Update list of weak boxes *after* processing finalizers.

It's a bit unlikely, but a finalizer could be arranged for a weak box.
Also add more assertions, clear old range in debug builds, and misc. cleanup.

gc.c

@@ -342,6 +342,8 @@ void gc_init(size_t min_size, size_t max_size)
 static inline void *_gc_alloc(size_t nbytes)
 {
   void *p = gc_free_ptr;
+  assert(nbytes == gc_align(nbytes));
+  assert(nbytes <= gc_free_space());
   gc_free_ptr += nbytes;
   return p;
 }
@@ -361,15 +363,11 @@ static void transfer_object(value_t *value)
 {
   if (is_object(*value))
   {
-    object_t *obj;
+    object_t *obj = _get_object(*value);
     size_t nbytes;
     void *newobj;
-    value_t new_value;
 
     assert(gc_range_of(obj) != gc_current_range);
-    assert(is_object(*value));
-
-    obj = _get_object(*value);
 
     if (obj->tag == BROKEN_HEART)
     {
@@ -396,27 +394,26 @@ static void transfer_object(value_t *value)
       nbytes = sizeof(will_t);
       break;
     case TYPE_TAG_BOX:
+      nbytes = sizeof(box_t);
+      break;
     default: /* pair */
       nbytes = sizeof(pair_t);
       break;
     }
 
     newobj = _gc_alloc(gc_align(nbytes));
-
     memcpy(newobj, obj, nbytes);
 
     /* Keep the original tag bits (pair or object) */
-    new_value = object_value(newobj) | (*value & 2);
-
     obj->tag = BROKEN_HEART;
-    obj->forward = new_value;
+    *value = obj->forward = (object_value(newobj) & ~2) | (*value & 2);
-
-    *value = new_value;
   }
 }
 
 static size_t transfer_vector(vector_t *vec)
 {
+  assert(vec->tag == TYPE_TAG_VECTOR);
+
   for (size_t i = 0; i < vec->size; ++i)
     transfer_object(&vec->elements[i]);
 
@@ -425,6 +422,8 @@ static size_t transfer_vector(vector_t *vec)
 
 static size_t transfer_struct(struct_t *s)
 {
+  assert(s->tag == TYPE_TAG_STRUCT);
+
   transfer_object(&s->type);
 
   for (size_t i = 0; i < s->nslots; ++i)
@@ -442,6 +441,8 @@ static size_t transfer_pair(pair_t *p)
 
 static size_t transfer_will(will_t *w)
 {
+  assert(w->tag == TYPE_TAG_WILL);
+
   transfer_object(&w->finalizer);
 
   /* Weak boxes are discarded when there are no other references,
@@ -497,39 +498,46 @@ static void transfer_roots(void)
 
 static void process_weak_boxes(void)
 {
-  value_t *wb = &gc_weak_box_list;
+  value_t wb = gc_weak_box_list;
 
-  while (!is_nil(*wb))
+  while (!is_nil(wb))
   {
-    if (is_broken_heart(*wb))
-    {
-      /* The box itself is reachable; need to update 'next' pointer to new location */
     weak_box_t *box;
 
-      *wb = _get_object(*wb)->forward;
+    if (is_broken_heart(wb))
-      assert(is_weak_box(*wb));
+    {
-      box = _get_weak_box(*wb);
+      /* Box has been moved; get a pointer to the new location, but don't update list yet. */
+      value_t fw = _get_object(wb)->forward;
+      assert(is_weak_box(fw));
+      box = _get_weak_box(fw);
+    }
+    else
+    {
+      /* Box hasn't been moved yet, but may live on as the value of a will. */
+      assert(is_weak_box(wb));
+      box = _get_weak_box(wb);
+    }
 
     if (is_broken_heart(box->value))
     {
-        /* The value in the box is also reachable; update w/ new location. */
+      /* The value in the box is reachable; update w/ new location. */
       box->value = _get_object(box->value)->forward;
     }
     else if (is_object(box->value))
     {
-        /* The value in the box is an unreachable object; change to #f */
+      /* The value in the box is an unreachable object; change to #f. */
+
+      /* Note that an object is considered unreachable via weak box when it could be finalized,
+       * even though it will be kept alive until the finalizer(s) is/are removed from the 'active'
+       * list and the finalizer(s) itself/themselves may restore the object to a reachable state. */
+
+      /* This last behavior is not recommended. */
+
       box->value = FALSE_VALUE;
     }
 
     /* Move on to this box's 'next' pointer */
-      wb = &_get_weak_box(*wb)->next;
+    wb = box->next;
-    }
-    else
-    {
-      /* Box is no longer reachable; remove it from the list by updating 'next' pointer. */
-      assert(is_weak_box(*wb));
-      *wb = _get_weak_box(*wb)->next;
-    }
   }
 }
 
