Bug #55498 SHOW CREATE TRIGGER takes wrong type of metadata lock

The first problem was that SHOW CREATE TRIGGER took a stronger metadata
lock than required. This caused the statement to be blocked when it was
not needed. For example, LOCK TABLE WRITE in one connection would block
SHOW CREATE TRIGGER in another connection.

Another problem was that a SHOW CREATE TRIGGER statement issued inside
a transaction did not release its metadata locks at the end of the
statement execution. This happened even if SHOW CREATE TRIGGER is an
information statement. The consequence was that SHOW CREATE TRIGGER
was able to block other connections from accessing the table
(e.g. using ALTER TABLE).

This patch fixes the problem by changing SHOW CREATE TRIGGER to take
a MDL_SHARED_HIGH_PRIO metadata lock similar to what is already done
for SHOW CREATE TABLE. The patch also changes SHOW CREATE TRIGGER to
explicitly release any metadata locks taken by the statement after
it completes.

Test case added to show_check.test.

1 files changed, 46 insertions, 0 deletions

diff --git a/mysql-test/t/show_check.test b/mysql-test/t/show_check.test
index 89a43f24296..0323f015e88 100644
--- a/mysql-test/t/show_check.test
+++ b/mysql-test/t/show_check.test
@@ -1279,6 +1279,52 @@ disconnect con1;
 DROP TABLE t1;
 
 
+--echo #
+--echo # Bug#55498 SHOW CREATE TRIGGER takes wrong type of metadata lock.
+--echo #
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+
+CREATE TABLE t1 (a INT);
+CREATE TRIGGER t1_bi BEFORE INSERT ON t1 FOR EACH ROW SET new.a = 1;
+
+--echo # Test 1: SHOW CREATE TRIGGER with WRITE locked table.
+
+--echo # Connection con1
+connect (con1, localhost, root);
+LOCK TABLE t1 WRITE;
+
+--echo # Connection default
+connection default;
+# Should not block.
+SHOW CREATE TRIGGER t1_bi;
+
+--echo # Connection con1
+connection con1;
+UNLOCK TABLES;
+
+--echo # Test 2: ALTER TABLE with SHOW CREATE TRIGGER in transaction
+
+--echo # Connection default
+connection default;
+START TRANSACTION;
+SHOW CREATE TRIGGER t1_bi;
+
+--echo # Connection con1
+connection con1;
+# Should not block.
+ALTER TABLE t1 CHARACTER SET = utf8;
+
+--echo # Connection default
+connection default;
+COMMIT;
+DROP TRIGGER t1_bi;
+DROP TABLE t1;
+disconnect con1;
+
+
 # Wait till all disconnects are completed
 --source include/wait_until_count_sessions.inc