SQLite

** space used by the RBU handle.
*/
struct rbu_vfs {
  sqlite3_vfs base;               /* rbu VFS shim methods */
  sqlite3_vfs *pRealVfs;          /* Underlying VFS */
  sqlite3_mutex *mutex;           /* Mutex to protect pMain */
  sqlite3rbu *pRbu;               /* Owner RBU object */
  rbu_file *pMain;                /* List of main db files */
  rbu_file *pMainRbu;             /* List of main db files with pRbu!=0 */
};

/*
** Each file opened by an rbu VFS is represented by an instance of
** the following structure.
**
** If this is a temporary file (pRbu!=0 && flags&DELETE_ON_CLOSE), variable

  int nShm;                       /* Number of entries in apShm[] array */
  char **apShm;                   /* Array of mmap'd *-shm regions */
  char *zDel;                     /* Delete this when closing file */

  const char *zWal;               /* Wal filename for this main db file */
  rbu_file *pWalFd;               /* Wal file descriptor for this main db */
  rbu_file *pMainNext;            /* Next MAIN_DB file */
  rbu_file *pMainRbuNext;         /* Next MAIN_DB file with pRbu!=0 */
};

/*
** True for an RBU vacuum handle, or false otherwise.
*/
#define rbuIsVacuum(p) ((p)->zTarget==0)

  i64 nDiff = nNew - pFd->sz;
  pRbu->szTemp += nDiff;
  pFd->sz = nNew;
  assert( pRbu->szTemp>=0 );
  if( pRbu->szTempLimit && pRbu->szTemp>pRbu->szTempLimit ) return SQLITE_FULL;
  return SQLITE_OK;
}

/*
** Add an item to the main-db lists, if it is not already present.
**
** There are two main-db lists. One for all file descriptors, and one
** for all file descriptors with rbu_file.pDb!=0. If the argument has
** rbu_file.pDb!=0, then it is assumed to already be present on the
** main list and is only added to the pDb!=0 list.
*/
static void rbuMainlistAdd(rbu_file *p){
  rbu_vfs *pRbuVfs = p->pRbuVfs;
  rbu_file *pIter;
  assert( (p->openFlags & SQLITE_OPEN_MAIN_DB) );
  sqlite3_mutex_enter(pRbuVfs->mutex);
  if( p->pRbu==0 ){
    for(pIter=pRbuVfs->pMain; pIter; pIter=pIter->pMainNext);
    p->pMainNext = pRbuVfs->pMain;
    pRbuVfs->pMain = p;
  }else{
    for(pIter=pRbuVfs->pMainRbu; pIter && pIter!=p; pIter=pIter->pMainRbuNext){}
    if( pIter==0 ){
      p->pMainRbuNext = pRbuVfs->pMainRbu;
      pRbuVfs->pMainRbu = p;
    }
  }
  sqlite3_mutex_leave(pRbuVfs->mutex);
}

/*
** Remove an item from the main-db lists.
*/
static void rbuMainlistRemove(rbu_file *p){
  rbu_file **pp;
  sqlite3_mutex_enter(p->pRbuVfs->mutex);
  for(pp=&p->pRbuVfs->pMain; *pp && *pp!=p; pp=&((*pp)->pMainNext)){}
  if( *pp ) *pp = p->pMainNext;
  p->pMainNext = 0;
  for(pp=&p->pRbuVfs->pMainRbu; *pp && *pp!=p; pp=&((*pp)->pMainRbuNext)){}
  if( *pp ) *pp = p->pMainRbuNext;
  p->pMainRbuNext = 0;
  sqlite3_mutex_leave(p->pRbuVfs->mutex);
}

/*
** Given that zWal points to a buffer containing a wal file name passed to 
** either the xOpen() or xAccess() VFS method, search the main-db list for
** a file-handle opened by the same database connection on the corresponding
** database file.
**
** If parameter bRbu is true, only search for file-descriptors with
** rbu_file.pDb!=0.
*/
static rbu_file *rbuFindMaindb(rbu_vfs *pRbuVfs, const char *zWal, int bRbu){
  rbu_file *pDb;
  sqlite3_mutex_enter(pRbuVfs->mutex);
  if( bRbu ){
    for(pDb=pRbuVfs->pMainRbu; pDb && pDb->zWal!=zWal; pDb=pDb->pMainRbuNext){}
  }else{
    for(pDb=pRbuVfs->pMain; pDb && pDb->zWal!=zWal; pDb=pDb->pMainNext){}
  }
  sqlite3_mutex_leave(pRbuVfs->mutex);
  return pDb;
}

