int eCond);
  static void async_sched_yield(void);
  
  /*
  ** There are also two definitions of the following. async_os_initialize()
  ** is called when the asynchronous VFS is first installed, and os_shutdown()
  ** is called when it is uninstalled (from within sqlite3async_shutdown()).
  **
  ** For pthreads builds, both of these functions are no-ops. For win32,
  ** they provide an opportunity to initialize and finalize the required
  ** mutex and condition variables.
  **
  ** If async_os_initialize() returns other than zero, then the initialization
  ** fails and SQLITE_ERROR is returned to the user.
  */
  static int async_os_initialize(void);
  static void async_os_shutdown(void);
  
  /* Values for use as the 'eMutex' argument of the above functions. The
  ** integer values assigned to these constants are important for assert()
  ** statements that verify that mutexes are locked in the correct order.
  ** Specifically, it is unsafe to try to lock mutex N while holding a lock
  ** on mutex M if (M<=N).
  */
  #define ASYNC_MUTEX_LOCK    0
  #define ASYNC_MUTEX_QUEUE   1
  #define ASYNC_MUTEX_WRITER  2
  
  /* Values for use as the 'eCond' argument of the above functions. */
  #define ASYNC_COND_QUEUE    0
  
  /*************************************************************************
  ** Start of OS specific code.
  */
  #if SQLITE_OS_WIN || defined(_WIN32) || defined(WIN32) || defined(__CYGWIN__) || defined(__MINGW32__) || defined(__BORLANDC__)
  
  #include <windows.h>
  
  /* The following block contains the win32 specific code. */
  
  #define mutex_held(X) (GetCurrentThreadId()==primitives.aHolder[X])
  
  static struct AsyncPrimitives {
    int isInit;
    DWORD aHolder[3];
    CRITICAL_SECTION aMutex[3];
    HANDLE aCond[1];
  } primitives = { 0 };
  
  static int async_os_initialize(void){
    if( !primitives.isInit ){
      primitives.aCond[0] = CreateEvent(NULL, TRUE, FALSE, 0);
      if( primitives.aCond[0]==NULL ){
        return 1;
      }
      InitializeCriticalSection(&primitives.aMutex[0]);
      InitializeCriticalSection(&primitives.aMutex[1]);
      InitializeCriticalSection(&primitives.aMutex[2]);
      primitives.isInit = 1;
    }
    return 0;
  }
  static void async_os_shutdown(void){
    if( primitives.isInit ){
      DeleteCriticalSection(&primitives.aMutex[0]);
      DeleteCriticalSection(&primitives.aMutex[1]);
      DeleteCriticalSection(&primitives.aMutex[2]);
      CloseHandle(primitives.aCond[0]);
      primitives.isInit = 0;
    }
  }
  
  /* The following block contains the Win32 specific code. */
  static void async_mutex_enter(int eMutex){
    assert( eMutex==0 || eMutex==1 || eMutex==2 );
    assert( eMutex!=2 || (!mutex_held(0) && !mutex_held(1) && !mutex_held(2)) );
    assert( eMutex!=1 || (!mutex_held(0) && !mutex_held(1)) );
    assert( eMutex!=0 || (!mutex_held(0)) );
    EnterCriticalSection(&primitives.aMutex[eMutex]);
    TESTONLY( primitives.aHolder[eMutex] = GetCurrentThreadId(); )
  }
  static void async_mutex_leave(int eMutex){
    assert( eMutex==0 || eMutex==1 || eMutex==2 );
    assert( mutex_held(eMutex) );
    TESTONLY( primitives.aHolder[eMutex] = 0; )
    LeaveCriticalSection(&primitives.aMutex[eMutex]);
  }
  static void async_cond_wait(int eCond, int eMutex){
    ResetEvent(primitives.aCond[eCond]);
    async_mutex_leave(eMutex);
    WaitForSingleObject(primitives.aCond[eCond], INFINITE);
    async_mutex_enter(eMutex);
  }
  static void async_cond_signal(int eCond){
    assert( mutex_held(ASYNC_MUTEX_QUEUE) );
    SetEvent(primitives.aCond[eCond]);
  }
  static void async_sched_yield(void){
    Sleep(0);
  }
  #else
  
