Fix indefinite wait when trying to stop pjmedia_clock (#3304)

pjmedia/include/pjmedia/clock.h

@@ -272,11 +272,15 @@ PJ_DECL(pj_status_t) pjmedia_clock_start(pjmedia_clock *clock);
 
 
 /**
- * Stop the clock.
+ * Stop the clock. When the function returns PJ_SUCCESS, it is guaranteed
+ * that the clock thread (if any) has stopped and the callback has completed.
+ * But if the function is called from within the clock's callback itself,
+ * the callback will still be running until completion. In that case, 
+ * the function will only stop future callbacks and returns PJ_EBUSY.
  *
  * @param clock             The media clock.
  *
- * @return                  PJ_SUCCES on success.
+ * @return                  PJ_SUCCES on success, or PJ_EBUSY.
  */
 PJ_DECL(pj_status_t) pjmedia_clock_stop(pjmedia_clock *clock);
 

pjmedia/src/pjmedia/clock_thread.c

@@ -218,7 +218,15 @@ PJ_DEF(pj_status_t) pjmedia_clock_start(pjmedia_clock *clock)
     clock->running = PJ_TRUE;
     clock->quitting = PJ_FALSE;
 
-    if ((clock->options & PJMEDIA_CLOCK_NO_ASYNC) == 0 && !clock->thread) {
+    if ((clock->options & PJMEDIA_CLOCK_NO_ASYNC) == 0) {
+        if (clock->thread) {
+            /* This is probably the leftover thread that failed to
+             * be cleaned up during the last clock stoppage.
+             */
+            pj_thread_destroy(clock->thread);
+            clock->thread = NULL;
+        }
+
         status = pj_thread_create(clock->pool, "clock", &clock_thread, clock,
                                   0, 0, &clock->thread);
         if (status != PJ_SUCCESS) {
@@ -247,7 +255,13 @@ PJ_DEF(pj_status_t) pjmedia_clock_stop(pjmedia_clock *clock)
             clock->thread = NULL;
             pj_pool_reset(clock->pool);
         } else {
-            clock->quitting = PJ_FALSE;
+            /* We are probably called from the clock thread itself.
+             * Do not cancel the thread's quitting though, since it
+             * may cause the clock thread to run indefinitely.
+             */ 
+            // clock->quitting = PJ_FALSE;
+
+            return PJ_EBUSY;
         }
     }