Closing methods should not interfere with returning values

A closing method cannot be called in its own stack slot, as there may
be returning values in the stack after that slot, and the call would
corrupt those values. Instead, the closing method must be copied to the
top of the stack to be called.

Moreover, even when a function returns no value, its return istruction
still has to have its position (which will set the stack top) after
the local variables, otherwise a closing method might corrupt another
not-yet-called closing method.

lfunc.c

@@ -98,27 +98,29 @@ UpVal *luaF_findupval (lua_State *L, StkId level) {
 
 
 static void callclose (lua_State *L, void *ud) {
-  luaD_callnoyield(L, cast(StkId, ud), 0);
+  UNUSED(ud);
+  luaD_callnoyield(L, L->top - 2, 0);
 }
 
 
 /*
-** Prepare closing method with its argument for object at
-** index 'func' in the stack. Assume there is an error message
-** (or nil) just below the object.
+** Prepare closing method plus its argument for object 'obj' with
+** error message 'err'. (This function assumes EXTRA_STACK.)
 */
-static int prepclosingmethod (lua_State *L, StkId func) {
-  if (ttisfunction(s2v(func))) {  /* object to-be-closed is a function? */
-    setobjs2s(L, func + 1, func - 1);  /* push error msg. as argument */
+static int prepclosingmethod (lua_State *L, TValue *obj, TValue *err) {
+  StkId top = L->top;
+  if (ttisfunction(obj)) {  /* object to-be-closed is a function? */
+    setobj2s(L, top, obj);  /* push function */
+    setobj2s(L, top + 1, err);  /* push error msg. as argument */
   }
   else {  /* try '__close' metamethod */
-    const TValue *tm = luaT_gettmbyobj(L, s2v(func), TM_CLOSE);
+    const TValue *tm = luaT_gettmbyobj(L, obj, TM_CLOSE);
     if (ttisnil(tm))  /* no metamethod? */
       return 0;  /* nothing to call */
-    setobjs2s(L, func + 1, func);  /* 'self' is the argument */
-    setobj2s(L, func, tm);  /* will call metamethod */
+    setobj2s(L, top, tm);  /* will call metamethod... */
+    setobj2s(L, top + 1, obj);  /* with 'self' as the argument */
   }
-  L->top = func + 2;  /* add function and argument */
+  L->top = top + 2;  /* add function and argument */
   return 1;
 }
 
@@ -129,22 +131,24 @@ static int prepclosingmethod (lua_State *L, StkId func) {
 ** will be handled there. Otherwise, a previous error already
 ** activated original protected call, and so the call to the
 ** closing method must be protected here.
+** If status is OK, the call to the closing method will be pushed
+** at the top of the stack. Otherwise, values are pushed after
+** the 'level' of the upvalue being closed, as everything after
+** that won't be used again.
 */
 static int closeupval (lua_State *L, TValue *uv, StkId level, int status) {
-  StkId func = level + 1;  /* save slot for old error message */
-  if (unlikely(status != LUA_OK))  /* was there an error? */
-    luaD_seterrorobj(L, status, level);  /* save error message */
-  else
-    setnilvalue(s2v(level));  /* no error message */
-  setobj2s(L, func, uv);  /* put object on top of error message */
-  if (!prepclosingmethod(L, func))
-    return status;  /* nothing to call */
-  if (likely(status == LUA_OK))  /* not in "error mode"? */
-    callclose(L, func);  /* call closing method */
-  else {  /* already inside error handler; cannot raise another error */
-    int newstatus = luaD_pcall(L, callclose, func, savestack(L, level), 0);
-    if (newstatus != LUA_OK)  /* error when closing? */
-      status = newstatus;  /* this will be the new error */
+  if (likely(status == LUA_OK)) {
+    if (prepclosingmethod(L, uv, &G(L)->nilvalue))  /* something to call? */
+      callclose(L, NULL);  /* call closing method */
+  }
+  else {  /* there was an error */
+    /* save error message and set stack top to 'level + 1' */
+    luaD_seterrorobj(L, status, level);
+    if (prepclosingmethod(L, uv, s2v(level))) {  /* something to call? */
+      int newstatus = luaD_pcall(L, callclose, NULL, savestack(L, level), 0);
+      if (newstatus != LUA_OK)  /* another error when closing? */
+        status = newstatus;  /* this will be the new error */
+    }
   }
   return status;
 }
@@ -169,12 +173,10 @@ static void trynewtbcupval (lua_State *L, void *ud) {
 void luaF_newtbcupval (lua_State *L, StkId level) {
   int status = luaD_rawrunprotected(L, trynewtbcupval, level);
   if (unlikely(status != LUA_OK)) {  /* memory error creating upvalue? */
-    StkId func = level + 1;
     lua_assert(status == LUA_ERRMEM);
-    setobjs2s(L, func, level);  /* open space for error message */
-    luaD_seterrorobj(L, status, level);  /* save error message */
-    if (prepclosingmethod(L, func))
-      callclose(L, func);  /* call closing method */
+    luaD_seterrorobj(L, LUA_ERRMEM, level + 1);  /* save error message */
+    if (prepclosingmethod(L, s2v(level), s2v(level + 1)))
+      callclose(L, NULL);  /* call closing method */
     luaD_throw(L, LUA_ERRMEM);  /* throw memory error */
   }
 }
@@ -201,7 +203,7 @@ int luaF_close (lua_State *L, StkId level, int status) {
     luaC_barrier(L, uv, slot);
     if (status >= 0 && uv->tt == LUA_TUPVALTBC) {  /* must be closed? */
       ptrdiff_t levelrel = savestack(L, level);
-      status = closeupval(L, uv->v, upl, status);  /* may reallocate the stack */
+      status = closeupval(L, uv->v, upl, status);  /* may realloc. the stack */
       level = restorestack(L, levelrel);
     }
   }