SQLite

** locks. These are not magic numbers as they are part of the SQLite file
** format.
*/
#define WAL_LOCK_WRITE  0
#define WAL_LOCK_CKPT   1
#define WAL_LOCK_READ0  3

#define SQLITE_FCNTL_RBUCNT    5149216

/*
** A structure to store values read from the rbu_state table in memory.
*/
struct RbuState {
  int eStage;
  char *zTbl;
  char *zIdx;

  sqlite3_file *pReal;            /* Underlying file handle */
  rbu_vfs *pRbuVfs;               /* Pointer to the rbu_vfs object */
  sqlite3rbu *pRbu;               /* Pointer to rbu object (rbu target only) */

  int openFlags;                  /* Flags this file was opened with */
  u32 iCookie;                    /* Cookie value for main db files */
  u8 iWriteVer;                   /* "write-version" value for main db files */
  u8 bNolock;

  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 */

/*
** Open the database handle and attach the RBU database as "rbu". If an
** error occurs, leave an error code and message in the RBU handle.
*/
static void rbuOpenDatabase(sqlite3rbu *p){
  int nRbu = 0;
  assert( p->rc==SQLITE_OK );
  assert( p->dbMain==0 && p->dbRbu==0 );
  assert( rbuIsVacuum(p) || p->zTarget!=0 );

  /* Open the RBU database */
  p->dbRbu = rbuOpenDbhandle(p, p->zRbu, 1);

  if( rbuIsVacuum(p) ){
    sqlite3_file_control(p->dbRbu, "main", SQLITE_FCNTL_RBUCNT, &nRbu);
  }

  /* If using separate RBU and state databases, attach the state database to
  ** the RBU db handle now.  */
  if( p->zState ){
    rbuMPrintfExec(p, p->dbRbu, "ATTACH %Q AS stat", p->zState);
    memcpy(p->zStateDb, "stat", 4);
  }else{

    }else{
      RbuState *pState = rbuLoadState(p);
      if( pState ){
        bOpen = (pState->eStage>RBU_STAGE_MOVE);
        rbuFreeState(pState);
      }
    }
    if( bOpen ) p->dbMain = rbuOpenDbhandle(p, p->zRbu, 1);
    if( bOpen ) p->dbMain = rbuOpenDbhandle(p, p->zRbu, nRbu<=1);
  }

  p->eStage = 0;
  if( p->dbMain==0 ){
    if( !rbuIsVacuum(p) ){
      p->dbMain = rbuOpenDbhandle(p, p->zTarget, 1);
    }else{
      int frc = sqlite3_file_control(p->dbRbu, "main", SQLITE_FCNTL_ZIPVFS, 0);
      char *zTarget = sqlite3_mprintf("file:%s-vacuum?rbu_memory=1", p->zRbu);
      if( zTarget==0 ){
        p->rc = SQLITE_NOMEM;
        return;
      }
      p->dbMain = rbuOpenDbhandle(p, zTarget, frc!=SQLITE_OK);
      p->dbMain = rbuOpenDbhandle(p, zTarget, nRbu<=1);
      sqlite3_free(zTarget);
      if( p->rc==SQLITE_OK ){
        p->rc = sqlite3_exec(p->dbMain, 
            "PRAGMA journal_mode=off; PRAGMA zipvfs_journal_mode = off;"
            "BEGIN EXCLUSIVE; COMMIT;", 0, 0, 0
        );
      }
    }
  }

  if( p->rc==SQLITE_OK ){
    p->rc = sqlite3_create_function(p->dbMain, 
        "rbu_tmp_insert", -1, SQLITE_UTF8, (void*)p, rbuTmpInsertFunc, 0, 0
    );

      if( p->eStage==RBU_STAGE_OAL ){
        sqlite3 *db = p->dbMain;

        /* Open transactions both databases. The *-oal file is opened or
        ** created at this point. */
        p->rc = sqlite3_exec(db, "BEGIN IMMEDIATE", 0, 0, &p->zErrmsg);
        if( p->rc==SQLITE_OK ){
          p->rc = sqlite3_exec(p->dbRbu, "BEGIN IMMEDIATE", 0, 0, &p->zErrmsg);
          p->rc = sqlite3_exec(p->dbRbu, "BEGIN", 0, 0, &p->zErrmsg);
        }

        /* Check if the main database is a zipvfs db. If it is, set the upper
        ** level pager to use "journal_mode=off". This prevents it from 
        ** generating a large journal using a temp file.  */
        if( p->rc==SQLITE_OK ){
          int frc = sqlite3_file_control(db, "main", SQLITE_FCNTL_ZIPVFS, 0);

*/
static void rbuPutU32(u8 *aBuf, u32 iVal){
  aBuf[0] = (iVal >> 24) & 0xFF;
  aBuf[1] = (iVal >> 16) & 0xFF;
  aBuf[2] = (iVal >>  8) & 0xFF;
  aBuf[3] = (iVal >>  0) & 0xFF;
}

static void rbuPutU16(u8 *aBuf, u16 iVal){
  aBuf[0] = (iVal >>  8) & 0xFF;
  aBuf[1] = (iVal >>  0) & 0xFF;
}

