Fixed bug of keys removed from tables vs 'next'

Fixed the bug that a key removed from a table might not be found
again by 'next'. (This is needed to allow keys to be removed during a
traversal.) This bug was introduced in commit 73ec04fc.

lgc.c

@@ -161,18 +161,17 @@ static void linkgclist_ (GCObject *o, GCObject **pnext, GCObject **list) {
 
 
 /*
-** Clear keys for empty entries in tables. If entry is empty
-** and its key is not marked, mark its entry as dead. This allows the
-** collection of the key, but keeps its entry in the table (its removal
-** could break a chain). The main feature of a dead key is that it must
-** be different from any other value, to do not disturb searches.
-** Other places never manipulate dead keys, because its associated empty
-** value is enough to signal that the entry is logically empty.
+** Clear keys for empty entries in tables. If entry is empty, mark its
+** entry as dead. This allows the collection of the key, but keeps its
+** entry in the table: its removal could break a chain and could break
+** a table traversal.  Other places never manipulate dead keys, because
+** its associated empty value is enough to signal that the entry is
+** logically empty.
 */
 static void clearkey (Node *n) {
   lua_assert(isempty(gval(n)));
-  if (keyiswhite(n))
-    setdeadkey(n);  /* unused and unmarked key; remove it */
+  if (keyiscollectable(n))
+    setdeadkey(n);  /* unused key; remove it */
 }
 
 

lobject.h

@@ -21,10 +21,12 @@
 */
 #define LUA_TUPVAL	LUA_NUMTYPES  /* upvalues */
 #define LUA_TPROTO	(LUA_NUMTYPES+1)  /* function prototypes */
+#define LUA_TDEADKEY	(LUA_NUMTYPES+2)  /* removed keys in tables */
+
 
 
 /*
-** number of all possible types (including LUA_TNONE)
+** number of all possible types (including LUA_TNONE but excluding DEADKEY)
 */
 #define LUA_TOTALTYPES		(LUA_TPROTO + 2)
 
@@ -555,7 +557,7 @@ typedef struct Proto {
 
 /*
 ** {==================================================================
-** Closures
+** Functions
 ** ===================================================================
 */
 
@@ -743,13 +745,13 @@ typedef struct Table {
 
 
 /*
-** Use a "nil table" to mark dead keys in a table. Those keys serve
-** to keep space for removed entries, which may still be part of
-** chains. Note that the 'keytt' does not have the BIT_ISCOLLECTABLE
-** set, so these values are considered not collectable and are different
-** from any valid value.
+** Dead keys in tables have the tag DEADKEY but keep their original
+** gcvalue. This distinguishes them from regular keys but allows them to
+** be found when searched in a special way. ('next' needs that to find
+** keys removed from a table during a traversal.)
 */
-#define setdeadkey(n)	(keytt(n) = LUA_TTABLE, gckey(n) = NULL)
+#define setdeadkey(node)	(keytt(node) = LUA_TDEADKEY)
+#define keyisdead(node)		(keytt(node) == LUA_TDEADKEY)
 
