bzr commit into mysql-trunk-runtime branch (jon.hauglid:3082) Bug#54905

[Jon Olav Hauglid]

#At file:///export/home/x/mysql-trunk-runtime-bug54905/ based on revid:jon.hauglid@stripped

 3082 Jon Olav Hauglid	2010-07-22
      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.

    modified:
      mysql-test/r/show_check.result
      mysql-test/t/show_check.test
      sql/sql_show.cc
=== modified file 'mysql-test/r/show_check.result'
--- a/mysql-test/r/show_check.result	2010-05-21 13:36:59 +0000
+++ b/mysql-test/r/show_check.result	2010-07-22 09:10:35 +0000
@@ -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;

=== modified file 'mysql-test/t/show_check.test'
--- a/mysql-test/t/show_check.test	2010-05-21 13:36:59 +0000
+++ b/mysql-test/t/show_check.test	2010-07-22 09:10:35 +0000
@@ -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
 

=== modified file 'sql/sql_show.cc'
--- a/sql/sql_show.cc	2010-07-15 15:46:41 +0000
+++ b/sql/sql_show.cc	2010-07-22 09:10:35 +0000
@@ -649,22 +649,30 @@ mysqld_show_create(THD *thd, 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 
   {
     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 
        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 
 
   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 
     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,


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