SQLite

       /* Normal reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B))

       /* Region D reader lock operations */
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_D))
       || (op==LOG_RDLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_D))

       /* Checkpointer lock operations */
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_WRLOCK && mRegion==(LOG_REGION_A))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_B|LOG_REGION_C))
       || (op==LOG_UNLOCK && mRegion==(LOG_REGION_A|LOG_REGION_B|LOG_REGION_C))

*/
void sqlite3LogMaxpgno(Log *pLog, Pgno *pPgno){
  assert( pLog->isLocked );
  *pPgno = pLog->hdr.nPage;
}

/* 



** This function returns SQLITE_OK if the caller may write to the database.
** Otherwise, if the caller is operating on a snapshot that has already
** been overwritten by another writer, SQLITE_BUSY is returned.
*/
int sqlite3LogWriteLock(Log *pLog, int op){
  assert( pLog->isLocked );
  if( op ){

    /* Obtain the writer lock */
    int rc = logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_WRLOCK);
    if( rc!=SQLITE_OK ){
      return rc;
    }

    /* If this is connection is a region D, then the SHARED lock on region
    ** D has just been upgraded to EXCLUSIVE. But no lock at all is held on
    ** region A. This means that if the write-transaction is committed
    ** and this connection downgrades to a reader, it will be left with no
    ** lock at all. And its snapshot could get clobbered by a checkpoint
    ** operation. 
    **
    ** To stop this from happening, grab a SHARED lock on region A now.
    ** This should always be successful, as the only time a client holds
    ** an EXCLUSIVE lock on region A, it must also be holding an EXCLUSIVE
    ** lock on region C (a checkpointer does this). This is not possible,
    ** as this connection currently has the EXCLUSIVE lock on region C.
    */
    if( pLog->isLocked==LOG_REGION_D ){
      logLockRegion(pLog, LOG_REGION_A, LOG_RDLOCK);
      pLog->isLocked = LOG_REGION_A;
    }

    if( memcmp(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr)) ){
      logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_UNLOCK);
      return SQLITE_BUSY;
    }
    pLog->isWriteLocked = 1;

  }else if( pLog->isWriteLocked ){
    logLockRegion(pLog, LOG_REGION_C|LOG_REGION_D, LOG_UNLOCK);
    memcpy(&pLog->hdr, pLog->pSummary->aData, sizeof(pLog->hdr));

  proc busyhandler x {
    if {$x==3} { sql3 { BEGIN; SELECT * FROM t1 } }
    if {$x==4} { sql2 COMMIT }
    if {$x<5}  { return 0 }
    return 1
  }
  db busy busyhandler
  do_test wal-10.$tn.17 {
    execsql { PRAGMA checkpoint } 
  } {}
  do_test wal-10.$tn.18 {
    sql3 { SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10 11 12}
  do_test wal-10.$tn.19 {
    catchsql { INSERT INTO t1 VALUES(13, 14) }
  } {1 {database is locked}}
  do_test wal-10.$tn.20 {
    execsql { SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10 11 12}
  do_test wal-10.$tn.21 {
    sql3 COMMIT
  } {}
  do_test wal-10.$tn.22 {
    execsql { INSERT INTO t1 VALUES(13, 14) }
    execsql { SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10 11 12 13 14}

  # Set [db3] up as a "region D" reader again. Then upgrade it to a writer
  # and back down to a reader. Then, check that a checkpoint is not possible
  # (as [db3] still has a snapshot locked).
  #
  do_test wal-10.$tn.23 {
    execsql { PRAGMA checkpoint }
  } {}
  do_test wal-10.$tn.24 {
    sql2 { BEGIN; SELECT * FROM t1; }
  } {1 2 3 4 5 6 7 8 9 10 11 12 13 14}
  do_test wal-10.$tn.25 {
    execsql { PRAGMA checkpoint }
  } {}
  do_test wal-10.$tn.26 {
    catchsql { INSERT INTO t1 VALUES(15, 16) }
  } {1 {database is locked}}
  do_test wal-10.$tn.27 {
    sql3 { INSERT INTO t1 VALUES(15, 16) }
  } {}
  do_test wal-10.$tn.28 {
    code3 {
      set ::STMT [sqlite3_prepare db3 "SELECT * FROM t1" -1 TAIL]
      sqlite3_step $::STMT
    }
    sql3 COMMIT
    execsql { SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16}
  db busy {}
  do_test wal-10.$tn.29 {
    execsql { INSERT INTO t1 VALUES(17, 18) }
    catchsql { PRAGMA checkpoint }
  } {1 {database is locked}}
  do_test wal-10.$tn.30 {
    code3 { sqlite3_finalize $::STMT }
    execsql { PRAGMA checkpoint }
  } {}

  # At one point, if a reader failed to upgrade to a writer because it
  # was reading an old snapshot, the write-locks were not being released.
  # Test that this bug has been fixed.
  #
  do_test wal-10.$tn.31 {
    execsql { BEGIN ; SELECT * FROM t1 }
    sql2 { INSERT INTO t1 VALUES(19, 20) }
    catchsql { INSERT INTO t1 VALUES(21, 22) }
  } {1 {database is locked}}
  do_test wal-10.$tn.32 {
    # This statement would fail when the bug was present.
    sql2 { INSERT INTO t1 VALUES(21, 22) }
  } {}
  do_test wal-10.$tn.33 {
    execsql { SELECT * FROM t1 ; COMMIT }
  } {1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18}
  do_test wal-10.$tn.34 {
    execsql { SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22}

  catch { db close }
  catch { code2 { db2 close } }
  catch { code3 { db3 close } }
  catch { close $::code2_chan }
  catch { close $::code3_chan }
}