Patch for Bug#52356: query_cache_debug fails on Linux.

There were two problems here:
  1. misleading error message
  2. abusing KILL QUERY in the test case

1. The server reported "'DELETE FROM t1' failed: 1689: Wait on a lock was
aborted due to a pending exclusive lock", while the proper error message
should be "'DELETE FROM t1' failed: 1317: Query execution was interrupted".

The problem is that the server has two different flags for
signalling that a query is being killed: THD::killed and
mysys_var::abort. The test case triggers a race: sometimes
mysys_var::abort is set earlier than THD::killed. That leads
to the following situation:

  - thr_lock() checks mysys_var::abort and returns error status,
    since mysys_var::abort is set;

  - the caller (mysql_lock_tables()) gets an error from thr_lock(),
    but THD::killed is not set, so it decides that thr_lock() couldn't
    get a lock due to a pending exclusive lock.

This is a known issue with the server and it's not going to be fixed soon.

5.5 differs from 5.1 here as follows: when thr_lock() returns an error:
  - 5.1 continues trying thr_lock() until success;
  - 5.5 propagates the error

2. The test case uses KILL QUERY is a highly concurent environment.

The fix is to wait for the dying statement to rest in peace before
executing another DELETE FROM t1.

mysql-test/collections/default.experimental

@@ -16,7 +16,6 @@ main.mysqlbinlog_row_innodb @solaris     # Bug#52202 2010-03-22 alik mysqlbinlog
 main.mysqlbinlog_row_myisam @solaris     # Bug#52202 2010-03-22 alik mysqlbinlog_row* fail in daily-trunk on Sol10 x86_64 debug_max
 main.outfile_loaddata @solaris           # Bug#46895 2010-01-20 alik Test "outfile_loaddata" fails (reproducible)
 main.plugin* @solaris                    # Bug#47146 Linking problem with example plugin when dtrace enabled
-main.query_cache_debug @linux            # Bug#52356 2010-04-10 alik query_cache_debug fails on Linux
 main.signal_demo3 @solaris               # Bug#47791 2010-01-20 alik Several test cases fail on Solaris with error Thread stack overrun
 main.sp @solaris                         # Bug#47791 2010-01-20 alik Several test cases fail on Solaris with error Thread stack overrun
 main.wait_timeout @solaris               # Bug#51244 2010-04-26 alik wait_timeout fails on OpenSolaris

mysql-test/t/query_cache_debug.test

@@ -221,6 +221,9 @@ KILL QUERY @thread_id;
 SELECT SQL_NO_CACHE id FROM information_schema.processlist WHERE state='wait_in_query_cache_invalidate2' LIMIT 1 INTO @flush_thread_id;
 KILL QUERY @flush_thread_id;
 
+LET $wait_condition= SELECT SQL_NO_CACHE COUNT(*) = 1 FROM INFORMATION_SCHEMA.PROCESSLIST WHERE Id = @flush_thread_id AND Command = 'Sleep';
+--source include/wait_condition.inc
+
 --echo **
 --echo *************************************************************************
 --echo ** No tables should be locked