 /* }================================================================== */
 

ltable.c

@@ -172,11 +172,17 @@ static Node *mainpositionTV (const Table *t, const TValue *key) {
 ** be equal to floats. It is assumed that 'eqshrstr' is simply
 ** pointer equality, so that short strings are handled in the
 ** default case.
-*/
-static int equalkey (const TValue *k1, const Node *n2) {
-  if (rawtt(k1) != keytt(n2))  /* not the same variants? */
+** A true 'deadok' means to accept dead keys as equal to their original
+** values, which can only happen if the original key was collectable.
+** All dead values are compared in the default case, by pointer
+** identity.  (Note that dead long strings are also compared by
+** identity).
+*/
+static int equalkey (const TValue *k1, const Node *n2, int deadok) {
+  if ((rawtt(k1) != keytt(n2)) &&  /* not the same variants? */
+       !(deadok && keyisdead(n2) && iscollectable(k1)))
    return 0;  /* cannot be same key */
-  switch (ttypetag(k1)) {
+  switch (keytt(n2)) {
     case LUA_VNIL: case LUA_VFALSE: case LUA_VTRUE:
       return 1;
     case LUA_VNUMINT:
@@ -187,7 +193,7 @@ static int equalkey (const TValue *k1, const Node *n2) {
       return pvalue(k1) == pvalueraw(keyval(n2));
     case LUA_VLCF:
       return fvalue(k1) == fvalueraw(keyval(n2));
-    case LUA_VLNGSTR:
+    case ctb(LUA_VLNGSTR):
       return luaS_eqlngstr(tsvalue(k1), keystrval(n2));
     default:
       return gcvalue(k1) == gcvalueraw(keyval(n2));
@@ -251,11 +257,12 @@ static unsigned int setlimittosize (Table *t) {
 /*
 ** "Generic" get version. (Not that generic: not valid for integers,
 ** which may be in array part, nor for floats with integral values.)
+** See explanation about 'deadok' in function 'equalkey'.
 */
-static const TValue *getgeneric (Table *t, const TValue *key) {
+static const TValue *getgeneric (Table *t, const TValue *key, int deadok) {
   Node *n = mainpositionTV(t, key);
   for (;;) {  /* check whether 'key' is somewhere in the chain */
-    if (equalkey(key, n))
+    if (equalkey(key, n, deadok))
       return gval(n);  /* that's it */
     else {
       int nx = gnext(n);
@@ -292,7 +299,7 @@ static unsigned int findindex (lua_State *L, Table *t, TValue *key,
   if (i - 1u < asize)  /* is 'key' inside array part? */
     return i;  /* yes; that's the index */
   else {
-    const TValue *n = getgeneric(t, key);
+    const TValue *n = getgeneric(t, key, 1);
     if (unlikely(isabstkey(n)))
       luaG_runerror(L, "invalid key to 'next'");  /* key not found */
     i = cast_int(nodefromval(n) - gnode(t, 0));  /* key index in hash table */
@@ -730,7 +737,7 @@ const TValue *luaH_getstr (Table *t, TString *key) {
   else {  /* for long strings, use generic case */
     TValue ko;
     setsvalue(cast(lua_State *, NULL), &ko, key);
-    return getgeneric(t, &ko);
+    return getgeneric(t, &ko, 0);
   }
 }
 
@@ -750,7 +757,7 @@ const TValue *luaH_get (Table *t, const TValue *key) {
       /* else... */
     }  /* FALLTHROUGH */
     default:
-      return getgeneric(t, key);
+      return getgeneric(t, key, 0);
   }
 }
 

testes/nextvar.lua

@@ -359,6 +359,38 @@ end
 assert(n == 5)
 
 
+do
+  print("testing next x GC of deleted keys")
+  -- bug in 5.4.1
+  local co = coroutine.wrap(function (t)
+    for k, v in pairs(t) do
+        local k1 = next(t)    -- all previous keys were deleted
+        assert(k == k1)       -- current key is the first in the table
+        t[k] = nil
+        local expected = (type(k) == "table" and k[1] or
+                          type(k) == "function" and k() or
+                          string.sub(k, 1, 1))
+        assert(expected == v)
+        coroutine.yield(v)
+    end
+  end)
+  local t = {}
+  t[{1}] = 1    -- add several unanchored, collectable keys
+  t[{2}] = 2
+  t[string.rep("a", 50)] = "a"    -- long string
+  t[string.rep("b", 50)] = "b"
+  t[{3}] = 3
+  t[string.rep("c", 10)] = "c"    -- short string
+  t[function () return 10 end] = 10
+  local count = 7
+  while co(t) do
+    collectgarbage("collect")   -- collect dead keys
+    count = count - 1
+  end
+  assert(count == 0 and next(t) == nil)    -- traversed the whole table
+end
+
+
 local function test (a)
   assert(not pcall(table.insert, a, 2, 20));
   table.insert(a, 10); table.insert(a, 2, 20);