Bug #11752803 SERVER CRASHES IF MAX_CONNECTIONS DECREASED BELOW

               CERTAIN LEVEL
      
Problem description: mysqld crashes when we update the max_connections 
variable to lesser value than the number of currently open connections.
      
Analysis: The "alarm_queue.max_elements" size will be decided at the 
server start time and it will get modified if we change max_connections 
value. In the current scenario the value of "alarm_queue.max_elements" 
is decremented when the max_connections is set to 2. When updating the  
"alarm_queue.max_elements" value we are not updating "max_used_alarms" 
value. Hence, instead of getting the warning "thr_alarm queue is full" 
it is ending up in asserting the server at the time of inserting new 
elements in the queue.
      
Fix: the fix is to dynamically increase the size of the alarm_queue.
In order to do that, queue_insert_safe() should be used instead if
queue_insert().

mysys/thr_alarm.c

@@ -52,6 +52,8 @@ static QUEUE alarm_queue;
 static uint max_used_alarms=0;
 pthread_t alarm_thread;
 
+#define MY_THR_ALARM_QUEUE_EXTENT 10
+
 #ifdef USE_ALARM_THREAD
 static void *alarm_handler(void *arg);
 #define reschedule_alarms() pthread_cond_signal(&COND_alarm)
@@ -74,8 +76,8 @@ void init_thr_alarm(uint max_alarms)
   DBUG_ENTER("init_thr_alarm");
   alarm_aborted=0;
   next_alarm_expire_time= ~ (time_t) 0;
-  init_queue(&alarm_queue,max_alarms+1,offsetof(ALARM,expire_time),0,
-	     compare_ulong,NullS);
+  init_queue_ex(&alarm_queue, max_alarms + 1, offsetof(ALARM,expire_time), 0,
+                compare_ulong, NullS, MY_THR_ALARM_QUEUE_EXTENT);
   sigfillset(&full_signal_set);			/* Neaded to block signals */
   pthread_mutex_init(&LOCK_alarm,MY_MUTEX_INIT_FAST);
   pthread_cond_init(&COND_alarm,NULL);
@@ -127,7 +129,10 @@ void resize_thr_alarm(uint max_alarms)
     than max_alarms
   */
   if (alarm_queue.elements < max_alarms)
+  {
     resize_queue(&alarm_queue,max_alarms+1);
+    max_used_alarms= alarm_queue.elements;
+  }
   pthread_mutex_unlock(&LOCK_alarm);
 }
 
@@ -182,17 +187,6 @@ my_bool thr_alarm(thr_alarm_t *alrm, uint sec, ALARM *alarm_data)
 
   if (alarm_queue.elements >= max_used_alarms)
   {
-    if (alarm_queue.elements == alarm_queue.max_elements)
-    {
-      DBUG_PRINT("info", ("alarm queue full"));
-      fprintf(stderr,"Warning: thr_alarm queue is full\n");
-      *alrm= 0;					/* No alarm */
-      pthread_mutex_unlock(&LOCK_alarm);
-#ifndef USE_ONE_SIGNAL_HAND
-      pthread_sigmask(SIG_SETMASK,&old_mask,NULL);
-#endif
-      DBUG_RETURN(1);
-    }
     max_used_alarms=alarm_queue.elements+1;
   }
   reschedule= (ulong) next_alarm_expire_time > (ulong) now + sec;
@@ -216,7 +210,7 @@ my_bool thr_alarm(thr_alarm_t *alrm, uint sec, ALARM *alarm_data)
   alarm_data->alarmed=0;
   alarm_data->thread=    current_my_thread_var->pthread_self;
   alarm_data->thread_id= current_my_thread_var->id;
-  queue_insert(&alarm_queue,(uchar*) alarm_data);
+  queue_insert_safe(&alarm_queue, (uchar*) alarm_data);
 
   /* Reschedule alarm if the current one has more than sec left */
   if (reschedule)