/*
** Close an rbu file.
*/
static int rbuVfsClose(sqlite3_file *pFile){
  rbu_file *p = (rbu_file*)pFile;
  int rc;
  int i;

  /* Free the contents of the apShm[] array. And the array itself. */
  for(i=0; i<p->nShm; i++){
    sqlite3_free(p->apShm[i]);
  }
  sqlite3_free(p->apShm);
  p->apShm = 0;
  sqlite3_free(p->zDel);

  if( p->openFlags & SQLITE_OPEN_MAIN_DB ){



    rbuMainlistRemove(p);

    rbuUnlockShm(p);
    p->pReal->pMethods->xShmUnmap(p->pReal, 0);
  }
  else if( (p->openFlags & SQLITE_OPEN_DELETEONCLOSE) && p->pRbu ){
    rbuUpdateTempSize(p, 0);
  }
  assert( p->pMainNext==0 && p->pRbuVfs->pMain!=p );

  /* Close the underlying file handle */
  rc = p->pReal->pMethods->xClose(p->pReal);
  return rc;
}

      rc = xControl(p->pReal, SQLITE_FCNTL_ZIPVFS, &dummy);
      if( rc==SQLITE_OK ){
        rc = SQLITE_ERROR;
        pRbu->zErrmsg = sqlite3_mprintf("rbu/zipvfs setup error");
      }else if( rc==SQLITE_NOTFOUND ){
        pRbu->pTargetFd = p;
        p->pRbu = pRbu;
        if( p->openFlags & SQLITE_OPEN_MAIN_DB ){
          rbuMainlistAdd(p);
        }
        if( p->pWalFd ) p->pWalFd->pRbu = pRbu;
        rc = SQLITE_OK;
      }
    }
    return rc;
  }
  else if( op==SQLITE_FCNTL_RBUCNT ){

    /* Release the checkpointer and writer locks */
    rbuUnlockShm(p);
    rc = p->pReal->pMethods->xShmUnmap(p->pReal, delFlag);
  }
  return rc;
}















/* 
** A main database named zName has just been opened. The following 
** function returns a pointer to a buffer owned by SQLite that contains
** the name of the *-wal file this db connection will use. SQLite
** happens to pass a pointer to this buffer when using xAccess()
** or xOpen() to operate on the *-wal file.  
*/

      ** (pFd->zWal) to point to a buffer owned by SQLite that contains
      ** the name of the *-wal file this db connection will use. SQLite
      ** happens to pass a pointer to this buffer when using xAccess()
      ** or xOpen() to operate on the *-wal file.  */
      pFd->zWal = rbuMainToWal(zName, flags);
    }
    else if( flags & SQLITE_OPEN_WAL ){
      rbu_file *pDb = rbuFindMaindb(pRbuVfs, zName, 0);
      if( pDb ){
        if( pDb->pRbu && pDb->pRbu->eStage==RBU_STAGE_OAL ){
          /* This call is to open a *-wal file. Intead, open the *-oal. This
          ** code ensures that the string passed to xOpen() is terminated by a
          ** pair of '\0' bytes in case the VFS attempts to extract a URI 
          ** parameter from it.  */
          const char *zBase = zName;

  }
  if( pFd->pReal->pMethods ){
    /* The xOpen() operation has succeeded. Set the sqlite3_file.pMethods
    ** pointer and, if the file is a main database file, link it into the
    ** mutex protected linked list of all such files.  */
    pFile->pMethods = &rbuvfs_io_methods;
    if( flags & SQLITE_OPEN_MAIN_DB ){


      rbuMainlistAdd(pFd);

    }
  }else{
    sqlite3_free(pFd->zDel);
  }

  return rc;
}

  **
  **   b) if the *-wal file does not exist, claim that it does anyway,
  **      causing SQLite to call xOpen() to open it. This call will also
  **      be intercepted (see the rbuVfsOpen() function) and the *-oal
  **      file opened instead.
  */
  if( rc==SQLITE_OK && flags==SQLITE_ACCESS_EXISTS ){
    rbu_file *pDb = rbuFindMaindb(pRbuVfs, zPath, 1);
    if( pDb && pDb->pRbu && pDb->pRbu->eStage==RBU_STAGE_OAL ){
      if( *pResOut ){
        rc = SQLITE_CANTOPEN;
      }else{
        sqlite3_int64 sz = 0;
        rc = rbuVfsFileSize(&pDb->base, &sz);
        *pResOut = (sz>0);