fix gc handling of ephemeron chains

collects/tests/racket/will.rktl

@@ -149,4 +149,22 @@
      
 ;; ----------------------------------------
 
+(let ([something (gensym)])
+  (define e-chain
+    (let loop ([n 100] [e #f])
+      (if (zero? n)
+          e
+          (loop (sub1 n)
+                (make-ephemeron something e)))))
+  (collect-garbage)
+  (test 100 'epehemeron-chain
+        (let loop ([e e-chain])
+          (if e
+              (add1 (loop (ephemeron-value e)))
+              0)))
+  ;; ensure that `something' is retained:
+  (test #t symbol? (list-ref (list something) (random 1))))
+     
+;; ----------------------------------------
+
 (report-errs)

src/racket/gc2/newgc.c

@@ -2974,6 +2974,14 @@ static void propagate_marks(NewGC *gc)
     propagate_marks_worker(gc, mark_table, p);
   }
 }
+
+static void propagate_marks_plus_ephemerons(NewGC *gc) 
+{
+  do {
+    propagate_marks(gc);
+  } while (mark_ready_ephemerons(gc));
+}
+
 #ifdef MZ_USE_PLACES
 static void promote_marked_gen0_big_pages(NewGC *gc) {
   mpage *page;
@@ -4162,17 +4170,13 @@ static void garbage_collect(NewGC *gc, int force_full, int switching_master)
 
   /* now propagate/repair the marks we got from these roots, and do the
      finalizer passes */
-  propagate_marks(gc);
-  mark_ready_ephemerons(gc); 
-  propagate_marks(gc); 
+  propagate_marks_plus_ephemerons(gc);
 
   check_finalizers(gc, 1);
-  mark_ready_ephemerons(gc);
-  propagate_marks(gc);
+  propagate_marks_plus_ephemerons(gc);
 
   check_finalizers(gc, 2);
-  mark_ready_ephemerons(gc);
-  propagate_marks(gc);
+  propagate_marks_plus_ephemerons(gc);
 
   if(gc->gc_full) zero_weak_finalizers(gc);
   do_ordered_level3(gc); propagate_marks(gc);

src/racket/gc2/weak.c

@@ -283,21 +283,25 @@ void init_ephemerons(GCTYPE *gc) {
   gc->num_last_seen_ephemerons = 0;
 }
 
-static void mark_ready_ephemerons(GCTYPE *gc)
+static int mark_ready_ephemerons(GCTYPE *gc)
 {
   GC_Ephemeron *waiting = NULL, *next, *eph;
+  int did_one = 0;
 
   for (eph = gc->ephemerons; eph; eph = next) {
     next = eph->next;
     if (is_marked(gc, eph->key)) {
       gcMARK2(eph->val, gc);
       gc->num_last_seen_ephemerons++;
+      did_one = 1;
     } else {
       eph->next = waiting;
       waiting = eph;
     }
   }
   gc->ephemerons = waiting;
+
+  return did_one;
 }
 
 static void zero_remaining_ephemerons(GCTYPE *gc)