MDEV-32728: Wrong mutex usage 'LOCK_thd_data' and 'wait_mutex'

Checking for kill with thd_kill_level() or check_killed() runs apc
requests, which takes the LOCK_thd_kill mutex. But this is dangerous,
as checking for kill needs to be called while holding many different
mutexes, and can lead to cyclic mutex dependency and deadlock.

But running apc is only "best effort", so skip running the apc if the
LOCK_thd_kill is not available. The apc will then be run on next check
of kill signal.

Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

Author: knielsen

sql/my_apc.cc

@@ -197,13 +197,16 @@ bool Apc_target::make_apc_call(THD *caller_thd, Apc_call *call,
   This should be called periodically by the APC target thread.
 */
 
-void Apc_target::process_apc_requests()
+void Apc_target::process_apc_requests(bool force)
 {
   while (1)
   {
     Call_request *request;
- 
-    mysql_mutex_lock(LOCK_thd_kill_ptr);
+
+    if (force)
+      mysql_mutex_lock(LOCK_thd_kill_ptr);
+    else if (mysql_mutex_trylock(LOCK_thd_kill_ptr))
+      break;                              // Mutex is blocked, try again later
     if (!(request= get_first_in_queue()))
     {
       /* No requests in the queue */

sql/my_apc.h

@@ -70,10 +70,10 @@ class Apc_target
     bool process= !--enabled && have_apc_requests();
     mysql_mutex_unlock(LOCK_thd_kill_ptr);
     if (unlikely(process))
-      process_apc_requests();
+      process_apc_requests(true);
   }
 
-  void process_apc_requests();
+  void process_apc_requests(bool force);
   /* 
     A lightweight function, intended to be used in frequent checks like this:
 

sql/sql_class.cc

@@ -4724,7 +4724,7 @@ extern "C" enum thd_kill_levels thd_kill_level(const MYSQL_THD thd)
     if (unlikely(apc_target->have_apc_requests()))
     {
       if (thd == current_thd)
-        apc_target->process_apc_requests();
+        apc_target->process_apc_requests(false);
     }
     return THD_IS_NOT_KILLED;
   }

sql/sql_class.h

@@ -3606,7 +3606,7 @@ class THD: public THD_count, /* this must be first */
       return TRUE;
     }
     if (apc_target.have_apc_requests())
-      apc_target.process_apc_requests(); 
+      apc_target.process_apc_requests(false);
     return FALSE;
   }
 

unittest/sql/my_apc-t.cc

@@ -100,7 +100,7 @@ void *test_apc_service_thread(void *ptr)
     //apc_target.enable();
     for (int i = 0; i < 10 && !service_should_exit; i++)
     {
-      apc_target.process_apc_requests();
+      apc_target.process_apc_requests(true);
       my_sleep(int_rand(30));
     }
   }