MDEV-18464 : Killing thread can cause mutex deadlock if done concurrently with Galera/replication victim killbb-10.2-MDEV-18464

Following issues here:

Whenever Galera BF (brute force) transaction decides to abort conflicting transaction it will kill that thread using thd::awake()
Whenever replication selects a thread as a victim it will call thd::awake()
User KILL [QUERY|CONNECTION] ... for a thread it will also call thd::awake()
Whenever one of these actions is executed we will hold number of InnoDB internal mutexes and thd mutexes.
Sometimes these mutexes are taken in different order causing mutex deadlock.

In this patch we will fix Galera BF and user kill cases so that we enqueue
victim thread to a list while we hold InnoDB mutexes and we then release them.
A new background thread will pick victim thread from this new list and uses
thd::awake() with no InnoDB mutexes. Idea is similar to replication background
kill. This fix enforces that we take LOCK_thd_data -> lock sys mutex -> trx mutex
always in this order.

wsrep_mysqld.cc
	Here we introduce a list where victim threads are stored,
	condition variable to be used to wake up background thread
	and mutex to protect list.

wsrep_thd.cc
	Create a new background thread to handle victim thread
	abort. We may take wsrep_thd_LOCK mutex here but not any
	InnoDB mutexes.

wsrep_innobase_kill_one_trx
	Remove all the wsrep code that was moved to wsrep_thd.cc
	We just enqueue required information to background kill
	list and cancel victim trx lock wait if there is such.
	Here we have InnoDB lock sys mutex and trx mutex so here
	we can't take wsrep_thd_LOCK mutex.

wsrep_abort_transaction
	Cleanup only.

1 files changed, 2 insertions, 0 deletions

diff --git a/sql/sql_class.h b/sql/sql_class.h
index b35f9a93238..95f08acf403 100644
--- a/sql/sql_class.h
+++ b/sql/sql_class.h
@@ -4409,6 +4409,8 @@ public:
 
 #ifdef WITH_WSREP
   const bool                wsrep_applier; /* dedicated slave applier thread */
+  bool                      wsrep_killer; /* dedicated background
+					  kill thread */
   bool                      wsrep_applier_closing; /* applier marked to close */
   bool                      wsrep_client_thread; /* to identify client threads*/
   bool                      wsrep_PA_safe;