  /* The following block contains the pthreads specific code. */
  #include <pthread.h>
  #include <sched.h>
  
  #define mutex_held(X) pthread_equal(primitives.aHolder[X], pthread_self())
  
  static int  async_os_initialize(void) {return 0;}
  static void async_os_shutdown(void) {}
  
  static struct AsyncPrimitives {
    pthread_mutex_t aMutex[3];
    pthread_cond_t aCond[1];
    pthread_t aHolder[3];
  } primitives = {
    { PTHREAD_MUTEX_INITIALIZER,
      PTHREAD_MUTEX_INITIALIZER,
      PTHREAD_MUTEX_INITIALIZER
    } , {
      PTHREAD_COND_INITIALIZER
    } , { 0, 0, 0 }
  };
  
  static void async_mutex_enter(int eMutex){
    assert( eMutex==0 || eMutex==1 || eMutex==2 );
    assert( eMutex!=2 || (!mutex_held(0) && !mutex_held(1) && !mutex_held(2)) );
    assert( eMutex!=1 || (!mutex_held(0) && !mutex_held(1)) );
    assert( eMutex!=0 || (!mutex_held(0)) );
    pthread_mutex_lock(&primitives.aMutex[eMutex]);
    TESTONLY( primitives.aHolder[eMutex] = pthread_self(); )
  }
  static void async_mutex_leave(int eMutex){
    assert( eMutex==0 || eMutex==1 || eMutex==2 );
    assert( mutex_held(eMutex) );
    TESTONLY( primitives.aHolder[eMutex] = 0; )
    pthread_mutex_unlock(&primitives.aMutex[eMutex]);
  }
  static void async_cond_wait(int eCond, int eMutex){
    assert( eMutex==0 || eMutex==1 || eMutex==2 );
    assert( mutex_held(eMutex) );
    TESTONLY( primitives.aHolder[eMutex] = 0; )
    pthread_cond_wait(&primitives.aCond[eCond], &primitives.aMutex[eMutex]);
    TESTONLY( primitives.aHolder[eMutex] = pthread_self(); )
  }
  static void async_cond_signal(int eCond){
    assert( mutex_held(ASYNC_MUTEX_QUEUE) );
    pthread_cond_signal(&primitives.aCond[eCond]);
  }
  static void async_sched_yield(void){
    sched_yield();
  }
  #endif
  /*
  ** End of OS specific code.
  *************************************************************************/
  
  #define assert_mutex_is_held(X) assert( mutex_held(X) )
  
  
  #ifndef SQLITE_ASYNC_TWO_FILEHANDLES
  /* #define SQLITE_ASYNC_TWO_FILEHANDLES 0 */
  #define SQLITE_ASYNC_TWO_FILEHANDLES 1
  #endif
  
  /*
  ** State information is held in the static variable "async" defined
  ** as the following structure.
  **
  ** Both async.ioError and async.nFile are protected by async.queueMutex.
  */
  static struct TestAsyncStaticData {
    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 eHalt;          /* One of the SQLITEASYNC_HALT_XXX values */
    volatile int bLockFiles;     /* Current value of "lockfiles" parameter */
    int ioError;                 /* True if an IO error has occurred */
    int nFile;                   /* Number of open files (from sqlite pov) */
  } async = { 0,0,0,0,0,1,0,0 };
  
  /* Possible values of AsyncWrite.op */
  #define ASYNC_NOOP          0
  #define ASYNC_WRITE         1
  #define ASYNC_SYNC          2
  #define ASYNC_TRUNCATE      3
  #define ASYNC_CLOSE         4
  #define ASYNC_DELETE        5
  #define ASYNC_OPENEXCLUSIVE 6
  #define ASYNC_UNLOCK        7
  
  /* Names of opcodes.  Used for debugging only.
  ** Make sure these stay in sync with the macros above!
  */
  static const char *azOpcodeName[] = {
    "NOOP", "WRITE", "SYNC", "TRUNCATE", "CLOSE", "DELETE", "OPENEX", "UNLOCK"
  };
  
