fix bug in `sync'

When `guard-evt' or `nack-guard-evt' is used, `sync' didn't
account for the possibility that a channel or semaphore
action might complete during the call to the guard, in which
case it might fail to select the event that has already
completed.

Merge to v5.3
(cherry picked from commit 880f84b24f)

src/racket/src/thread.c

@@ -6136,6 +6136,13 @@ static int syncing_ready(Scheme_Object *s, Scheme_Schedule_Info *sinfo)
 
       sleep_end = r_sinfo.sleep_end;
 
+      /* Calling a guard can allow thread swap, which might choose a
+         semaphore or a channel, so check for a result: */
+      if (syncing->result) {
+        result = 1;
+        goto set_sleep_end_and_return;
+      }
+
       if ((i > r_sinfo.w_i) && sinfo->false_positive_ok) {
 	/* There was a redirection. Assert: !yep. 
 	   Give up if we've chained too much. */
@@ -6181,6 +6188,9 @@ static int syncing_ready(Scheme_Object *s, Scheme_Schedule_Info *sinfo)
       set_sync_target(syncing, i, sema, o, NULL, repost, 1, NULL);
       j--; /* try again with this sema */
     }
+
+    if (syncing->result)
+      scheme_signal_error("internal error: sync result set unexpectedly");
   }
 
   if (syncing->timeout >= 0.0) {