rc = sqlite3OsSync(pWal->pDbFd, sync_flags);
        }
      }
    }

    /* Release the reader lock held while backfilling */
    walUnlockExclusive(pWal, WAL_READ_LOCK(0), 1);





  }

  walIteratorFree(pIter);
  return rc;
}

/*

          rc = sqlite3OsSync(pWal->pDbFd, sync_flags);
        }
      }
    }

    /* Release the reader lock held while backfilling */
    walUnlockExclusive(pWal, WAL_READ_LOCK(0), 1);
  }else{
    /* Reset the return code so as not to report a checkpoint failure
    ** just because active readers prevent any backfill.
    */
    rc = SQLITE_OK;
  }

  walIteratorFree(pIter);
  return rc;
}

/*

  # checkpointing the database. But not from writing to it.
  #
  do_test wal-10.$tn.11 {
    sql2 { BEGIN; SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10}
  do_test wal-10.$tn.12 {
    catchsql { PRAGMA wal_checkpoint } 
  } {1 {database is locked}}
  do_test wal-10.$tn.13 {
    execsql { INSERT INTO t1 VALUES(11, 12) }
    sql2 {SELECT * FROM t1}
  } {1 2 3 4 5 6 7 8 9 10}

  # Connection [db2] is holding a lock on a snapshot, preventing [db] from
  # checkpointing the database. Add a busy-handler to [db]. If [db2] completes
  # its transaction from within the busy-handler, [db] is able to complete
  # the checkpoint operation.
  #
  proc busyhandler x {
    if {$x==4} { sql2 COMMIT }
    if {$x<5} { return 0 }
    return 1
  }
  db busy busyhandler
  do_test wal-10.$tn.14 {
    execsql { PRAGMA wal_checkpoint } 
  } {}

  # Similar to the test above. Except this time, a new read transaction is
  # started (db3) while the checkpointer is waiting for an old one (db2) to 
  # finish. The checkpointer can finish, but any subsequent write operations 
  # must wait until after db3 has closed the read transaction, as db3 is a
  # "region D" writer.
  #
  db busy {}
  do_test wal-10.$tn.15 {
    sql2 { BEGIN; SELECT * FROM t1; }
  } {1 2 3 4 5 6 7 8 9 10 11 12}
  do_test wal-10.$tn.16 {
    catchsql { PRAGMA wal_checkpoint } 
  } {1 {database is locked}}
  proc busyhandler x {
    if {$x==3} { sql3 { BEGIN; SELECT * FROM t1 } }
    if {$x==4} { sql2 COMMIT }

  # checkpointing the database. But not from writing to it.
  #
  do_test wal-10.$tn.11 {
    sql2 { BEGIN; SELECT * FROM t1 }
  } {1 2 3 4 5 6 7 8 9 10}
  do_test wal-10.$tn.12 {
    catchsql { PRAGMA wal_checkpoint } 
  } {0 {}}   ;# Reader no longer block checkpoints
  do_test wal-10.$tn.13 {
    execsql { INSERT INTO t1 VALUES(11, 12) }
    sql2 {SELECT * FROM t1}
  } {1 2 3 4 5 6 7 8 9 10}

  # Writers do not block checkpoints any more either.



  #






  do_test wal-10.$tn.14 {
    catchsql { PRAGMA wal_checkpoint } 
  } {0 {}}

  # Similar to the test above. Except this time, a new read transaction is
  # started (db3) while the checkpointer is waiting for an old one (db2) to 
  # finish. The checkpointer can finish, but any subsequent write operations 
  # must wait until after db3 has closed the read transaction, as db3 is a
  # "region D" writer.
  #

  do_test wal-10.$tn.15 {
    sql2 { COMMIT; BEGIN; SELECT * FROM t1; }
  } {1 2 3 4 5 6 7 8 9 10 11 12}
  do_test wal-10.$tn.16 {
    catchsql { PRAGMA wal_checkpoint } 
  } {1 {database is locked}}
  proc busyhandler x {
    if {$x==3} { sql3 { BEGIN; SELECT * FROM t1 } }
    if {$x==4} { sql2 COMMIT }