cs: wake up main thread blocked on fsemaphore

If a future posts to an fsemaphore that the main thread is waiting on
but went to sleep, then be sure to wake up the main thread.

Relevant to #2957

racket/src/thread/fsemaphore.rkt

@@ -53,12 +53,13 @@
           (set-fsemaphore-dependents! fs (hash-remove deps f))
           (future-notify-dependent f)]
          [else
+          (set-fsemaphore-c! fs 1)
           (when b
             ;; This is a kind of broadcast wakeup, and then the
             ;; awakened threads will compete for the fsemaphore:
             (set-fsemaphore-dep-box! fs #f)
-            (set-box! b #t))
-          (set-fsemaphore-c! fs 1)])]
+            (set-box! b #t)
+            (wakeup-this-place))])]
       [else
        (set-fsemaphore-c! fs (add1 c))])))
 

racket/src/thread/future.rkt

@@ -43,7 +43,8 @@
   (provide future*-lock
            set-future*-state!
            future-suspend
-           future-notify-dependent))
+           future-notify-dependent
+           wakeup-this-place))
 
 (define (init-future-place!)
   (init-future-logging-place!))