MDEV-14581 Warning info not cleared when caching THD

In thread caching code, clear THD's warnings before reuse.

Author: vaintroub

mysql-test/t/warnings-master.opt

@@ -1 +1 @@
---loose-skip-innodb
+--loose-skip-innodb --thread-handling=one-thread-per-connection

mysql-test/t/warnings.test

@@ -286,3 +286,15 @@ SHOW WARNINGS;
 
 DROP TABLE t1;
 DROP FUNCTION f1;
+
+# MDEV-14581 Warning info not cleared when caching THD
+connect (con1,localhost,root,,);
+SELECT TIME('10:10:10.11111111111');
+disconnect con1;
+
+connect (con2,localhost,root,,);
+SHOW WARNINGS;
+disconnect con2;
+
+connection default;
+

sql/mysqld.cc

@@ -2961,6 +2961,10 @@ static bool cache_thread(THD *thd)
       _db_pop_();
 #endif
 
+    /* Clear warnings. */
+    if (!thd->get_stmt_da()->is_warning_info_empty())
+      thd->get_stmt_da()->clear_warning_info(thd->query_id);
+
     set_timespec(abstime, THREAD_CACHE_TIMEOUT);
     while (!abort_loop && ! wake_thread && ! kill_cached_threads)
     {