  /*
  ** Entries on the write-op queue are instances of the AsyncWrite
  ** structure, defined here.
  **
  ** The interpretation of the iOffset and nByte variables varies depending
  ** on the value of AsyncWrite.op:
  **
  ** ASYNC_NOOP:
  **     No values used.
  **
  ** 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_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).
  **
  ** ASYNC_UNLOCK:
  **     nByte   -> Argument to sqlite3OsUnlock().
  **
  **
  ** 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. */
    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
  ** (i.e. if two handles are opened on the one file, only one of these
  ** structures is allocated) and stored in the async.aLock hash table. The
  ** keys for async.aLock are the full pathnames of the opened files.
  **
  ** AsyncLock.pList points to the head of a linked list of AsyncFileLock
  ** structures, one for each handle currently open on the file.
  **
  ** If the opened file is not a main-database (the SQLITE_OPEN_MAIN_DB is
  ** not passed to the sqlite3OsOpen() call), or if async.bLockFiles is
  ** false, variables AsyncLock.pFile and AsyncLock.eLock are never used.
  ** Otherwise, pFile is a file handle 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.
  */
  struct AsyncFileLock {
    int eLock;                /* Internally visible lock state (sqlite pov) */
    int eAsyncLock;           /* Lock-state with write-queue unlock */
    AsyncFileLock *pNext;
  };
  
  /*
  ** 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 */
    AsyncFileLock lock;        /* Lock state for this handle */
    AsyncLock *pLock;          /* AsyncLock object for this file system entry */
    AsyncWrite closeOp;        /* Preallocated close operation */
  };
  
  /*
  ** 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.
  **
  ** Once an AsyncWrite structure has been added to the list, it becomes the
  ** property of the writer thread and must not be read or modified by the
  ** caller.
  */
  static void addAsyncWrite(AsyncWrite *pWrite){
    /* We must hold the queue mutex in order to modify the queue pointers */
    if( pWrite->op!=ASYNC_UNLOCK ){
      async_mutex_enter(ASYNC_MUTEX_QUEUE);
    }
  
    /* Add the record to the end of the write-op queue */
    assert( !pWrite->pNext );
    if( async.pQueueLast ){
      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--;
    }
  
    /* The writer thread might have been idle because there was nothing
    ** on the write-op queue for it to do.  So wake it up. */
    async_cond_signal(ASYNC_COND_QUEUE);
  
    /* Drop the queue mutex */
    if( pWrite->op!=ASYNC_UNLOCK ){
      async_mutex_leave(ASYNC_MUTEX_QUEUE);
    }
  }
  
  /*
  ** Increment async.nFile in a thread-safe manner.
  */
  static void incrOpenFileCount(void){
    /* We must hold the queue mutex in order to modify async.nFile */
    async_mutex_enter(ASYNC_MUTEX_QUEUE);
    if( async.nFile==0 ){
      async.ioError = SQLITE_OK;
    }
    async.nFile++;
    async_mutex_leave(ASYNC_MUTEX_QUEUE);
  }
  
  /*
  ** 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;
    }
    p = sqlite3_malloc(sizeof(AsyncWrite) + (zByte?nByte:0));
    if( !p ){
      /* The upper layer does not expect operations like OsWrite() to
      ** return SQLITE_NOMEM. This is partly because under normal conditions
      ** SQLite is required to do rollback without calling malloc(). So
      ** if malloc() fails here, treat it as an I/O error. The above
      ** layer knows how to handle that.
      */
      return SQLITE_IOERR;
    }
    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;
  
    /* Unlock the file, if it is locked */
    async_mutex_enter(ASYNC_MUTEX_LOCK);
    p->lock.eLock = 0;
    async_mutex_leave(ASYNC_MUTEX_LOCK);
  
    addAsyncWrite(&p->closeOp);
    return SQLITE_OK;
  }
  
  /*
  ** 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 = 0;
    sqlite3_file *pBase = p->pBaseRead;
    sqlite3_int64 iAmt64 = (sqlite3_int64)iAmt;
  
    /* Grab the write queue mutex for the duration of the call */
    async_mutex_enter(ASYNC_MUTEX_QUEUE);
  
    /* 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 ){
      sqlite3_int64 nRead;
      rc = pBase->pMethods->xFileSize(pBase, &filesize);
      if( rc!=SQLITE_OK ){
        goto asyncread_out;
      }
      nRead = MIN(filesize - iOffset, iAmt64);
      if( nRead>0 ){
        rc = pBase->pMethods->xRead(pBase, zOut, (int)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)
        )){
          sqlite3_int64 nCopy;
          sqlite3_int64 nByte64 = (sqlite3_int64)pWrite->nByte;
  
