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47e687b109
Set solution is to check if transaction, which modified a record, is
still active in lock_clust_rec_read_check_and_lock(). if yes, then just
request a lock. If no, then, depending on if the current transaction read
view can see the changes, return eighter DB_RECORD_CHANGED or request a
lock.
We can do the check in lock_clust_rec_read_check_and_lock() because
transaction tries to set a lock on the record which cursor points to after
transaction resuming and cursor position restoring. If the lock already
exists, then we don't request the lock again. But for the current commit
it's important that lock_clust_rec_read_check_and_lock() will be invoked
again for the same record, so we can do the check again after
transaction, which modified a record, was committed or rolled back.
MDEV-33802(4aa9291) is partially reverted. If some transaction holds
implicit lock on some record and transaction with snapshot isolation level
requests conflicting lock on the same record, it should be blocked instead
of returning DB_RECORD_CHANGED to have ability to continue execution when
implicit lock owner is rolled back.
The construction
--------------------------------------------------------------------------
let $wait_condition=
select count(*) = 1 from information_schema.processlist
where state = 'Updating' and info = 'UPDATE t SET b = 2 WHERE a';
--source include/wait_condition.inc
--------------------------------------------------------------------------
is not reliable enought to make sure transaction is blocked in test
case, the test failed sporadically with
--------------------------------------------------------------------------
./mtr --max-test-fail=1 --parallel=96 lock_isolation{,,,,,,,}{,,,}{,,} \
--repeat=500
--------------------------------------------------------------------------
command. That's why it was replaced with debug sync-points.
Reviewed by: Marko Mäkelä
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