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MariaDB server is a community developed fork of MySQL server. Started by core members of the original MySQL team, MariaDB actively works with outside developers to deliver the most featureful, stable, and sanely licensed open SQL server in the industry.
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This commit implements mysql/mysql-server@7037a0bdc8 functionality, i.e. if some transaction A holds not-gap S-lock on some record, and some other transactions B={b1, b2, ..., bn} have not-gap X-locks waiting for the S-lock of transaction A, and transaction A requests not-gap and not insert intention X-lock which conflicts with the X-locks of transactions B and does not conflict with another locks in the queue, then grant the X-lock to transaction A. MySQL's commit contains the following explanation of why insert-intention locks must not overtake a waiting ordinary or gap locks: "It is important that this decission rule doesn't allow INSERT_INTENTION locks to overtake WAITING locks on gaps (`S`, `S|GAP`, `X`, `X|GAP`), as inserting a record into a gap would split such WAITING lock, violating the invariant that each transaction can have at most single WAITING lock at any time." I would add to the explanation the following. Suppose we have trx 1 which holds ordinary X-lock on some record. And trx 2 executes "DELETE FROM t" or "SELECT * FOR UPDATE" in RR(see lock_delete_updated.test and MDEV-27992), i.e. it creates waiting ordinary X-lock on the same record. And then trx 1 wants to insert some record just before the locked record. It requests insert-intention lock, and if the lock overtakes trx 2 lock, there will be phantom records for trx 2 in RR. lock_delete_updated.test shows how "DELETE" allows to insert some records in already scanned gap and misses some records to delete. The current implementation differs from MySQL implementation. There are two key differences: 1. Lock queue ordering. In MySQL all waiting locks precede all granted locks. A new waiting lock is added to the head of the queue, a new granted lock is added to the end of the queue, if some waiting lock is granted, it's moved to the end of the queue. In MariaDB any new lock is added to the end of the queue and waiting lock does not change its position in the queue where the lock is granted. The rule is that blocking lock must be located before blocked lock in lock queue. We maintain the rule with inserting bypassing lock just before bypassed one. 2. MySQL implementation uses some object(locksys::Trx_locks_cache) which can be passed to consecutive calls to rec_lock_has_to_wait() for the same trx and heap_no to cache the result of checking if trx has a granted lock which is blocking the waiting lock(see locksys::Trx_locks_cache::has_granted_blocker()). The current implementation does not use such object, because it looks for such granted lock on the level of lock_rec_other_has_conflicting() and lock_rec_has_to_wait_in_queue(). I.e. there is no need in additional lock queue iteration in locksys::Trx_locks_cache::has_granted_blocker(), as we already iterate it in lock_rec_other_has_conflicting() and lock_rec_has_to_wait_in_queue(). During the testing the following case was found. Suppose we have delete-marked record and going to do inplace insert into that delete-marked record. Usually we don't create explicit lock if there are no conlicting with not gap X-lock locks(see lock_clust_rec_modify_check_and_lock(), btr_cur_update_in_place()). The implicit lock will be converted to explicit one by demand. That can happen during INSERT, the not-gap S-lock can be acquired on searching for duplicates(see row_ins_duplicate_error_in_clust()), and, if delete-marked record is found, inplace insert(see btr_cur_upd_rec_in_place()) modifies the record, what is treated as implicit lock. But there can be a case when some transaction trx1 holds not-gap S-lock, another transaction trx2 creates waiting X-lock, and then trx2 tries to do inplace insert. Before the fix the waiting X-lock of trx2 would be conflicting lock, and trx1 would try to create explicit X-lock, what would cause deadlock, and one of the transactions whould be rolled back. But after the fix, trx2 waiting X-lock is not treated as conflicting with trx1 X-lock anymore, as trx1 already holds S-lock. If we don't create explicit lock, then some other transaction trx3 can create it during implicit to explicit lock conversion and place it at the end of the queue. So there can be the following locks order in the queue: S1(granted) X2(waiting) X1(granted) The above queue is not valid, because all granted trx1 locks must be placed before waiting trx2 lock. Besides, lock_rec_release_try() can remove S(granted, trx1) lock and grant X lock to trx 2, and there can be two granted X-locks on the same record: X2(granted) X1(granted) Taking into account that lock_rec_release_try() can release cell and lock_sys latches leaving some locks unreleased, the queue validation function can fail in any unexpected place. It can be fixed with two ways: 1) Place explicit X(granted, trx1) lock before X(waiting, trx2) lock during implicit to explicit lock conversion. This option is implemented in MySQL, as granted lock is always placed at the top of locks queue, and waiting locks are placed at the bottom of the queue. MariaDB does not do this, and implementing this variant would require conflicting locks search before converting implicit to explicit lock, what, in turns, would require cell and/or lock_sys latch acquiring. 