MDEV-14526: MariaDB keeps crashing under load when query_cache_type is changed

The problem was in such scenario:
T1 - starts registering query and locked QC
T2 - starts disabling QC and wait for UNLOCK
T1 - unlock QC
T2 - disable QC and destroy signals without waiting for query unlock
T1 a) - not yet unlocked query in qc and crash on attempt to unlock because
        QC signals are destroyed
   b) if above was done before destruction, it execute end_of results first
      time at exit on after try_lock which see QC disables and return TRUE.
      But it do not reset query_cache_tls->first_query_block which lead to
      second call of end_of_result when diagnostic arena has already
      inappropriate status (not is_eof()).

Fix is:
  1) wait for all queries unlocked before destroying them by locking and
     unlocking
  2) remove query_cache_tls->first_query_block if QC disabled

1 files changed, 52 insertions, 0 deletions

diff --git a/mysql-test/t/query_cache_debug.test b/mysql-test/t/query_cache_debug.test
index 854af85f3fb..02a1689f807 100644
--- a/mysql-test/t/query_cache_debug.test
+++ b/mysql-test/t/query_cache_debug.test
@@ -319,3 +319,55 @@ RESET QUERY CACHE;
 DROP TABLE t1;
 SET GLOBAL query_cache_size= DEFAULT;
 SET GLOBAL query_cache_type= DEFAULT;
+
+--echo #
+--echo # MDEV-14526: MariaDB keeps crashing under load when
+--echo # query_cache_type is changed
+--echo #
+
+CREATE TABLE t1 (
+  `id` int(10) NOT NULL AUTO_INCREMENT,
+  `k` int(10) default '0',
+  PRIMARY KEY (`id`))
+ENGINE=MyISAM;
+
+INSERT IGNORE INTO t1 VALUES
+  (NULL,1),(NULL,8),(NULL,NULL),(NULL,NULL),(NULL,4),(NULL,9),(NULL,7),
+  (NULL,3),(NULL,NULL),(NULL,2),(NULL,3),(NULL,NULL),(NULL,2),(NULL,7),
+  (NULL,1),(NULL,2),(NULL,4),(NULL,NULL),(NULL,1),(NULL,1),(NULL,4);
+
+SET GLOBAL query_cache_size= 1024*1024;
+SET GLOBAL query_cache_type= 1;
+
+--connect (con2,localhost,root,,test)
+--connect (con1,localhost,root,,test)
+set debug_sync="wait_in_query_cache_store_query SIGNAL parked WAIT_FOR go";
+--send
+
+	SELECT DISTINCT id FROM t1 WHERE id BETWEEN 5603 AND 16218 ORDER BY k;
+
+--connection default
+
+set debug_sync="now WAIT_FOR parked";
+--connection con2
+--send
+        SET GLOBAL query_cache_type= 0;
+
+--connection default
+set debug_sync="now SIGNAL go";
+
+--connection con1
+--reap
+--connection con2
+--reap
+
+# Cleanup
+--disconnect con1
+--disconnect con2
+--connection default
+set debug_sync= 'RESET';
+DROP TABLE t1;
+SEt GLOBAL query_cache_size= DEFAULT;
+SEt GLOBAL query_cache_type= DEFAULT;
+
+--echo # End of 5.5 tests