fix handling of choice evt returned by a guard evt

Closes PR 14195

Merge to v6.0

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

@@ -241,6 +241,28 @@
   (test 0.15 sync/timeout 18 
 	(choice-evt (make-delay 0.2) (make-delay 0.15))))
 
+;;check flattening of choice evts returned by a guard:
+(let ()
+  (define s1 (make-semaphore))
+  (define s2 (make-semaphore))
+  (define s3 (make-semaphore))
+  (define s4 (make-semaphore))
+  
+  (define evt1 (choice-evt s1 s2))
+  (define evt2 (choice-evt s3 s4))
+
+  (thread (lambda () 
+            (sync (system-idle-evt))
+            (semaphore-post
+             (list-ref (list s1 s2 s3 s4)
+                       (random 4)))))
+
+  (test #t
+        semaphore?
+        (sync (guard-evt
+               (lambda ()
+                 (choice-evt evt1 evt2))))))
+
 ;; ----------------------------------------
 ;; Wrapped waitables
 

racket/src/racket/src/thread.c

@@ -5947,6 +5947,7 @@ void scheme_clear_thread_sync(Scheme_Thread *p)
 /*========================================================================*/
 
 static void syncing_needs_wakeup(Scheme_Object *s, void *fds);
+static Evt_Set *make_evt_set(const char *name, int argc, Scheme_Object **argv, int delta, int flatten);
 
 typedef struct Evt {
   MZTAG_IF_REQUIRED
@@ -6168,7 +6169,15 @@ static void set_sync_target(Syncing *syncing, int i, Scheme_Object *target,
 
   if (SCHEME_EVTSETP(target) && retry) {
     /* Flatten the set into this one */
-    Evt_Set *wts = (Evt_Set *)target;
+    Evt_Set *wts;
+
+    if (SCHEME_EVTSET_UNFLATTENEDP(target)) {
+      Scheme_Object *a[1];
+      a[0] = target;
+      wts = make_evt_set("sync", 1, a, 0, 1);
+    } else
+      wts = (Evt_Set *)target;
+
     if (wts->argc == 1) {
       /* 1 thing in set? Flattening is easy! */
       evt_set->argv[i] = wts->argv[0];
@@ -6486,7 +6495,7 @@ static int evt_set_flatten(Evt_Set *e, int pos, Scheme_Object **args, Evt **ws)
   return pos;
 }
 
-Evt_Set *make_evt_set(const char *name, int argc, Scheme_Object **argv, int delta, int flatten)
+static Evt_Set *make_evt_set(const char *name, int argc, Scheme_Object **argv, int delta, int flatten)
 {
   Evt *w, **iws, **ws;
   Evt_Set *evt_set, *subset;