Use a condition variable for manking an external wait on a FEB call.

src/feb.c

@@ -61,6 +61,7 @@ typedef enum bt {
 } blocker_type;
 typedef struct {
     pthread_mutex_t lock;
+    pthread_cond_t condition;
     void           *a;
     void           *b;
     blocker_type    type;
@@ -230,20 +231,23 @@ static aligned_t qthread_feb_blocker_thread(void *arg)
             a->retval = qthread_empty(a->a);
             break;
     }
-    pthread_mutex_unlock(&(a->lock));
+    pthread_mutex_lock(&a->lock);
+    pthread_cond_signal(&a->condition);
+    pthread_mutex_unlock(&a->lock);
     return 0;
 }                                      /*}}} */
 
 static int qthread_feb_blocker_func(void        *dest,
                                     void        *src,
                                     blocker_type t)
 {   /*{{{*/
-    qthread_feb_blocker_t args = { PTHREAD_MUTEX_INITIALIZER, dest, src, t, QTHREAD_SUCCESS };
+    qthread_feb_blocker_t args = { PTHREAD_MUTEX_INITIALIZER, PTHREAD_COND_INITIALIZER, dest, src, t, QTHREAD_SUCCESS };
 
-    pthread_mutex_lock(&args.lock);
     qthread_fork(qthread_feb_blocker_thread, &args, NULL);
     pthread_mutex_lock(&args.lock);
+    pthread_cond_wait(&args.condition, &args.lock);
     pthread_mutex_unlock(&args.lock);
+    pthread_cond_destroy(&args.condition);
     pthread_mutex_destroy(&args.lock);
     return args.retval;
 } /*}}}*/