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MDEV-30225 RR isolation violation with locking unique search

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Before the fix next-key lock was requested only if a record was
delete-marked for locking unique search in RR isolation level.
There can be several delete-marked records for the same unique key,
that's why InnoDB scans the records until eighter non-delete-marked record
is reached or all delete-marked records with the same unique key are
scanned.

For range scan next-key locks are used for RR to protect scanned range from
inserting new records by other transactions. And this is the reason of why
next-key locks are used for delete-marked records for unique searches.

If a record is not delete-marked, the requested lock type was "not-gap".
When a record is not delete-marked during lock request by trx 1, and
some other transaction holds conflicting lock, trx 1 creates waiting
not-gap lock on the record and suspends. During trx 1 suspending the
record can be delete-marked. And when the lock is granted on conflicting
transaction commit or rollback, its type is still "not-gap". So we have
"not-gap" lock on delete-marked record for RR. And this let some other
transaction to insert some record with the same unique key when trx 1 is
not committed, what can cause isolation level violation.

The fix is to set next-key locks for both delete-marked and
non-delete-marked records for unique search in RR.
This commit is contained in:
Vlad Lesin
2022-12-13 16:06:13 +03:00
parent 3f63aa18a7
commit 3ddc00dc3b
7 changed files with 119 additions and 2 deletions
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4
mysql-test/suite/innodb/t/cursor-restore-locking.test
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@ -27,6 +27,10 @@ SET DEBUG_SYNC = 'ib_after_row_insert SIGNAL first_ins_row_inserted WAIT_FOR fir
--send INSERT INTO t VALUES(10, 20)
--connect(con_del_2,localhost,root,,)
# After MDEV-30225 is fixed, the following DELETE creates next-key lock for
# unqique search for RR, and the above INSERT kills it as deadlock victim.
# But it still requests not-gap lock for RC.
SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
SET DEBUG_SYNC = 'now WAIT_FOR first_ins_locked';
SET DEBUG_SYNC = 'lock_wait_suspend_thread_enter SIGNAL second_del_locked';
###############################################################################