Cherry-picking patch for Bug#56137 from mysql-5.5-runtime.

```---------------------------------------------------------
revno: 3124
revision-id: dlenev@mysql.com-20100831090419-rzr5ktekby2gspm1
parent: alik@sun.com-20100827083901-x4wvtc10u9p7gcs9
committer: Dmitry Lenev <dlenev@mysql.com>
branch nick: mysql-5.5-rt-56137
timestamp: Tue 2010-08-31 13:04:19 +0400
message:
  Bug #56137 "Assertion `thd->lock == 0' failed on upgrading 
  from 5.1.50 to 5.5.6".
  
  Debug builds of the server aborted due to an assertion
  failure when DROP DATABASE statement was run on an
  installation which had outdated or corrupt mysql.proc table.
  Particularly this affected the mysql_upgrade tool which is
  run as part of 5.1 to 5.5 upgrade.
  
  The problem was that sp_drop_db_routines(), which was invoked
  during dropping of the database, could have returned without
  closing and unlocking mysql.proc table in cases when this
  table was not up-to-date with the current server. As a result
  further attempt to open and lock the mysql.event table, which
  was necessary to complete dropping of the database, ended up
  with an assert.
  
  This patch solves this problem by ensuring that
  sp_drop_db_routines() always closes mysql.proc table and
  releases metadata locks on it. This is achieved by changing
  open_proc_table_for_update() function to close tables and
  release metadata locks acquired by it in case of failure.
  This step also makes behavior of the latter function
  consistent with behavior of open_proc_table_for_read()/
  open_and_lock_tables().
  
  
  Test case for this bug was added to sp-destruct.test.
```

---------------------------------------------------------

mysql-test/r/sp-destruct.result

@@ -134,3 +134,19 @@ Warning	1405	Failed to revoke all privileges to dropped routine
 # Restore the procs_priv table
 RENAME TABLE procs_priv_backup TO mysql.procs_priv;
 FLUSH TABLE mysql.procs_priv;
+#
+# Bug #56137 "Assertion `thd->lock == 0' failed on upgrading from
+#             5.1.50 to 5.5.6".
+#
+drop database if exists mysqltest;
+# Backup mysql.proc.
+flush table mysql.proc;
+create database mysqltest;
+# Corrupt mysql.proc to make it unusable by current version of server.
+alter table mysql.proc drop column type;
+# The below statement should not cause assertion failure.
+drop database mysqltest;
+Warnings:
+Error	1547	Column count of mysql.proc is wrong. Expected 20, found 19. The table is probably corrupted
+# Restore mysql.proc.
+drop table mysql.proc;

mysql-test/t/sp-destruct.test

@@ -222,3 +222,33 @@ SHOW WARNINGS;
 --echo # Restore the procs_priv table
 RENAME TABLE procs_priv_backup TO mysql.procs_priv;
 FLUSH TABLE mysql.procs_priv;
+
+
+--echo #
+--echo # Bug #56137 "Assertion `thd->lock == 0' failed on upgrading from
+--echo #             5.1.50 to 5.5.6".
+--echo #
+--disable_warnings
+drop database if exists mysqltest;
+--enable_warnings
+--echo # Backup mysql.proc.
+flush table mysql.proc;
+let $MYSQLD_DATADIR= `select @@datadir`;
+--copy_file $MYSQLD_DATADIR/mysql/proc.frm $MYSQLTEST_VARDIR/tmp/proc.frm
+--copy_file $MYSQLD_DATADIR/mysql/proc.MYD $MYSQLTEST_VARDIR/tmp/proc.MYD
+--copy_file $MYSQLD_DATADIR/mysql/proc.MYI $MYSQLTEST_VARDIR/tmp/proc.MYI
+
+create database mysqltest;
+--echo # Corrupt mysql.proc to make it unusable by current version of server.
+alter table mysql.proc drop column type;
+--echo # The below statement should not cause assertion failure.
+drop database mysqltest;
+
+--echo # Restore mysql.proc.
+drop table mysql.proc;
+--copy_file $MYSQLTEST_VARDIR/tmp/proc.frm $MYSQLD_DATADIR/mysql/proc.frm
+--copy_file $MYSQLTEST_VARDIR/tmp/proc.MYD $MYSQLD_DATADIR/mysql/proc.MYD
+--copy_file $MYSQLTEST_VARDIR/tmp/proc.MYI $MYSQLD_DATADIR/mysql/proc.MYI 
+--remove_file $MYSQLTEST_VARDIR/tmp/proc.frm
+--remove_file $MYSQLTEST_VARDIR/tmp/proc.MYD
+--remove_file $MYSQLTEST_VARDIR/tmp/proc.MYI

sql/sp.cc

@@ -440,6 +440,7 @@ static TABLE *open_proc_table_for_update(THD *thd)
 {
   TABLE_LIST table_list;
   TABLE *table;
+  MDL_ticket *mdl_savepoint= thd->mdl_context.mdl_savepoint();
   DBUG_ENTER("open_proc_table_for_update");
 
   table_list.init_one_table("mysql", 5, "proc", 4, "proc", TL_WRITE);
@@ -450,6 +451,9 @@ static TABLE *open_proc_table_for_update(THD *thd)
   if (!proc_table_intact.check(table, &proc_table_def))
     DBUG_RETURN(table);
 
+  close_thread_tables(thd);
+  thd->mdl_context.rollback_to_savepoint(mdl_savepoint);
+
   DBUG_RETURN(NULL);
 }