MDEV-19198 - DBUG assert in CREATE IF NOT EXIST under LOCK TABLES WRITE

Relax the assert condition. A locked table that did existed prior to
CREATE IF NOT EXIST, retains the MDL_NO_SHARED_READ_WRITE MDL lock prio.

mysql-test/r/mdev19198.result

+CREATE TABLE t1 (c INT);
+CREATE TABLE t2 (c INT);
+LOCK TABLES t1 WRITE, t2 READ;
+CREATE TABLE IF NOT EXISTS t1 LIKE t2;
+Warnings:
+Note	1050	Table 't1' already exists
+UNLOCK TABLES;
+LOCK TABLES t1 READ , t2 READ;
+CREATE TABLE IF NOT EXISTS t1 LIKE t2;
+ERROR HY000: Table 't1' was locked with a READ lock and can't be updated
+UNLOCK TABLES;
+CREATE TABLE IF NOT EXISTS t1 LIKE t2;
+Warnings:
+Note	1050	Table 't1' already exists
+DROP TABLES t1,t2;

mysql-test/t/mdev19198.test

+CREATE TABLE t1 (c INT);
+CREATE TABLE t2 (c INT);
+
+LOCK TABLES t1 WRITE, t2 READ;
+CREATE TABLE IF NOT EXISTS t1 LIKE t2;
+UNLOCK TABLES;
+
+LOCK TABLES t1 READ , t2 READ;
+--error ER_TABLE_NOT_LOCKED_FOR_WRITE
+CREATE TABLE IF NOT EXISTS t1 LIKE t2;
+UNLOCK TABLES;
+
+CREATE TABLE IF NOT EXISTS t1 LIKE t2;
+
+DROP TABLES t1,t2;

sql/sql_table.cc

@@ -5615,11 +5615,18 @@ bool mysql_create_like_table(THD* thd, TABLE_LIST* table,
     /*
       Ensure that we have an exclusive lock on target table if we are creating
       non-temporary table.
+      If we're creating non-temporary table, then either
+     - there is an exclusive lock on the table
+     or
+     - there was CREATE IF EXIST, and the table was not created
+      (it existed), and was previously locked
     */
     DBUG_ASSERT((create_info->tmp_table()) ||
                 thd->mdl_context.is_lock_owner(MDL_key::TABLE, table->db,
                                                table->table_name,
-                                               MDL_EXCLUSIVE));
+                                               MDL_EXCLUSIVE) ||
+                (thd->locked_tables_mode && pos_in_locked_tables &&
+                 create_info->if_not_exists()));
   }
 
   DEBUG_SYNC(thd, "create_table_like_before_binlog");