Bug #27811 The variable 'join_tab' is being used without being defined

Description:
Compile mysqld-debug.exe using full debug libraries on windows using visual studio 2005.

Run the testcase, and get the message which I'll type:

Debug Error!
Program: C:\build\mysql-5.0.40\sql\debug\mysqld-debug.exe
Module: C:\build\mysql-5.0.40\sql\debug\mysqld-debug.exe
File: c:\build\mysql-5.0.40\sql\sql_select.cpp
Line: 10204

Run-Time Check Failure #3 - The variable 'join_tab' is being used without being defined.

(Press Retry to debug the application)

Stack:
ntdll.dll!DbgBreakPoint
mysqld-debug.exe!failwithmessage
mysqld-debug.exe!_RTC_UninitUse
mysqld-debug.exe!do_select
mysqld-debug.exe!JOIN::exec
mysqld-debug.exe!subselect_single_select_engine::exec
mysqld-debug.exe!Item_subselect::exec
mysqld-debug.exe!Item_singlerow_subselect::val_int
mysqld-debug.exe!Arg_comparator::compare_int_signed
mysqld-debug.exe!Arg_comparator::compare
mysqld-debug.exe!Item_func_eq::val_int
mysqld-debug.exe!Item_equal::add
mysqld-debug.exe!Item_equal::update_const
mysqld-debug.exe!update_const_equal_items
mysqld-debug.exe!join_read_const_table
mysqld-debug.exe!make_join_statistics
mysqld-debug.exe!JOIN::optimize
mysqld-debug.exe!mysql_select
mysqld-debug.exe!mysql_explain_union
mysqld-debug.exe!mysql_execute_command
mysqld-debug.exe!mysql_parse
mysqld-debug.exe!dispatch_command
mysqld-debug.exe!do_command
mysqld-debug.exe!handle_one_connection
mysqld-debug.exe!pthread_start
mysqld-debug.exe!_callthreadstart
mysqld-debug.exe!_threadstart
kernel32.dll!FlsSetValue

Here's the code place mentioned:

if (!join->conds || join->conds->val_int())
{
  error= (*end_select)(join,join_tab,0);
  if (error == NESTED_LOOP_OK || error == NESTED_LOOP_QUERY_LIMIT)
    error= (*end_select)(join,join_tab,1);
}

join_tab has a value of 0xcccccccc
Although the function that is called has join_tab declared as
__attribute__((unused)), how can this always be guaranteed ?

At least it can make our full debug binaries difficult to
use when doing normal testing.

This testcase doesn't cause a crash if the warning message is ignored.

How to repeat:
DROP TABLE IF EXISTS `t1`;
CREATE TABLE `t1` ( `c1` char(50), `c2` int) ENGINE=MyISAM;
INSERT INTO `t1` VALUES ('a',1);
EXPLAIN SELECT * FROM `t1` WHERE `c2` =  (SELECT max(2));

Suggested fix:
initialize the join_tab to NULL if it's not going to be used.

stack from the debugger break

Attachment: bug27811_full_stack_5.0.40_debug_windows.txt (text/plain), 3.15 KiB.

A patch for this bug has been committed. After review, it may
be pushed to the relevant source trees for release in the next
version. You can access the patch from:

  http://lists.mysql.com/commits/25022

ChangeSet@1.2459, 2007-04-20 19:01:30+03:00, gkodinov@magare.gmz +3 -0
  Bug #27811: 
   FORCE_INIT_OF_VARS was not defined for the 
   debug builds on Windows. This caused LINT_INIT
   macro to be defined as NOP and this triggers
   false alarms about use of uninitialized with
   the runtime libs of some Visual Studio versions.
   Fixed by defining FORCE_INIT_OF_VARS to match the
   state of the Windows

Pushed into 5.0.42

Pushed into 5.1.18-beta

Noted in 5.0.42, 5.1.18 changelogs.

Debug builds on Windows generated false alarms about uninitialized
variables with some Visual Studio runtime libraries.