From: Jon Olav Hauglid
Date: July 20 2010 1:40pm
Subject: bzr commit into mysql-trunk-runtime branch (jon.hauglid:3081) Bug#55223
List-Archive: http://lists.mysql.com/commits/113953
X-Bug: 55223
Message-Id: <201007201340.o6KBbP7W016011@rcsinet15.oracle.com>
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#At file:///export/home/x/mysql-trunk-runtime-bug55223/ based on revid:jon.hauglid@stripped

 3081 Jon Olav Hauglid	2010-07-20
      Bug #55223 assert in Protocol::end_statement during CREATE DATABASE
      
      The problem was that a statement could cause an assert if it was aborted by
      KILL QUERY while it waited on a metadata lock. This assert checks that a
      statement either sends OK or an error to the client. If the bug was triggered
      on release builds, it caused OK to be sent to the client instead of
      ER_QUERY_INTERRUPTED.
      
      The root cause of the problem is that there are two separate ways to tell if a
      statement is killed: thd->killed and mysys_var->abort. KILL QUERY causes both
      to be set, thd->killed before mysys_var->abort. The latter was used to check
      if a wait for a metadata lock should be aborted. If waiting is aborted, no OK
      message is sent and thd->killed is checked to see if ER_QUERY_INTERRUPTED
      should be sent to the client. The assert could be triggered if the compiler
      reordered the KILL QUERY code so that mysys_var->abort was set before
      thd->killed, and the aborted statement reached the thd->killed check before
      thd->killed was set by KILL QUERY.
      
      This patch fixes the problem by changing the metadata lock waiting code to
      check thd->killed.
      
      No test case added as reproducing the assert is dependent on very exact timing
      of two (or more) threads and compiler reording of instructions. The patch has
      been checked using RQG and the grammar posted on the bug report.

    modified:
      sql/mdl.cc
=== modified file 'sql/mdl.cc'
--- a/sql/mdl.cc	2010-07-13 18:01:54 +0000
+++ b/sql/mdl.cc	2010-07-20 13:39:59 +0000
@@ -1029,7 +1029,7 @@ MDL_wait::timed_wait(THD *thd, struct ti
   old_msg= MDL_ENTER_COND(thd, mysys_var, &m_COND_wait_status,
                           &m_LOCK_wait_status);
 
-  while (!m_wait_status && !mysys_var->abort &&
+  while (!m_wait_status && !thd_killed(thd) &&
          wait_result != ETIMEDOUT && wait_result != ETIME)
     wait_result= mysql_cond_timedwait(&m_COND_wait_status, &m_LOCK_wait_status,
                                       abs_timeout);
@@ -1048,7 +1048,7 @@ MDL_wait::timed_wait(THD *thd, struct ti
       false, which means that the caller intends to restart the
       wait.
     */
-    if (mysys_var->abort)
+    if (thd_killed(thd))
       m_wait_status= KILLED;
     else if (set_status_on_timeout)
       m_wait_status= TIMEOUT;


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