WL#4284: Transactional DDL locking
Affects: Server-5.5 — Status: Complete — Priority: Medium
Split from WL#3726. Implement transaction-long metadata locks. I.e. if a table is used in a transaction, a metadata lock should be kept for this table until the transaction is committed. This will prevent concurrent transactions from issuing a DDL against this table. Necessary to fix BUG#989 If DROP TABLE while there's an active transaction, wrong binlog order
In order to ensure transaction serializability, the server must not allow a concurrent connection to perform a DDL statement on a table that is used in an uncompleted transaction. Currently, for each table used in a statement, the server acquires a shared metadata lock till the end of the statement. When the statement is complete, the shared metadata lock is released. A simple deadlock avoidance technique is employed to address the problem of deadlocks in case of a lock conflict (see http://forge.mysql.com/wiki/MySQL_Internals_Data_and_meta-data_locking for details). In order to ensure serializability, the server must defer the release of metadata locks from the end of a statement till the end of the encompassing transaction. For a statement that doesn't start a transaction, e.g. a statement on a non-transactional table/tables, this brings no change. For a connection that runs in autocommit mode, the change also has no effect. Since each DDL statement commits the transaction, if any, in the beginning, the change has no effect on lock conflicts between concurrent DDL statements. The other class of conflicts is between DDL and DML statements. When a connection doesn't use autocommit and utilizes multi-statement transactions, a new scenario of lock conflicts is introduced. A conflict may happen between the acquirer of a shared lock and a concurrent DDL, at the time when the acquirer has executed several statements and possibly modified or retrieved data. Let's consider an example. Transaction 1 modifies table t1, then table t2, and then commits. The first statement of the transaction will acquire a shared metadata lock on table t1, and it will be kept till COMMIT, in order to ensure serializability. At the moment when the second statement attempts to acquire a shared metadata lock on t2, a concurrent ALTER or DROP statement may have locked t2 exclusively. The prescription of the current locking protocol is that the acquirer of the shared lock backs off -- gives up all his current locks and retries. For a multiple-statement scenario that implies that the entire multi-statement transaction has to be rolled back. Indeed, imagine that the concurrent connection issued RENAME t2 to t3, t1 to t2, t3 to t1, rather than a simple ALTER. In that case it will have acquired an exclusive lock on t2 at the moment it encounters that t1 is locked in shared mode, and the owner is not about to give away its locks. Unless the DML transaction back off, a deadlock is unavoidable. In future we may consider implementing a deadlock detection algorithm that would allow us to detect the scenario described above while allowing for other scenarios of lock conflicts. One possible solution is given in Appendix A. The scope of this task, however, is to simply abort the transaction that attempts to acquire a shared lock. Note, that the abortion only takes place if the transaction has executed statements -- in other words, has acquired shared metadata locks that ought to be released. If a lock conflict happens in the first statement of the transaction, the locking algorithm back off and retries as before. If a user has set a savepoint, ROLLBACK TO SAVEPOINT should release all metadata locks acquired after the savepoint has been set. Apart from these two changes, the locking protocol should be left intact. Appendix A: a possible approach to deadlock detection ----------------------------------------------------- If a shared/exclusive lock conflict occurs when attempting to acquire an exclusive lock, a "pending exclusive" lock is placed instead. This lock is "upgraded" to exclusive when all transactions that own the conflicting lock have committed. Existence of a pending exclusive lock prevents all new transactions from acquiring the lock: they back off. Existing transaction, still, can proceed, as long as new locks that they attempt to acquire do not lead to deadlocks. This latter fact can be checked easily by maintaining the list of waiters on a given lock, and checking if some transaction is present twice in the list.
Extend the metadata locking subsystem to keep metadata locks till the end of a transaction. Changes to existing APIs: - none Changed files and functions: 1) Introduce ha_in_transaction() that would return TRUE if there is a started transaction in a given connection. 2) Store metadata locks in transaction memory root, rather than statement memory root. 3) Modify close_thread_tables() to call mdl_release_locks() only if we're outside the context of some transaction. Since close_thread_tables() is called after each statement, including COMMIT and ROLLBACK, that will ensure that metadata locks are released automatically whenever a transaction has either ended automatically, or was committed/aborted manually. BEGIN word is a no-op at MySQL. Operations on non-transactional engines do not start a transaction either. Therefore, for statements that use non-transactional tables outside the context of some transaction, locks will be released at the end of each statement, just like before. 3) Modify open_table() to raise rollback the transaction and return ER_LOCK_DEADLOCK error in case of a shared/exclusive lock conflict in a non-first statement of a transaction. 4) Modify ha_savepoint() to remember, and ha_rollback_to_savepoint() to rollback to the remembered set of metadata locks. Effect on LOCK TABLES TRANSACTIONAL Effect on other statements None Bug reports: BUG#989 If DROP TABLE while there's an active transaction, wrong binlog order BUG#12347 InnoDB: DROP TABLE ignores locks BUG#39945 "WITH CONSISTENT SNAPSHOT" does not isolate against table changes (i.e. RENAME)
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