Bug #54905 Connection with WRITE lock cannot ALTER table due to

           concurrent SHOW CREATE

The problem was that a SHOW CREATE TABLE statement issued inside
a transaction did not release its metadata locks at the end of the
statement execution. This happened even if SHOW CREATE TABLE is an
information statement. 

The consequence was that SHOW CREATE TABLE was able to block other
connections from accessing the table (e.g. using ALTER TABLE).

This patch fixes the problem by explicitly releasing any metadata
locks taken by SHOW CREATE TABLE after the statement completes.

Test case added to show_check.test.

mysql-test/r/show_check.result

@@ -1466,3 +1466,24 @@ t1	CREATE TABLE `t1` (
 # Switching to connection 'default'.
 UNLOCK TABLES;
 DROP TABLE t1;
+#
+# Bug#54905 Connection with WRITE lock cannot ALTER table due to
+#           concurrent SHOW CREATE
+#
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t1(a INT);
+# Connection con1
+LOCK TABLE t1 WRITE;
+# Connection default
+START TRANSACTION;
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `a` int(11) DEFAULT NULL
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+# Connection con1
+ALTER TABLE t1 CHARACTER SET = utf8;
+UNLOCK TABLES;
+# Connection default
+COMMIT;
+DROP TABLE t1;

mysql-test/t/show_check.test

@@ -1246,6 +1246,39 @@ UNLOCK TABLES;
 DROP TABLE t1;
 
 
+--echo #
+--echo # Bug#54905 Connection with WRITE lock cannot ALTER table due to
+--echo #           concurrent SHOW CREATE
+--echo #
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+
+CREATE TABLE t1(a INT);
+
+--echo # Connection con1
+connect (con1,localhost,root);
+LOCK TABLE t1 WRITE;
+
+--echo # Connection default
+connection default;
+START TRANSACTION;
+SHOW CREATE TABLE t1;
+
+--echo # Connection con1
+connection con1;
+# Used to block
+ALTER TABLE t1 CHARACTER SET = utf8;
+UNLOCK TABLES;
+
+--echo # Connection default
+connection default;
+COMMIT;
+disconnect con1;
+DROP TABLE t1;
+
+
 # Wait till all disconnects are completed
 --source include/wait_until_count_sessions.inc
 

sql/sql_show.cc

@@ -649,22 +649,30 @@ mysqld_show_create(THD *thd, TABLE_LIST *table_list)
   Protocol *protocol= thd->protocol;
   char buff[2048];
   String buffer(buff, sizeof(buff), system_charset_info);
+  List<Item> field_list;
+  bool error= TRUE;
   DBUG_ENTER("mysqld_show_create");
   DBUG_PRINT("enter",("db: %s  table: %s",table_list->db,
                       table_list->table_name));
 
+  /*
+    Metadata locks taken during SHOW CREATE should be released when
+    the statmement completes as it is an information statement.
+  */
+  MDL_ticket *mdl_savepoint= thd->mdl_context.mdl_savepoint();
+
   /* We want to preserve the tree for views. */
   thd->lex->view_prepare_mode= TRUE;
 
   {
     Show_create_error_handler view_error_suppressor(thd, table_list);
     thd->push_internal_handler(&view_error_suppressor);
-    bool error=
+    bool open_error=
       open_normal_and_derived_tables(thd, table_list,
                                      MYSQL_OPEN_FORCE_SHARED_HIGH_PRIO_MDL);
     thd->pop_internal_handler();
-    if (error && (thd->killed || thd->is_error()))
-      DBUG_RETURN(TRUE);
+    if (open_error && (thd->killed || thd->is_error()))
+      goto exit;
   }
 
   /* TODO: add environment variables show when it become possible */
@@ -672,7 +680,7 @@ mysqld_show_create(THD *thd, TABLE_LIST *table_list)
   {
     my_error(ER_WRONG_OBJECT, MYF(0),
              table_list->db, table_list->table_name, "VIEW");
-    DBUG_RETURN(TRUE);
+    goto exit;
   }
 
   buffer.length(0);
@@ -684,9 +692,8 @@ mysqld_show_create(THD *thd, TABLE_LIST *table_list)
        view_store_create_info(thd, table_list, &buffer) :
        store_create_info(thd, table_list, &buffer, NULL,
                          FALSE /* show_database */)))
-    DBUG_RETURN(TRUE);
+    goto exit;
 
-  List<Item> field_list;
   if (table_list->view)
   {
     field_list.push_back(new Item_empty_string("View",NAME_CHAR_LEN));
@@ -707,7 +714,8 @@ mysqld_show_create(THD *thd, TABLE_LIST *table_list)
 
   if (protocol->send_result_set_metadata(&field_list,
                             Protocol::SEND_NUM_ROWS | Protocol::SEND_EOF))
-    DBUG_RETURN(TRUE);
+    goto exit;
+
   protocol->prepare_for_resend();
   if (table_list->view)
     protocol->store(table_list->view_name.str, system_charset_info);
@@ -735,10 +743,16 @@ mysqld_show_create(THD *thd, TABLE_LIST *table_list)
     protocol->store(buffer.ptr(), buffer.length(), buffer.charset());
 
   if (protocol->write())
-    DBUG_RETURN(TRUE);
+    goto exit;
 
+  error= FALSE;
   my_eof(thd);
-  DBUG_RETURN(FALSE);
+
+exit:
+  close_thread_tables(thd);
+  /* Release any metadata locks taken during SHOW CREATE. */
+  thd->mdl_context.rollback_to_savepoint(mdl_savepoint);
+  DBUG_RETURN(error);
 }
 
 bool mysqld_show_create_db(THD *thd, char *dbname,