Introduce thd_query_safe() from MySQL 5.7

commit 565d20b44f24fcc855dc616164d87b03cfad10bc

Author: Jon Olav Hauglid <jon.hauglid@oracle.com>

Date:   Thu Mar 6 12:15:07 2014 +0100

    Bug#17606098: DEADLOCK INVOLVING SRV_MONITOR_THREAD AND LOCK_THD_DATA

    This deadlock involved LOCK_thd_data and InnoDB's trx_sys mutex.

    It could occur if InnoDB code (e.g. the srv_monitor_thread) called

    thd_security_context() while having trx_sys locked and at the same time

    the server tried to notify InnoDB about a deadlock (which involves calling

    mysql_lock_abort_for_thread() while holding LOCK_thd_data).

    This could lead to deadlock if mysql_lock_abort_for_thread() lead to

    trx_allocate_for_mysql() being called inside InnoDB.

    This very rarely happens - the problem was found with RQG and is very

    difficult to reproduce.

    This patch solves the problem by splitting LOCK_thd_data so that a

    separate new mutex LOCK_thd_query protects the query string

    accessed by thd_security_context().

    The patch also strengthens the protection of the query string by

    enforcing that it can only be set by the owner thread and that this

    requires locking of LOCK_thd_query. Reading the query string can

    be done by the owner thread without holding LOCK_thd_query but

    other threads reading the query string have to have LOCK_thd_query

    locked.

    This also solves a separate problem where other threads could

    read the query string while it was being deleted by the owner thread.

    Finally, the patch updates the performance schema mutex

    heuristics - including correcting an issue introduced by WL#6369.