          /* Set variable iBeginIn to the offset in buffer pWrite->zBuf[] from
          ** which data should be copied. Set iBeginOut to the offset within
          ** the output buffer to which data should be copied. If either of
          ** these offsets is a negative number, set them to 0.
          */
          sqlite3_int64 iBeginOut = (pWrite->iOffset-iOffset);
          sqlite3_int64 iBeginIn = -iBeginOut;
          if( iBeginIn<0 ) iBeginIn = 0;
          if( iBeginOut<0 ) iBeginOut = 0;
  
          filesize = MAX(filesize, pWrite->iOffset+nByte64);
  
          nCopy = MIN(nByte64-iBeginIn, iAmt64-iBeginOut);
          if( nCopy>0 ){
            memcpy(&((char *)zOut)[iBeginOut], &pWrite->zBuf[iBeginIn], (size_t)nCopy);
            ASYNC_TRACE(("OVERREAD %d bytes at %d\n", nCopy, iBeginOut+iOffset));
          }
        }
      }
    }
  
  asyncread_out:
    async_mutex_leave(ASYNC_MUTEX_QUEUE);
    if( rc==SQLITE_OK && filesize<(iOffset+iAmt) ){
      rc = SQLITE_IOERR_SHORT_READ;
    }
    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;
  
    async_mutex_enter(ASYNC_MUTEX_QUEUE);
  
    /* Read the filesystem size from the base file. If pMethods 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;
          }
        }
      }
      *piSize = s;
    }
    async_mutex_leave(ASYNC_MUTEX_QUEUE);
    return rc;
  }
  
  /*
  ** Lock or unlock the actual file-system entry.
  */
  static int getFileLock(AsyncLock *pLock){
    int rc = SQLITE_OK;
    AsyncFileLock *pIter;
    int eRequired = 0;
  
    if( pLock->pFile ){
      for(pIter=pLock->pList; pIter; pIter=pIter->pNext){
        assert(pIter->eAsyncLock>=pIter->eLock);
        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 ){
      async_mutex_enter(ASYNC_MUTEX_LOCK);
      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);
        }
      }
      async_mutex_leave(ASYNC_MUTEX_LOCK);
    }
  
    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;
      async_mutex_enter(ASYNC_MUTEX_QUEUE);
      async_mutex_enter(ASYNC_MUTEX_LOCK);
      pLock->eLock = MIN(pLock->eLock, eLock);
      rc = addNewAsyncWrite(p, ASYNC_UNLOCK, 0, eLock, 0);
      async_mutex_leave(ASYNC_MUTEX_LOCK);
      async_mutex_leave(ASYNC_MUTEX_QUEUE);
    }
    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;
  
    async_mutex_enter(ASYNC_MUTEX_LOCK);
    for(pIter=p->pLock->pList; pIter; pIter=pIter->pNext){
      if( pIter->eLock>=SQLITE_LOCK_RESERVED ){
        ret = 1;
        break;
      }
    }
    async_mutex_leave(ASYNC_MUTEX_LOCK);
  
    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: {
        async_mutex_enter(ASYNC_MUTEX_LOCK);
        *(int*)pArg = ((AsyncFile*)id)->pData->lock.eLock;
        async_mutex_leave(ASYNC_MUTEX_LOCK);
        return SQLITE_OK;
      }
    }
    return SQLITE_NOTFOUND;
  }
  
  /*
  ** Return the device characteristics and sector-size of the device. It
  ** is tricky to implement these correctly, as this backend might
  ** not have an open file handle at this point.
  */
  static int asyncSectorSize(sqlite3_file *pFile){
    UNUSED_PARAMETER(pFile);
    return 512;
  }
  static int asyncDeviceCharacteristics(sqlite3_file *pFile){
    UNUSED_PARAMETER(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;
  }
  
