Update reader to fix corner-cases in the placeholder/back-reference system.

gc.h

@@ -224,6 +224,11 @@ static inline bool is_nil(value_t v)
   return v == NIL;
 }
 
+static inline bool is_undefined(value_t v)
+{
+  return v == UNDEFINED;
+}
+
 static inline value_t boolean_value(bool b)
 {
   return b ? TRUE_VALUE : FALSE_VALUE;

reader.c

@@ -35,10 +35,12 @@ static value_t read_backref(reader_state_t *state);
 static void next_char(reader_state_t *state);
 static void skip_whitespace(reader_state_t *state);
 
-static value_t make_placeholder(reader_state_t *state, fixnum_t ref);
+static bool    is_placeholder(reader_state_t *state, value_t value);
 static value_t get_placeholder(reader_state_t *state, fixnum_t ref);
+static void    set_placeholder(reader_state_t *state, value_t place, value_t value);
+static value_t finalize_placeholder(reader_state_t *state, value_t place);
 static value_t patch_placeholders(reader_state_t *state, value_t v);
-static void set_placeholder(value_t place, value_t value);
+
 static void tree_replace(value_t *in, value_t oldval, value_t newval);
 
 value_t read_value(FILE *f)
@@ -489,9 +491,10 @@ static value_t read_definition(reader_state_t *state)
   release_assert(state->ch == '=');
   next_char(state);
 
-  register_gc_root(&place_root, make_placeholder(state, ref));
+  register_gc_root(&place_root, get_placeholder(state, ref));
   v = read_one_value(state);
-  set_placeholder(place_root.value, v);
+  release_assert(!is_placeholder(state, v));
+  set_placeholder(state, place_root.value, v);
   unregister_gc_root(&place_root);
 
   return v;
@@ -516,48 +519,63 @@ static value_t read_backref(reader_state_t *state)
   }
 }
 
-static value_t make_placeholder(reader_state_t *state, fixnum_t ref)
+static bool is_placeholder(reader_state_t *state, value_t value)
 {
-  state->ref_alist.value = cons(UNDEFINED, state->ref_alist.value);
+  for (value_t item = state->ref_alist.value; !is_nil(item); item = _CDDR(item))
-  state->ref_alist.value = cons(fixnum_value(ref), state->ref_alist.value);
+  {
-  return state->ref_alist.value;
+    if (value == item)
+      return true;
+  }
+  return false;
 }
 
 static value_t get_placeholder(reader_state_t *state, fixnum_t ref)
 {
   value_t refval = fixnum_value(ref);
-  value_t item = state->ref_alist.value;
 
-  while (!is_nil(item))
+  for (value_t item = state->ref_alist.value; !is_nil(item); item = _CDDR(item))
   {
     if (_CAR(item) == refval)
       return item;
-    else
-      item = _CDDR(item);
   }
 
-  release_assert(NOTREACHED("Back-reference without definition!"));
+  /* No existing placeholder with that number; create a new one. */
-  return UNDEFINED;
+  state->ref_alist.value = cons(UNDEFINED, state->ref_alist.value);
+  state->ref_alist.value = cons(refval,    state->ref_alist.value);
+  return state->ref_alist.value;
+}
+
+static void set_placeholder(reader_state_t *state, value_t place, value_t value)
+{
+  assert(is_placeholder(state, place));
+  release_assert(is_undefined(_CADR(place)));
+  _CADR(place) = value;
+}
+
+static value_t finalize_placeholder(reader_state_t *state, value_t place)
+{
+  assert(is_placeholder(state, place));
+
+  if (is_placeholder(state, _CADR(place)))
+  {
+    value_t other_place = _CADR(place);
+    _CADR(place) = UNDEFINED;
+    _CADR(place) = finalize_placeholder(state, other_place);
+  }
+
+  return _CADR(place);
 }
 
 static value_t patch_placeholders(reader_state_t *state, value_t v)
 {
-  value_t item = state->ref_alist.value;
+  for (value_t item = state->ref_alist.value; !is_nil(item); item = _CDDR(item))
-
-  while (!is_nil(item))
   {
-    tree_replace(&v, item, _CADR(item));
+    tree_replace(&v, item, finalize_placeholder(state, item));
-    item = _CDDR(item);
   }
 
   return v;
 }
 
-static void set_placeholder(value_t place, value_t value)
-{
-  CAR(CDR(place)) = value;
-}
-
 typedef struct seen_value
 {
   value_t value;