SQLite

  if( !sqlite3SafetyCheckOk(db) ) return SQLITE_MISUSE_BKPT;
#endif

  /* Initialize the output variables to -1 in case an error occurs. */
  if( pnLog ) *pnLog = -1;
  if( pnCkpt ) *pnCkpt = -1;

  assert( SQLITE_CHECKPOINT_PASSIVE==0 );
  assert( SQLITE_CHECKPOINT_FULL==1 );
  assert( SQLITE_CHECKPOINT_RESTART==2 );
  assert( SQLITE_CHECKPOINT_TRUNCATE==3 );
  if( eMode<SQLITE_CHECKPOINT_PASSIVE || eMode>SQLITE_CHECKPOINT_TRUNCATE ){
    return SQLITE_MISUSE;
  }

  sqlite3_mutex_enter(db->mutex);
  if( zDb && zDb[0] ){
    iDb = sqlite3FindDbName(db, zDb);
  }

    }
  }
  break;
#endif /* SQLITE_OMIT_COMPILEOPTION_DIAGS */

#ifndef SQLITE_OMIT_WAL
  /*
  **   PRAGMA [database.]wal_checkpoint = passive|full|restart|truncate
  **
  ** Checkpoint the database.
  */
  case PragTyp_WAL_CHECKPOINT: {
    int iBt = (pId2->z?iDb:SQLITE_MAX_ATTACHED);
    int eMode = SQLITE_CHECKPOINT_PASSIVE;
    if( zRight ){
      if( sqlite3StrICmp(zRight, "full")==0 ){
        eMode = SQLITE_CHECKPOINT_FULL;
      }else if( sqlite3StrICmp(zRight, "restart")==0 ){
        eMode = SQLITE_CHECKPOINT_RESTART;
      }else if( sqlite3StrICmp(zRight, "truncate")==0 ){
        eMode = SQLITE_CHECKPOINT_TRUNCATE;
      }
    }
    sqlite3VdbeSetNumCols(v, 3);
    pParse->nMem = 3;
    sqlite3VdbeSetColName(v, 0, COLNAME_NAME, "busy", SQLITE_STATIC);
    sqlite3VdbeSetColName(v, 1, COLNAME_NAME, "log", SQLITE_STATIC);
    sqlite3VdbeSetColName(v, 2, COLNAME_NAME, "checkpointed", SQLITE_STATIC);

**   This mode works the same way as SQLITE_CHECKPOINT_FULL, except after 
**   checkpointing the log file it blocks (calls the 
**   [sqlite3_busy_handler|busy-handler callback])
**   until all readers are reading from the database file only. This ensures 
**   that the next client to write to the database file restarts the log file 
**   from the beginning. This call blocks database writers while it is running,
**   but not database readers.
**
** <dt>SQLITE_CHECKPOINT_TRUNCATE<dd>
**   This mode works the same way as SQLITE_CHECKPOINT_RESTART except that,
**   if successful, it also truncates the log file to zero bytes in size.
** </dl>
**
** If pnLog is not NULL, then *pnLog is set to the total number of frames in
** the log file before returning. If pnCkpt is not NULL, then *pnCkpt is set to
** the total number of checkpointed frames (including any that were already
** checkpointed when this function is called). *pnLog and *pnCkpt may be
** populated even if sqlite3_wal_checkpoint_v2() returns other than SQLITE_OK.
** If no values are available because of an error, they are both set to -1
** before returning to communicate this to the caller.
**
** All calls obtain an exclusive "checkpoint" lock on the database file. If
** any other process is running a checkpoint operation at the same time, the 
** lock cannot be obtained and SQLITE_BUSY is returned. Even if there is a 
** busy-handler configured, it will not be invoked in this case.
**
** The SQLITE_CHECKPOINT_FULL, RESTART and TRUNCATE modes also obtain the 
** exclusive "writer" lock on the database file. If the writer lock cannot be
** obtained immediately, and a busy-handler is configured, it is invoked and
** the writer lock retried until either the busy-handler returns 0 or the lock
** is successfully obtained. The busy-handler is also invoked while waiting for
** database readers as described above. If the busy-handler returns 0 before
** the writer lock is obtained or while waiting for database readers, the
** checkpoint operation proceeds from that point in the same way as 
** SQLITE_CHECKPOINT_PASSIVE - checkpointing as many frames as possible 
** without blocking any further. SQLITE_BUSY is returned in this case.
**
** If parameter zDb is NULL or points to a zero length string, then the

** CAPI3REF: Checkpoint operation parameters
**
** These constants can be used as the 3rd parameter to
** [sqlite3_wal_checkpoint_v2()].  See the [sqlite3_wal_checkpoint_v2()]
** documentation for additional information about the meaning and use of
** each of these values.
*/
#define SQLITE_CHECKPOINT_PASSIVE  0
#define SQLITE_CHECKPOINT_FULL     1
#define SQLITE_CHECKPOINT_RESTART  2
#define SQLITE_CHECKPOINT_TRUNCATE 3