  /*
  ** The parameter passed to this function is a copy of a 'flags' parameter
  ** passed to this modules xOpen() method. This function returns true
  ** if the file should be opened asynchronously, or false if it should
  ** be opened immediately.
  **
  ** If the file is to be opened asynchronously, then asyncOpen() will add
  ** an entry to the event queue and the file will not actually be opened
  ** until the event is processed. Otherwise, the file is opened directly
  ** by the caller.
  */
  static int doAsynchronousOpen(int flags){
    return (flags&SQLITE_OPEN_CREATE) && (
        (flags&SQLITE_OPEN_MAIN_JOURNAL) ||
        (flags&SQLITE_OPEN_TEMP_JOURNAL) ||
        (flags&SQLITE_OPEN_DELETEONCLOSE)
    );
  }
  
  /*
  ** 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 */
      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 isAsyncOpen = doAsynchronousOpen(flags);
  
    /* If zName is NULL, then the upper layer is requesting an anonymous file.
    ** Otherwise, allocate enough space to make a copy of the file name (along
    ** with the second nul-terminator byte required by xOpen).
    */
    if( zName ){
      nName = (int)strlen(zName);
    }
  
    nByte = (
      sizeof(AsyncFileData) +        /* AsyncFileData structure */
      2 * pVfs->szOsFile +           /* AsyncFileData.pBaseRead and pBaseWrite */
      nName + 2                      /* 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->closeOp.pFileData = pData;
    pData->closeOp.op = ASYNC_CLOSE;
  
    if( zName ){
      z += pVfs->szOsFile;
      pData->zName = z;
      pData->nName = nName;
      memcpy(pData->zName, zName, nName);
    }
  
    if( !isAsyncOpen ){
      int flagsout;
      rc = pVfs->xOpen(pVfs, pData->zName, pData->pBaseRead, flags, &flagsout);
      if( rc==SQLITE_OK
       && (flagsout&SQLITE_OPEN_READWRITE)
       && (flags&SQLITE_OPEN_EXCLUSIVE)==0
      ){
        rc = pVfs->xOpen(pVfs, pData->zName, pData->pBaseWrite, flags, 0);
      }
      if( pOutFlags ){
        *pOutFlags = flagsout;
      }
    }
  
    async_mutex_enter(ASYNC_MUTEX_LOCK);
  
    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);
          if( async.bLockFiles && (flags&SQLITE_OPEN_MAIN_DB) ){
            pLock->pFile = (sqlite3_file *)&pLock[1];
            rc = pVfs->xOpen(pVfs, pData->zName, pLock->pFile, flags, 0);
            if( rc!=SQLITE_OK ){
              sqlite3_free(pLock);
              pLock = 0;
            }
          }
          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);
    }
  
    async_mutex_leave(ASYNC_MUTEX_LOCK);
  
    if( rc==SQLITE_OK ){
      pData->pLock = pLock;
    }
  
    if( rc==SQLITE_OK && isAsyncOpen ){
      rc = addNewAsyncWrite(pData, ASYNC_OPENEXCLUSIVE, (sqlite3_int64)flags,0,0);
      if( rc==SQLITE_OK ){
        if( pOutFlags ) *pOutFlags = flags;
      }else{
        async_mutex_enter(ASYNC_MUTEX_LOCK);
        unlinkAsyncFile(pData);
        async_mutex_leave(ASYNC_MUTEX_LOCK);
        sqlite3_free(pData);
      }
    }
    if( rc!=SQLITE_OK ){
      p->pMethod = 0;
    }else{
      incrOpenFileCount();
    }
  
    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){
    UNUSED_PARAMETER(pAsyncVfs);
    return addNewAsyncWrite(0, ASYNC_DELETE, syncDir, (int)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 *pResOut
  ){
    int rc;
    int ret;
    AsyncWrite *p;
    sqlite3_vfs *pVfs = (sqlite3_vfs *)pAsyncVfs->pAppData;
  
    assert(flags==SQLITE_ACCESS_READWRITE
        || flags==SQLITE_ACCESS_READ
        || flags==SQLITE_ACCESS_EXISTS
    );
  
    async_mutex_enter(ASYNC_MUTEX_QUEUE);
    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",
      flags==SQLITE_ACCESS_READWRITE?"read-write":
      flags==SQLITE_ACCESS_READ?"read":"exists"
      , zName, ret)
    );
    async_mutex_leave(ASYNC_MUTEX_QUEUE);
    *pResOut = ret;
    return rc;
  }
  
