Fixed possible mutex-wrong-order with KILL USER

The old code collected a list of THD's, locked the THD's from getting
deleted by locking two mutex and then later in a separate loop
sent a kill signal to each THD.

The problem with this approach is that, as THD's can be reused,
the second time the THD is killed, the mutex can be taken in
different order, which signals failures in safe_mutex.

Fixed by sending the kill signal directly and not collect the THD's
in a list to be signaled later.  This is the same approach we are using
in kill_zombie_dump_threads().

Other things:
- Reset safe_mutex_t->locked_mutex when freed (Safety fix)

mysys/thr_mutex.c

@@ -665,6 +665,7 @@ void safe_mutex_free_deadlock_data(safe_mutex_t *mp)
     my_hash_free(mp->used_mutex);
     my_hash_free(mp->locked_mutex);
     my_free(mp->locked_mutex);
+    mp->locked_mutex= 0;
     mp->create_flags|= MYF_NO_DEADLOCK_DETECTION;
   }
 }

sql/sql_parse.cc

@@ -9371,11 +9371,13 @@ kill_one_thread(THD *thd, my_thread_id id, killed_state kill_signal, killed_type
 
 struct kill_threads_callback_arg
 {
-  kill_threads_callback_arg(THD *thd_arg, LEX_USER *user_arg):
-    thd(thd_arg), user(user_arg) {}
+  kill_threads_callback_arg(THD *thd_arg, LEX_USER *user_arg,
+                            killed_state kill_signal_arg):
+    thd(thd_arg), user(user_arg), kill_signal(kill_signal_arg), counter(0) {}
   THD *thd;
   LEX_USER *user;
-  List<THD> threads_to_kill;
+  killed_state kill_signal;
+  uint counter;
 };
 
 
@@ -9398,11 +9400,12 @@ static my_bool kill_threads_callback(THD *thd, kill_threads_callback_arg *arg)
       {
         return MY_TEST(arg->thd->security_ctx->master_access & PROCESS_ACL);
       }
-      if (!arg->threads_to_kill.push_back(thd, arg->thd->mem_root))
-      {
-        mysql_mutex_lock(&thd->LOCK_thd_kill); // Lock from delete
-        mysql_mutex_lock(&thd->LOCK_thd_data);
-      }
+      arg->counter++;
+      mysql_mutex_lock(&thd->LOCK_thd_kill); // Lock from delete
+      mysql_mutex_lock(&thd->LOCK_thd_data);
+      thd->awake_no_mutex(arg->kill_signal);
+      mysql_mutex_unlock(&thd->LOCK_thd_data);
+      mysql_mutex_unlock(&thd->LOCK_thd_kill);
     }
   }
   return 0;
@@ -9412,42 +9415,17 @@ static my_bool kill_threads_callback(THD *thd, kill_threads_callback_arg *arg)
 static uint kill_threads_for_user(THD *thd, LEX_USER *user,
                                   killed_state kill_signal, ha_rows *rows)
 {
-  kill_threads_callback_arg arg(thd, user);
+  kill_threads_callback_arg arg(thd, user, kill_signal);
   DBUG_ENTER("kill_threads_for_user");
-
-  *rows= 0;
-
-  if (unlikely(thd->is_fatal_error))        // If we run out of memory
-    DBUG_RETURN(ER_OUT_OF_RESOURCES);
-
   DBUG_PRINT("enter", ("user: %s  signal: %u", user->user.str,
                        (uint) kill_signal));
 
+  *rows= 0;
+
   if (server_threads.iterate(kill_threads_callback, &arg))
     DBUG_RETURN(ER_KILL_DENIED_ERROR);
 
-  if (!arg.threads_to_kill.is_empty())
-  {
-    List_iterator_fast<THD> it2(arg.threads_to_kill);
-    THD *next_ptr;
-    THD *ptr= it2++;
-    do
-    {
-      ptr->awake_no_mutex(kill_signal);
-      /*
-        Careful here: The list nodes are allocated on the memroots of the
-        THDs to be awakened.
-        But those THDs may be terminated and deleted as soon as we release
-        LOCK_thd_kill, which will make the list nodes invalid.
-        Since the operation "it++" dereferences the "next" pointer of the
-        previous list node, we need to do this while holding LOCK_thd_kill.
-      */
-      next_ptr= it2++;
-      mysql_mutex_unlock(&ptr->LOCK_thd_kill);
-      mysql_mutex_unlock(&ptr->LOCK_thd_data);
-      (*rows)++;
-    } while ((ptr= next_ptr));
-  }
+  *rows= arg.counter;
   DBUG_RETURN(0);
 }