/*
** CAPI3REF: Virtual Table Interface Configuration
**
** This function may be called by either the [xConnect] or [xCreate] method
** of a [virtual table] implementation to configure
** various facets of the virtual table interface.

  int rc;

  int eMode;
  int nLog = -555;
  int nCkpt = -555;
  Tcl_Obj *pRet;

  const char * aMode[] = { "passive", "full", "restart", "truncate", 0 };
  assert( SQLITE_CHECKPOINT_PASSIVE==0 );
  assert( SQLITE_CHECKPOINT_FULL==1 );
  assert( SQLITE_CHECKPOINT_RESTART==2 );
  assert( SQLITE_CHECKPOINT_TRUNCATE==3 );

  if( objc!=3 && objc!=4 ){
    Tcl_WrongNumArgs(interp, 1, objv, "DB MODE ?NAME?");
    return TCL_ERROR;
  }

  if( objc==4 ){

  assert( p->readOnly==0 );
  aRes[0] = 0;
  aRes[1] = aRes[2] = -1;
  assert( pOp->p2==SQLITE_CHECKPOINT_PASSIVE
       || pOp->p2==SQLITE_CHECKPOINT_FULL
       || pOp->p2==SQLITE_CHECKPOINT_RESTART
       || pOp->p2==SQLITE_CHECKPOINT_TRUNCATE
  );
  rc = sqlite3Checkpoint(db, pOp->p1, pOp->p2, &aRes[1], &aRes[2]);
  if( rc==SQLITE_BUSY ){
    rc = SQLITE_OK;
    aRes[0] = 1;
  }
  for(i=0, pMem = &aMem[pOp->p3]; i<3; i++, pMem++){

/*
** The cache of the wal-index header must be valid to call this function.
** Return the page-size in bytes used by the database.
*/
static int walPagesize(Wal *pWal){
  return (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
}

/*
** The following is guaranteed when this function is called:
**
**   a) the WRITER lock is held,
**   b) the entire log file has been checkpointed, and
**   c) any existing readers are reading exclusively from the database
**      file - there are no readers that may attempt to read a frame from
**      the log file.
**
** This function updates the shared-memory structures so that the next
** client to write to the database (which may be this one) does so by
** writing frames into the start of the log file.
**
** The value of parameter salt1 is used as the aSalt[1] value in the 
** new wal-index header. It should be passed a pseudo-random value (i.e. 
** one obtained from sqlite3_randomness()).
*/
static void walRestartHdr(Wal *pWal, u32 salt1){
  volatile WalCkptInfo *pInfo = walCkptInfo(pWal);
  int i;                          /* Loop counter */
  u32 *aSalt = pWal->hdr.aSalt;   /* Big-endian salt values */
  pWal->nCkpt++;
  pWal->hdr.mxFrame = 0;
  sqlite3Put4byte((u8*)&aSalt[0], 1 + sqlite3Get4byte((u8*)&aSalt[0]));
  memcpy(&pWal->hdr.aSalt[1], &salt1, 4);
  walIndexWriteHdr(pWal);
  pInfo->nBackfill = 0;
  pInfo->aReadMark[1] = 0;
  for(i=2; i<WAL_NREADER; i++) pInfo->aReadMark[i] = READMARK_NOT_USED;
  assert( pInfo->aReadMark[0]==0 );
}

/*
** Copy as much content as we can from the WAL back into the database file
** in response to an sqlite3_wal_checkpoint() request or the equivalent.
**
** The amount of information copies from WAL to database might be limited
** by active readers.  This routine will never overwrite a database page

  if( rc==SQLITE_BUSY ){
    /* Reset the return code so as not to report a checkpoint failure
    ** just because there are active readers.  */
    rc = SQLITE_OK;
  }

  /* If this is an SQLITE_CHECKPOINT_RESTART or TRUNCATE operation, and the
  ** entire wal file has been copied into the database file, then block 
  ** until all readers have finished using the wal file. This ensures that 
  ** the next process to write to the database restarts the wal file.
  */
  if( rc==SQLITE_OK && eMode!=SQLITE_CHECKPOINT_PASSIVE ){
    assert( pWal->writeLock );
    if( pInfo->nBackfill<pWal->hdr.mxFrame ){
      rc = SQLITE_BUSY;
    }else if( eMode>=SQLITE_CHECKPOINT_RESTART ){
      u32 salt1;
      sqlite3_randomness(4, &salt1);
      assert( mxSafeFrame==pWal->hdr.mxFrame );
      rc = walBusyLock(pWal, xBusy, pBusyArg, WAL_READ_LOCK(1), WAL_NREADER-1);
      if( rc==SQLITE_OK ){
        if( eMode==SQLITE_CHECKPOINT_TRUNCATE ){
          /* If this is a TRUNCATE checkpoint, also truncate the wal file
          ** to zero bytes in size on disk. 
          **
          ** In theory, it might be safe to do this without updating the
          ** wal-index header in shared memory, as all subsequent reader or
          ** writer clients should see that the entire log file has been
          ** checkpointed and behave accordingly. This seems unsafe though,
          ** as it would leave the system in a state where the contents of
          ** the wal-index header do not match the contents of the 
          ** file-system. To avoid this, update the wal-index header to
          ** indicate that the log file contains zero valid frames.  */
          walRestartHdr(pWal, salt1);
          rc = sqlite3OsTruncate(pWal->pWalFd, 0);
        }
        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }
    }
  }

