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.

1 files changed, 23 insertions, 9 deletions

diff --git a/sql/sql_show.cc b/sql/sql_show.cc
index a8ae5832a2a..8647237ce72 100644
--- a/sql/sql_show.cc
+++ b/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,