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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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35
mysql-test/suite/innodb/r/insert-before-delete.result Normal file
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connect pause_purge,localhost,root;
START TRANSACTION WITH CONSISTENT SNAPSHOT;
connection default;
CREATE TABLE t (pk int PRIMARY KEY, sk INT UNIQUE) ENGINE=InnoDB;
INSERT INTO t VALUES (10, 100);
connect con1,localhost,root;
BEGIN;
SELECT * FROM t WHERE sk = 100 FOR UPDATE;
pk sk
10 100
connect con2,localhost,root;
SET DEBUG_SYNC="lock_wait_suspend_thread_enter SIGNAL insert_wait_started";
INSERT INTO t VALUES (5, 100) # trx 1;
connect con3,localhost,root;
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ;
SET DEBUG_SYNC="now WAIT_FOR insert_wait_started";
SET DEBUG_SYNC="lock_wait_suspend_thread_enter SIGNAL delete_started_waiting";
DELETE FROM t WHERE sk = 100 # trx 2;
connection con1;
SET DEBUG_SYNC="now WAIT_FOR delete_started_waiting";
DELETE FROM t WHERE sk=100;
COMMIT;
disconnect con1;
connection con2;
disconnect con2;
connection con3;
ERROR 40001: Deadlock found when trying to get lock; try restarting transaction
disconnect con3;
connection default;
SELECT * FROM t;
pk sk
5 100
disconnect pause_purge;
SET DEBUG_SYNC="RESET";
DROP TABLE t;