 walcheckpoint_out:
  walIteratorFree(pIter);

    pWal->hdr.aFrameCksum[1] = aWalData[2];
    walCleanupHash(pWal);
  }

  return rc;
}


/*
** This function is called just before writing a set of frames to the log
** file (see sqlite3WalFrames()). It checks to see if, instead of appending
** to the current log file, it is possible to overwrite the start of the
** existing log file with the new frames (i.e. "reset" the log). If so,
** it sets pWal->hdr.mxFrame to 0. Otherwise, pWal->hdr.mxFrame is left
** unchanged.

        ** readers are currently using the WAL), then the transactions
        ** frames will overwrite the start of the existing log. Update the
        ** wal-index header to reflect this.
        **
        ** In theory it would be Ok to update the cache of the header only
        ** at this point. But updating the actual wal-index header is also
        ** safe and means there is no special case for sqlite3WalUndo()
        ** to handle if this transaction is rolled back.  */



        walRestartHdr(pWal, salt1);









        walUnlockExclusive(pWal, WAL_READ_LOCK(1), WAL_NREADER-1);
      }else if( rc!=SQLITE_BUSY ){
        return rc;
      }
    }
    walUnlockShared(pWal, WAL_READ_LOCK(0));
    pWal->readLock = -1;

    proc do_wal_checkpoint { dbhandle args } {
      set a(-mode) passive
      array set a $args
      foreach key [array names a] {
        if {[lsearch {-mode -db} $key]<0} { error "unknown switch: $key" }
      }

      set vals {restart full truncate}
      if {[lsearch -exact $vals $a(-mode)]<0} { set a(-mode) passive }

      set cmd [list sqlite3_wal_checkpoint_v2 $dbhandle $a(-mode)]
      if {[info exists a(-db)]} { lappend sql $a(-db) }

      uplevel $cmd
    }
  }

    6   FULL      3   {0 4 4}   2

    7   RESTART   -   {0 4 4}   3
    8   RESTART   1   {1 3 3}   1
    9   RESTART   2   {1 4 3}   2
    10  RESTART   3   {1 4 4}   3

    11  TRUNCATE  -   {0 0 0}   3
    12  TRUNCATE  1   {1 3 3}   1
    13  TRUNCATE  2   {1 4 3}   2
    14  TRUNCATE  3   {1 4 4}   3

  } {
    do_multiclient_test tn {
      setup_and_attach_aux

      proc busyhandler {x} {
        set ::max_busyhandler $x
        if {$::busy_on!="-" && $x==$::busy_on} { return 1 }

    code1 {sqlite3 db  test.db}
    code2 {sqlite3 db2 test.db}
    code3 {sqlite3 db3 test.db}

    do_test 3.$tn.5 { sql3 { PRAGMA journal_mode } } {wal}

    do_test 3.$tn.6 { code3 { do_wal_checkpoint db3 } } {0 0 0}
  }

  # Test SQLITE_CHECKPOINT_TRUNCATE.
  #
  do_multiclient_test tn {

    code1 $do_wal_checkpoint
    code2 $do_wal_checkpoint
    code3 $do_wal_checkpoint

    do_test 3.$tn.1 {
      sql1 {
        PRAGMA page_size = 1024;
        PRAGMA journal_mode = WAL;
        PRAGMA synchronous = normal;
        CREATE TABLE t1(x, y);
        CREATE INDEX i1 ON t1(x, y);
        INSERT INTO t1 VALUES(1, 2);
        INSERT INTO t1 VALUES(3, 4);
      }
      file size test.db-wal
    } [wal_file_size 8 1024]

    do_test 3.$tn.2 { do_wal_checkpoint db -mode truncate } {0 0 0}
    do_test 3.$tn.3 { file size test.db-wal } 0

    do_test 3.$tn.4 {
      sql2 { SELECT * FROM t1 }
    } {1 2 3 4}

    do_test 3.$tn.5 {
      sql2 { INSERT INTO t1 VALUES('a', 'b') }
      file size test.db-wal
    } [wal_file_size 2 1024]

  }
}


finish_test