fix a bug related to the implicit prompt in a thread taking over the role of an explicit prompt

svn: r5887

collects/tests/mzscheme/prompt-tests.ss

@@ -1667,6 +1667,27 @@
   (test '(1 2 3 4 5 1 2 3 4 5 1 6 6 2 3 4 5 1 6 6) go (default-continuation-prompt-tag) #f)
   (test '(1 2 3 4 5 1 2 3 4 5 1 6 6 2 3 4 5 1 6 6) go (make-continuation-prompt-tag) #f))
 
+;; ----------------------------------------
+;; Run two levels of continuations where an explicit
+;; prompt in a capturing thread is represented by an
+;; implicit prompt in the calling thread.
+
+(let ()
+  (define (foo thunk)
+    (call-with-continuation-prompt
+     (lambda ()
+       (let/cc ret
+         (let ([run? #f])
+           (let/cc run
+             (thread (lambda ()
+                       (sync (system-idle-evt))
+                       (set! run? #t)
+                       (run))))
+           (when run? (ret (thunk))))))))
+  (define s (make-semaphore))
+  (foo (lambda () (semaphore-post s)))
+  (test s sync s))
+
 ;; ----------------------------------------
 ;; Try long chain of composable continuations
 

src/mzscheme/src/eval.c

@@ -5961,7 +5961,7 @@ Scheme_Object *scheme_jump_to_continuation(Scheme_Object *obj, int num_rands, Sc
          created with a new thread or a barrier prompt. */
       p->meta_continuation = NULL; /* since prompt wasn't in any meta-continuation */
       p->meta_prompt = NULL;
-      if (c->barrier_prompt == barrier_prompt) {
+      if ((c->barrier_prompt == barrier_prompt) && barrier_prompt) {
         /* Barrier determines continuation end. */
         c->resume_to = NULL;
         p->stack_start = c->stack_start;