fix sync on inaccessible place channel

Commit 5ea4c2ab68 broke GCing of a thread that is blocked
via `sync` (as opposed to `place-channel-get`) on a place
channel whose write end is inaccessible.

pkgs/racket-pkgs/racket-test/tests/racket/place-channel.rkt

@@ -421,27 +421,30 @@
   ;; can be GCed if the other end of the channel is
   ;; unreachable --- where a place's channels should
   ;; all count as "unreachable" when the place ends
-  (displayln "checking place-channel and thread GC interaction")
-  (let ([N 40])
-    (define weaks (make-weak-hash))
-    (for ([i (in-range N)])
-      (define s (make-semaphore))
-      (hash-set!
-       weaks
-       (thread (lambda ()
-                 (define-values (i o) (place-channel))
-                 (define p (place ch (place-channel-get ch)))
-                 (place-channel-put p o)
-                 (place-wait p)
-                 (semaphore-post s)
-                 (sync i)))
-       #t)
-      (sync s))
-    (for ([i 3])
-      (sync (system-idle-evt))
-      (collect-garbage))
-    (unless ((hash-count weaks) . < . (/ N 2))
-      (error "thread-gc test failed")))
+  (define (check-thread sync-ch)
+    (displayln "checking place-channel and thread GC interaction")
+    (let ([N 40])
+      (define weaks (make-weak-hash))
+      (for ([i (in-range N)])
+        (define s (make-semaphore))
+        (hash-set!
+         weaks
+         (thread (lambda ()
+                   (define-values (i o) (place-channel))
+                   (define p (place ch (place-channel-get ch)))
+                   (place-channel-put p o)
+                   (place-wait p)
+                   (semaphore-post s)
+                   (sync-ch i)))
+         #t)
+        (sync s))
+      (for ([i 3])
+        (sync (system-idle-evt))
+        (collect-garbage))
+      (unless ((hash-count weaks) . < . (/ N 2))
+        (error "thread-gc test failed"))))
+  (check-thread place-channel-get)
+  (check-thread sync)
 
 )
   

racket/src/racket/src/place.c

@@ -3547,7 +3547,14 @@ static int place_channel_ready(Scheme_Object *so, Scheme_Schedule_Info *sinfo) {
 
     return 1;
   }
-  
+
+  if (no_writers) {
+    /* block on a semaphore that is not accessible, which may allow the thread
+       to be GCed */
+    scheme_set_sync_target(sinfo, scheme_make_sema(0), scheme_void, NULL, 0, 0, NULL);
+    return 0;
+  }
+
   return 0;
 }