@@ -577,6 +585,30 @@ static void process_wills(void)
   }
 }
 
+static void update_weak_box_list(void)
+{
+  value_t *wb = &gc_weak_box_list;
+
+  while (!is_nil(*wb))
+  {
+    if (is_broken_heart(*wb))
+    {
+      /* The box itself is reachable; need to update 'next' pointer to new location */
+      *wb = _get_object(*wb)->forward;
+
+      /* Move on to next box's 'next' pointer */
+      assert(is_weak_box(*wb));
+      wb = &_get_weak_box(*wb)->next;
+    }
+    else
+    {
+      /* Box is no longer reachable; remove it from the list by updating 'next' pointer. */
+      assert(is_weak_box(*wb));
+      *wb = _get_weak_box(*wb)->next;
+    }
+  }
+}
+
 #define GC_DEFLATE_SIZE (64*1024)
 
 static void update_soft_limit(size_t min_free)
@@ -585,50 +617,56 @@ static void update_soft_limit(size_t min_free)
   size_t min_limit = bytes_used + min_free;
   size_t new_limit = (4 * min_limit) / 3;
 
+  if (gc_soft_limit > GC_DEFLATE_SIZE)
+  {
+    size_t deflate_limit = gc_soft_limit - GC_DEFLATE_SIZE;
+
+    if (new_limit < deflate_limit)
+      new_limit = deflate_limit;
+  }
+
   if (new_limit > gc_max_size)
     new_limit = gc_max_size;
-#if 1
   else if (new_limit < gc_min_size)
     new_limit = gc_min_size;
 
-  if (gc_soft_limit > GC_DEFLATE_SIZE)
-  {
-    if (new_limit < (gc_soft_limit - GC_DEFLATE_SIZE))
-      new_limit = gc_soft_limit - GC_DEFLATE_SIZE;
-  }
-
   gc_soft_limit = new_limit;
-#else
-  if (new_limit > gc_soft_limit)
-    gc_soft_limit = new_limit;
-#endif
 
   /* Update end of range to reflect new limit */
   gc_range_end = gc_ranges[gc_current_range] + gc_soft_limit;
 
+#ifndef NO_STATS
   if (gc_soft_limit > gc_stats.high_water)
   {
     gc_stats.high_water = gc_soft_limit;
   }
+#endif
 }
 
 static void _collect_garbage(size_t min_free)
 {
   if (gc_enabled)
   {
-    struct timespec start_time;
     char *object_ptr;
 
+#ifndef NO_STATS
+#ifndef NO_TIMING_STATS
+    struct timespec start_time;
     clock_gettime(CLOCK_MONOTONIC, &start_time);
+#endif
+
     gc_stats.total_freed -= gc_free_space();
     ++gc_stats.collections;
+#endif
 
     //debug(("Collecting garbage...\n"));
 
-    /* Swap ranges; new "current" range is initially empty, old one is full */
     swap_gc_ranges();
+
+    /* New "current" range is initially empty, old one is full */
     object_ptr = gc_free_ptr;
 
+    /* Prime the pump */
     transfer_roots();
 
     /* Keep transferring until no more objects in the new range refer to the old one,
@@ -638,20 +676,31 @@ static void _collect_garbage(size_t min_free)
       object_ptr += gc_align(transfer_children((object_t*)object_ptr));
     }
 
+    /* These have to be examined after normal reachability has been determined */
     process_weak_boxes();
     process_wills();
 
     /* Keep transferring until no more objects in the new range refer to the old one.
      * This is so that values which are otherwise unreachable, but have finalizers which
-     * may be able to reach them, are not collected prematurely. Note that these values may
+     * may be able to reach them, are not collected prematurely. process_wills() transfers
+     * the value of any will newly placed on the active list. Note that these values may
      * be finalized in any order, and that any weak references have already been cleared. */
     while (object_ptr < gc_free_ptr)
     {
       object_ptr += gc_align(transfer_children((object_t*)object_ptr));
     }
 
+    update_weak_box_list();
+
+#ifndef NDEBUG
+    /* Clear old range, to make it easier to detect bugs. */
+    memset(gc_ranges[1-gc_current_range], 0, gc_soft_limit);
+#endif
+    
     //debug(("Finished collection with %d bytes to spare (out of %d bytes).\n", gc_free_space(), gc_soft_limit));
 
+#ifndef NO_STATS
+#ifndef NO_TIMING_STATS
     {
       struct timespec end_time;
       nsec_t nsec;
@@ -662,12 +711,15 @@ static void _collect_garbage(size_t min_free)
       nsec += (end_time.tv_nsec - start_time.tv_nsec);
 
       gc_stats.total_ns += nsec;
-      gc_stats.total_freed += gc_free_space();
 
       if (nsec > gc_stats.max_ns)
         gc_stats.max_ns = nsec;
     }
   }
+#endif
+
+  gc_stats.total_freed += gc_free_space();
+#endif
 
   update_soft_limit(min_free);
 
@@ -679,10 +731,9 @@ static void _collect_garbage(size_t min_free)
 
 void collect_garbage(size_t min_free)
 {
-  bool was_enabled = gc_enabled;
+  bool was_enabled = set_gc_enabled(true);
-  gc_enabled = true;
   _collect_garbage(min_free);
-  gc_enabled = was_enabled;
+  set_gc_enabled(was_enabled);
 }
 
 bool set_gc_enabled(bool enable)