cs & thread: repair for sync/timeout

If a `sync/timeout` on a semaphore (or simiilar asynchronous event)
succeeds simultaneously with the timeout, the success could get lost.

pkgs/racket-test-core/tests/racket/sync.rktl

@@ -1528,6 +1528,22 @@
   (void (sync t))
   (test 'ok values v))
 
+;; ----------------------------------------
+;; Try to make a semaphore-post succeed at exactly
+;; the same time that a `sync/timeout` times out
+
+(for ([i 10])
+  (define s (make-semaphore))
+  (define t (thread
+             (lambda ()
+               (sleep (- 0.1 (* 0.001 (random))))
+               (semaphore-post s))))
+  (define r (sync/timeout 0.1 s))
+  (unless r
+    ;; This will get stuck if the success of time sync got lost
+    (sync s))
+  (thread-wait t))
+
 ;; ----------------------------------------
 
 (report-errs)

racket/src/thread/sync.rkt

@@ -121,13 +121,13 @@
                     timeout
                     (timeout-at . <= . (current-inexact-milliseconds)))
                (start-atomic)
-               (syncing-done! s none-syncer)
                (cond
                  [(syncing-selected s)
                   ;; Selected after all:
                   (end-atomic)
                   (loop #f #f)]
                  [else
+                  (syncing-done! s none-syncer)
                   (end-atomic)
                   ;; Return result in a thunk:
                   (lambda () #f)])]