bc: fix use of fsemaphores in the main thread

Fix a locking mistake in the implementation of fsemaphores.

pkgs/racket-test/tests/future/fsema-lock.rkt

@@ -0,0 +1,28 @@
+#lang racket/base
+(require racket/future)
+
+;; Make sure fsemaphores can work like locks, as
+;; long as every future that might hold a lock is
+;; demanded by a thread (so it must continue to
+;; run if it takes a lock)
+
+(for ([N (in-range 4 20)])
+  (define f (make-fsemaphore 1))
+  (define working (box 'ok))
+  (define fts
+    (for/list ([i (in-range N)])
+      (define ft
+        (future
+         (lambda ()
+           (for ([i (in-range 5000)])
+             (fsemaphore-wait f)
+             (unless (box-cas! working 'ok 'not-ok)
+               (printf "FAIL\n")
+               (exit 1))
+             (unless (box-cas! working 'not-ok 'ok)
+               (printf "FAIL\n")
+               (exit 1))
+             (fsemaphore-post f)))))
+      (thread (lambda () (touch ft)))
+      ft))
+  (for-each touch fts))

racket/src/racket/src/future.c

@@ -1625,9 +1625,11 @@ Scheme_Object *scheme_fsemaphore_wait(int argc, Scheme_Object **argv)
         return scheme_void;
       }
 
+      do {
         mzrt_mutex_unlock(sema->mut);
         sema = block_until_sema_ready(sema);
         mzrt_mutex_lock(sema->mut);
+      } while (!sema->ready);
     } else {
       /* On a future thread, suspend the future (to be 
         resumed whenever the fsema becomes ready */