/*
** Read data from an rbuVfs-file.
*/
static int rbuVfsRead(
  sqlite3_file *pFile, 
  void *zBuf, 

     && (p->openFlags & SQLITE_OPEN_WAL) 
     && iOfst>=pRbu->iOalSz 
    ){
      rc = SQLITE_OK;
      memset(zBuf, 0, iAmt);
    }else{
      rc = p->pReal->pMethods->xRead(p->pReal, zBuf, iAmt, iOfst);
#if 1
      /* If this is being called to read the first page of the target 
      ** database as part of an rbu vacuum operation, synthesize the 
      ** contents of the first page if it does not yet exist. Otherwise,
      ** SQLite will not check for a *-wal file.  */
      if( p->pRbu && rbuIsVacuum(p->pRbu) 
          && rc==SQLITE_IOERR_SHORT_READ && iOfst==0
          && (p->openFlags & SQLITE_OPEN_MAIN_DB)
      ){
        sqlite3_file *pFd = 0;
        rc = sqlite3_file_control(
            p->pRbu->dbRbu, "main", SQLITE_FCNTL_FILE_POINTER, (void*)&pFd
        );
        if( rc==SQLITE_OK ){
          rc = pFd->pMethods->xRead(pFd, zBuf, iAmt, iOfst);
        }
        if( rc==SQLITE_OK ){
          u8 *aBuf = (u8*)zBuf;
          rbuPutU32(&aBuf[52], 0);          /* largest root page number */
          rbuPutU32(&aBuf[36], 0);          /* number of free pages */
          rbuPutU32(&aBuf[32], 0);          /* first page on free list trunk */
          rbuPutU32(&aBuf[28], 1);          /* size of db file in pages */

          if( iAmt>100 ){
            assert( iAmt>=101 );
            memset(&aBuf[101], 0, iAmt-101);
            rbuPutU16(&aBuf[105], iAmt & 0xFFFF);
          }
        }
      }
#endif
    }
    if( rc==SQLITE_OK && iOfst==0 && (p->openFlags & SQLITE_OPEN_MAIN_DB) ){
      /* These look like magic numbers. But they are stable, as they are part
       ** of the definition of the SQLite file format, which may not change. */
      u8 *pBuf = (u8*)zBuf;
      p->iCookie = rbuGetU32(&pBuf[24]);
      p->iWriteVer = pBuf[19];

}

/*
** Return the current file-size of an rbuVfs-file.
*/
static int rbuVfsFileSize(sqlite3_file *pFile, sqlite_int64 *pSize){
  rbu_file *p = (rbu_file *)pFile;
  int rc;
  return p->pReal->pMethods->xFileSize(p->pReal, pSize);
  rc = p->pReal->pMethods->xFileSize(p->pReal, pSize);

  /* If this is an RBU vacuum operation and this is the target database,
  ** pretend that it has at least one page. Otherwise, SQLite will not
  ** check for the existance of a *-wal file. rbuVfsRead() contains 
  ** similar logic.  */
  if( rc==SQLITE_OK && *pSize==0 
   && p->pRbu && rbuIsVacuum(p->pRbu) 
   && (p->openFlags & SQLITE_OPEN_MAIN_DB)
  ){
    *pSize = 1024;
  }
  return rc;
}

/*
** Lock an rbuVfs-file.
*/
static int rbuVfsLock(sqlite3_file *pFile, int eLock){
  rbu_file *p = (rbu_file*)pFile;
  sqlite3rbu *pRbu = p->pRbu;
  int rc = SQLITE_OK;

  assert( p->openFlags & (SQLITE_OPEN_MAIN_DB|SQLITE_OPEN_TEMP_DB) );
  if( pRbu && eLock==SQLITE_LOCK_EXCLUSIVE && pRbu->eStage!=RBU_STAGE_DONE ){
  if( eLock==SQLITE_LOCK_EXCLUSIVE 
   && (p->bNolock || (pRbu && pRbu->eStage!=RBU_STAGE_DONE))
  ){
    /* Do not allow EXCLUSIVE locks. Preventing SQLite from taking this 
    ** prevents it from checkpointing the database from sqlite3_close(). */
    rc = SQLITE_BUSY;
  }else{
    rc = p->pReal->pMethods->xLock(p->pReal, eLock);
  }

        pRbu->pTargetFd = p;
        p->pRbu = pRbu;
        if( p->pWalFd ) p->pWalFd->pRbu = pRbu;
        rc = SQLITE_OK;
      }
    }
    return rc;
  }
  else if( op==SQLITE_FCNTL_RBUCNT ){
    int *pnRbu = (int*)pArg;
    (*pnRbu)++;
    p->bNolock = 1;
  }

  rc = xControl(p->pReal, op, pArg);
  if( rc==SQLITE_OK && op==SQLITE_FCNTL_VFSNAME ){
    rbu_vfs *pRbuVfs = p->pRbuVfs;
    char *zIn = *(char**)pArg;
    char *zOut = sqlite3_mprintf("rbu(%s)/%z", pRbuVfs->base.zName, zIn);