  /*
  ** Fill in zPathOut with the full path to the file identified by zPath.
  */
  static int asyncFullPathname(
    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 i, j;
      char *z = zPathOut;
      int n = (int)strlen(z);
      while( n>1 && z[n-1]=='/' ){ n--; }
      for(i=j=0; i<n; i++){
        if( z[i]=='/' ){
          if( z[i+1]=='/' ) continue;
          if( z[i+1]=='.' && i+2<n && z[i+2]=='/' ){
            i += 1;
            continue;
          }
          if( z[i+1]=='.' && i+3<n && z[i+2]=='.' && z[i+3]=='/' ){
            while( j>0 && z[j-1]!='/' ){ j--; }
            if( j>0 ){ j--; }
            i += 2;
            continue;
          }
        }
        z[j++] = z[i];
      }
      z[j] = 0;
    }
  
    return rc;
  }
  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
  ))(void){
    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 */
    SQLITEASYNC_VFSNAME,  /* zName */
    0,                    /* pAppData */
    asyncOpen,            /* xOpen */
    asyncDelete,          /* xDelete */
    asyncAccess,          /* xAccess */
    asyncFullPathname,    /* xFullPathname */
    asyncDlOpen,          /* xDlOpen */
    asyncDlError,         /* xDlError */
    asyncDlSym,           /* xDlSym */
    asyncDlClose,         /* xDlClose */
    asyncRandomness,      /* xDlError */
    asyncSleep,           /* xDlSym */
    asyncCurrentTime      /* xDlClose */
  };
  
  /*
  ** This procedure runs in a separate thread, reading messages off of the
  ** write queue and processing them one by one.
  **
  ** If async.writerHaltNow is true, then this procedure exits
  ** after processing a single message.
  **
  ** If async.writerHaltWhenIdle is true, then this procedure exits when
  ** the write queue is empty.
  **
  ** If both of the above variables are false, this procedure runs
  ** indefinately, waiting for operations to be added to the write queue
  ** and processing them in the order in which they arrive.
  **
  ** 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){
    sqlite3_vfs *pVfs = (sqlite3_vfs *)(async_vfs.pAppData);
    AsyncWrite *p = 0;
    int rc = SQLITE_OK;
    int holdingMutex = 0;
  
    async_mutex_enter(ASYNC_MUTEX_WRITER);
  
    while( async.eHalt!=SQLITEASYNC_HALT_NOW ){
      int doNotFree = 0;
      sqlite3_file *pBase = 0;
  
      if( !holdingMutex ){
        async_mutex_enter(ASYNC_MUTEX_QUEUE);
      }
      while( (p = async.pQueueFirst)==0 ){
        if( async.eHalt!=SQLITEASYNC_HALT_NEVER ){
          async_mutex_leave(ASYNC_MUTEX_QUEUE);
          break;
        }else{
          ASYNC_TRACE(("IDLE\n"));
          async_cond_wait(ASYNC_COND_QUEUE, ASYNC_MUTEX_QUEUE);
          ASYNC_TRACE(("WAKEUP\n"));
        }
      }
      if( p==0 ) break;
      holdingMutex = 1;
  
      /* Right now this thread is holding the mutex on the write-op queue.
      ** Variable 'p' points to the first entry in the write-op queue. In
      ** the general case, we hold on to the mutex for the entire body of
      ** the loop.
      **
      ** However in the cases enumerated below, we relinquish the mutex,
      ** perform the IO, and then re-request the mutex before removing 'p' from
      ** the head of the write-op queue. The idea is to increase concurrency with
      ** sqlite threads.
      **
      **     * An ASYNC_CLOSE operation.
      **     * An ASYNC_OPENEXCLUSIVE operation. For this one, we relinquish
      **       the mutex, call the underlying xOpenExclusive() function, then
      **       re-aquire the mutex before seting the AsyncFile.pBaseRead
      **       variable.
      **     * 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) )
        ){
          async_mutex_leave(ASYNC_MUTEX_QUEUE);
          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 = 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.
          */
          async_mutex_enter(ASYNC_MUTEX_LOCK);
          rc = unlinkAsyncFile(pData);
          async_mutex_leave(ASYNC_MUTEX_LOCK);
  
          if( !holdingMutex ){
            async_mutex_enter(ASYNC_MUTEX_QUEUE);
            holdingMutex = 1;
          }
          assert_mutex_is_held(ASYNC_MUTEX_QUEUE);
          async.pQueueFirst = p->pNext;
          sqlite3_free(pData);
          doNotFree = 1;
          break;
        }
  
        case ASYNC_UNLOCK: {
          AsyncWrite *pIter;
          AsyncFileData *pData = p->pFileData;
          int eLock = p->nByte;
  