2) Create and place X(granted, trx1) lock before X(waiting, trx2) during inplace INSERT, i.e. when lock_rec_lock() is invoked from lock_clust_rec_modify_check_and_lock() or lock_sec_rec_modify_check_and_lock(), if X(waiting, trx2) is bypassed. Such a way we don't need in additional conflicting locks search, as they are searched anyway in lock_rec_low(). This fix implements the second variant(see the changes around c_lock_info.insert_after in lock_rec_lock). I.e. if some record was delete-marked and we do inplace insert in such a record, and some lock for bypass was found, create explicit lock to avoid conflicting lock search on each implicit to explicit lock conversion. We can remove it if MDEV-35624 is implemented. lock_rec_other_has_conflicting(), lock_rec_has_to_wait_in_queue(): search locks to bypass along with conflicting locks searching in the same loop. The result is returned in conflicting_lock_info object. There can be several locks to bypass, only the first one is returned to limit lock_rec_find_similar_on_page() with the first bypassed lock to preserve "blocking before blocked" invariant. conflicting_lock_info also contains a pointer to the lock, after which we can insert bypassing lock. This lock precedes bypassed one. Bypassing lock can be next-key lock, and the following cases are possible: 1. S1(not-gap, granted) II2(granted) X3(waiting for S1), When new X1(ordinary) lock is acquired, there will be the following locks queue: S1(not-gap, granted) II2(granted) X1(ordinary, granted) X3(waiting for S1) If we had inserted new X1 lock just after S1, and S1 had been released on transaction commit or rollback, we would have the following sequence in the locks queue: X1(ordinary, granted) II2(granted) X3(waiting for X1) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ This is not a real issue as II lock once granted can be ignored but it could possibly hit some assert(taking into account that lock_release_try() can release lock_sys latch, and other threads can acquire the latch and validate lock queue) as it breaks our design constraint that any granted lock in the queue should not conflict with locks ahead in the queue. But lock_rec_queue_validate() does not check the above constraint. We place new bypassing lock just before bypassed one, but there still can be the case when lock bitmap is used instead of creating new lock object(see lock_rec_add_to_queue() and lock_rec_find_similar_on_page()), and the lock, which owns the bitmap, can precede II2(granted). We can either disable lock_rec_find_similar_on_page() space optimization for bypassing locks or treat "X1(ordinary, granted) II2(granted)" sequence as valid. As we don't currently have the function which would fail on the above sequence, let treat it as valid for the case, when lock_release() execution is in process. 2. S1(ordinary, granted) II2(waiting for S1) X3(waiting for S1) When new X1(ordinary) lock is acquired, there will be the following locks queue: S1(ordinary, granted) II2(waiting for S1) X1(ordinary, granted) X3(waiting for S1). After S1 releasing there will be: II2(granted) X1(ordinary, granted) X3(waiting for X1) ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The above queue is valid because ordinary lock does not conflict with II-lock(see lock_rec_has_to_wait()). lock_rec_create_low(): insert new lock to the position which lock_rec_other_has_conflicting(), lock_rec_has_to_wait_in_queue() return if the lock is bypassing. lock_rec_find_similar_on_page(): add ability to limit similiar lock search with the certain lock to preserve "blocking before blocked" invariant for all bypassed locks. lock_rec_add_to_queue(): don't treat bypassed locks as waiting ones to let lock bitmap reusing for bypassing locks. lock_rec_lock(): fix inplace insert case, explained above. lock_rec_dequeue_from_page(), lock_rec_rebuild_waiting_queue: move bypassing lock to the correct place to preserve "blocking before blocked" invariant. Reviewed by: Debarun Banerjee, Marko Mäkelä. |
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MariaDB: The innovative open source database
MariaDB was designed as a drop-in replacement of MySQL(R) with more features, new storage engines, fewer bugs, and better performance.
MariaDB is brought to you by the MariaDB Foundation and the MariaDB Corporation. Please read the CREDITS file for details about the MariaDB Foundation, and who is developing MariaDB.
MariaDB is developed by many of the original developers of MySQL who now work for the MariaDB Corporation, the MariaDB Foundation and by many people in the community.
MySQL, which is the base of MariaDB, is a product and trademark of Oracle Corporation, Inc. For a list of developers and other contributors, see the Credits appendix. You can also run 'SHOW authors' to get a list of active contributors.
A description of the MariaDB project and a manual can be found at:
https://mariadb.com/kb/en/mariadb-vs-mysql-features/
https://mariadb.com/kb/en/mariadb-versus-mysql-compatibility/
https://mariadb.com/kb/en/new-and-old-releases/
Getting the code, building it and testing it
Refer to the following guide: https://mariadb.org/get-involved/getting-started-for-developers/get-code-build-test/ which outlines how to build the source code correctly and run the MariaDB testing framework, as well as which branch to target for your contributions.
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