Chez Scheme: add test case for recent continuation GC repair

racket/src/ChezScheme/mats/4.ms

@@ -4822,6 +4822,30 @@
                  ;; Check that the GC update the reference to `key2` in `e`:
                  (eq? (car e) key2)))))))
 
+   ;; ----------------------------------------
+   ;; Check GC interaction with zero-sized contnuations - unlikely
+   ;; to crash even if things are wrong, but it's worth a try
+
+   (let ()
+     (define (loop i)
+       (if (zero? i)
+           (call/cc (lambda (k) (k '())))
+           (let ([keeps (loop (sub1 i))])
+             (call/cc
+              (lambda (k)
+                (cons k keeps))))))
+
+     (with-interrupts-disabled
+      (collect 0)
+      (let ([keeps (filter (lambda (c)
+                             (eqv? 0 (#%$continuation-stack-clength c)))
+                           (loop 1000))])
+        ;; The `keeps` list is probably non-empty
+        (collect 0 0)
+        (collect 0 0)
+        (keep-live keeps)
+        #t)))
+
    ;; ----------------------------------------
    ;; Check fasl:
    (let ([s (gensym)])

racket/src/ChezScheme/s/cpnanopass.ss

@@ -12962,6 +12962,8 @@
                                      ,(if 1-shot?
                                           `(immediate ,(constant opportunistic-1-shot-flag))
                                           %ac0))
+                               ;; NB: clength will be 0 if we're reifying to hide a continuation link
+                               ;; where the attachments field is #f. Make sure the GC is ok with that.
                                (set! ,(%mref ,%xp ,(constant continuation-stack-clength-disp)) ,%ac0)
                                (set! ,(%tc-ref scheme-stack-size) ,(%inline - ,(%tc-ref scheme-stack-size) ,%ac0))
                                ,(finish %xp)))]