          /* When a file is locked by SQLite using the async backend, it is
          ** locked within the 'real' file-system synchronously. When it is
          ** unlocked, an ASYNC_UNLOCK event is added to the write-queue to
          ** unlock the file asynchronously. The design of the async backend
          ** requires that the 'real' file-system file be locked from the
          ** time that SQLite first locks it (and probably reads from it)
          ** until all asynchronous write events that were scheduled before
          ** SQLite unlocked the file have been processed.
          **
          ** This is more complex if SQLite locks and unlocks the file multiple
          ** times in quick succession. For example, if SQLite does:
          **
          **   lock, write, unlock, lock, write, unlock
          **
          ** Each "lock" operation locks the file immediately. Each "write"
          ** and "unlock" operation adds an event to the event queue. If the
          ** second "lock" operation is performed before the first "unlock"
          ** operation has been processed asynchronously, then the first
          ** "unlock" cannot be safely processed as is, since this would mean
          ** the file was unlocked when the second "write" operation is
          ** processed. To work around this, when processing an ASYNC_UNLOCK
          ** operation, SQLite:
          **
          **   1) Unlocks the file to the minimum of the argument passed to
          **      the xUnlock() call and the current lock from SQLite's point
          **      of view, and
          **
          **   2) Only unlocks the file at all if this event is the last
          **      ASYNC_UNLOCK event on this file in the write-queue.
          */
          assert( holdingMutex==1 );
          assert( async.pQueueFirst==p );
          for(pIter=async.pQueueFirst->pNext; pIter; pIter=pIter->pNext){
            if( pIter->pFileData==pData && pIter->op==ASYNC_UNLOCK ) break;
          }
          if( !pIter ){
            async_mutex_enter(ASYNC_MUTEX_LOCK);
            pData->lock.eAsyncLock = MIN(
                pData->lock.eAsyncLock, MAX(pData->lock.eLock, eLock)
            );
            assert(pData->lock.eAsyncLock>=pData->lock.eLock);
            rc = getFileLock(pData->pLock);
            async_mutex_leave(ASYNC_MUTEX_LOCK);
          }
          break;
        }
  
        case ASYNC_DELETE:
          ASYNC_TRACE(("DELETE %s\n", p->zBuf));
          rc = pVfs->xDelete(pVfs, p->zBuf, (int)p->iOffset);
          if( rc==SQLITE_IOERR_DELETE_NOENT ) rc = SQLITE_OK;
          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 );
          async_mutex_enter(ASYNC_MUTEX_QUEUE);
          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
      ** so that the AsyncWrite structure can be safely removed from the
      ** global write-op queue.
      */
      if( !holdingMutex ){
        async_mutex_enter(ASYNC_MUTEX_QUEUE);
        holdingMutex = 1;
      }
      /* ASYNC_TRACE(("UNLINK %p\n", p)); */
      if( p==async.pQueueLast ){
        async.pQueueLast = 0;
      }
      if( !doNotFree ){
        assert_mutex_is_held(ASYNC_MUTEX_QUEUE);
        async.pQueueFirst = p->pNext;
        sqlite3_free(p);
      }
      assert( holdingMutex );
  
      /* An IO error has occurred. We cannot report the error back to the
      ** connection that requested the I/O since the error happened
      ** asynchronously.  The connection has already moved on.  There
      ** really is nobody to report the error to.
      **
      ** The file for which the error occurred may have been a database or
      ** journal file. Regardless, none of the currently queued operations
      ** associated with the same database should now be performed. Nor should
      ** any subsequently requested IO on either a database or journal file
      ** handle for the same database be accepted until the main database
      ** file handle has been closed and reopened.
      **
      ** Furthermore, no further IO should be queued or performed on any file
      ** handle associated with a database that may have been part of a
      ** multi-file transaction that included the database associated with
      ** the IO error (i.e. a database ATTACHed to the same handle at some
      ** point in time).
      */
      if( rc!=SQLITE_OK ){
        async.ioError = rc;
      }
  
      if( async.ioError && !async.pQueueFirst ){
        async_mutex_enter(ASYNC_MUTEX_LOCK);
        if( 0==async.pLock ){
          async.ioError = SQLITE_OK;
        }
        async_mutex_leave(ASYNC_MUTEX_LOCK);
      }
  
