thread: fix resume of suspended sync

racket/src/thread/schedule.rkt

@@ -40,7 +40,7 @@
                (all-threads-poll-done?)
                (waiting-on-external-or-idle?))
       (or (check-external-events 'slow)
-          (post-idle)
+          (try-post-idle)
           (process-sleep)))
     (define child (thread-group-next! g))
     (cond
@@ -196,6 +196,12 @@
   ;; Maybe some thread can proceed:
   (thread-did-work!))
 
+(define (try-post-idle)
+  (and (post-idle)
+       (begin
+         (thread-did-work!)
+         #t)))
+
 ;; ----------------------------------------
 
 (define (accum-cpu-time! t)

racket/src/thread/sync.rkt

@@ -129,7 +129,8 @@
                   ;; Return result in a thunk:
                   (lambda () #f)])]
               [(and (all-asynchronous? s)
-                    (not (syncing-selected s)))
+                    (not (syncing-selected s))
+                    (not (syncing-need-retry? s)))
                (suspend-syncing-thread s timeout-at)
                (set-syncing-wakeup! s void)
                (loop #f #t)]