bzr push into mysql-5.6 branch (harin.vadodaria:4643 to 4644) Bug#13904906 Bug#15883127

[Harin Vadodaria]

 4644 Harin Vadodaria	2012-11-21 [merge]
      Bug#15883127: PORT FIX FOR BUG #13904906 TO MYSQL 5.1
      
      Description: Null merge from 5.5 to 5.6

 4643 Andrei Elkin	2012-11-21
      Bug#15867985 	STRESS TESTS CAUSES DBUG SERVER TO EXIT WITH SIGNAL 6
      
      When run with --mysqld=--binlog-order-commits=1 (default) mysqld executes a 
      piece of BGC logics that tames a group of threads in a way that they give up
      their commit execution path to a leader thread.
      This preemption appeared to have a specific to DBUG_ON build flaw in that
      the leader and a follower thread could modify a thread-specific area
      concurrently which led to a segfault.
      
      Fixed with refining the preemption logics to be thread-safe for the DBUG_ON build.

    modified:
      sql/binlog.cc
      sql/binlog.h
      sql/sql_class.h
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