      /* 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 ){
        async_mutex_leave(ASYNC_MUTEX_QUEUE);
        holdingMutex = 0;
        if( async.ioDelay>0 ){
          pVfs->xSleep(pVfs, async.ioDelay*1000);
        }else{
          async_sched_yield();
        }
      }
    }
  
    async_mutex_leave(ASYNC_MUTEX_WRITER);
    return;
  }
  
  /*
  ** Install the asynchronous VFS.
  */
  int sqlite3async_initialize(const char *zParent, int isDefault){
    int rc = SQLITE_OK;
    if( async_vfs.pAppData==0 ){
      sqlite3_vfs *pParent = sqlite3_vfs_find(zParent);
      if( !pParent || async_os_initialize() ){
        rc = SQLITE_ERROR;
      }else if( SQLITE_OK!=(rc = sqlite3_vfs_register(&async_vfs, isDefault)) ){
        async_os_shutdown();
      }else{
        async_vfs.pAppData = (void *)pParent;
        async_vfs.mxPathname = ((sqlite3_vfs *)async_vfs.pAppData)->mxPathname;
      }
    }
    return rc;
  }
  
  /*
  ** Uninstall the asynchronous VFS.
  */
  void sqlite3async_shutdown(void){
    if( async_vfs.pAppData ){
      async_os_shutdown();
      sqlite3_vfs_unregister((sqlite3_vfs *)&async_vfs);
      async_vfs.pAppData = 0;
    }
  }
  
  /*
  ** Process events on the write-queue.
  */
  void sqlite3async_run(void){
    asyncWriterThread();
  }
  
  /*
  ** Control/configure the asynchronous IO system.
  */
  int sqlite3async_control(int op, ...){
    int rc = SQLITE_OK;
    va_list ap;
    va_start(ap, op);
    switch( op ){
      case SQLITEASYNC_HALT: {
        int eWhen = va_arg(ap, int);
        if( eWhen!=SQLITEASYNC_HALT_NEVER
         && eWhen!=SQLITEASYNC_HALT_NOW
         && eWhen!=SQLITEASYNC_HALT_IDLE
        ){
          rc = SQLITE_MISUSE;
          break;
        }
        async.eHalt = eWhen;
        async_mutex_enter(ASYNC_MUTEX_QUEUE);
        async_cond_signal(ASYNC_COND_QUEUE);
        async_mutex_leave(ASYNC_MUTEX_QUEUE);
        break;
      }
  
      case SQLITEASYNC_DELAY: {
        int iDelay = va_arg(ap, int);
        if( iDelay<0 ){
          rc = SQLITE_MISUSE;
          break;
        }
        async.ioDelay = iDelay;
        break;
      }
  
      case SQLITEASYNC_LOCKFILES: {
        int bLock = va_arg(ap, int);
        async_mutex_enter(ASYNC_MUTEX_QUEUE);
        if( async.nFile || async.pQueueFirst ){
          async_mutex_leave(ASYNC_MUTEX_QUEUE);
          rc = SQLITE_MISUSE;
          break;
        }
        async.bLockFiles = bLock;
        async_mutex_leave(ASYNC_MUTEX_QUEUE);
        break;
      }
  
      case SQLITEASYNC_GET_HALT: {
        int *peWhen = va_arg(ap, int *);
        *peWhen = async.eHalt;
        break;
      }
      case SQLITEASYNC_GET_DELAY: {
        int *piDelay = va_arg(ap, int *);
        *piDelay = async.ioDelay;
        break;
      }
      case SQLITEASYNC_GET_LOCKFILES: {
        int *piDelay = va_arg(ap, int *);
        *piDelay = async.bLockFiles;
        break;
      }
  
      default:
        rc = SQLITE_ERROR;
        break;
    }
    va_end(ap);
    return rc;
  }
  
  #endif /* !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_ASYNCIO) */
  
  