Re: [Dev-luatex] append_to_vlist with single return value broken

7 Mar 2020


      On 3/7/2020 2:13 PM, Marcel Fabian Krüger wrote:
...
Hi,
according to the documentation for `append_to_vlist_filter`:
...
The prevdepth is also optional
so it should be allowed to only return a box in which case the prevdepth
value shouldn't change.
But if we actually try this:
\directlua{
   callback.register("append_to_vlist_filter", function(b)
     return b
   end)
}
abc
\bye
LuaTeX answers with
warning  (pdf backend): no pages of output.
because it discards all boxes. This is caused by a bug in luanode.c, lua_appendtovlist_callback:
The callback is called with lua_pcall(Luas, 4, 2, 0). Then LuaTeX tries
to determine if one or two values have been returned by looking at the
type of slot -1: If this is a number, the second return value have been
a number and it is evaluated as new prevdepth. Otherwise
else if (lua_type(Luas, -1) != LUA_TNIL) {
         p = check_isnode(Luas, -1);
         *result = *p;
     }
is evaluated to scan for a single return value which is a node. But because
lua_pcall(Luas, 4, 2, 0) explicitly requested two return values, a
callback with one return values would result in a stack where the one
return value is followed by `nil`. Therefore this `if` block is not
entered.
The problem can be fixed  by always looking at stack slot -2 for the
returned node:
actually we can as well accept nil then so that one can discard something
if (lua_type(Luas, -2) == LUA_TUSERDATA) {
         halfword* p = lmt_check_isnode(Luas, -2);
         *result = *p;
     } else if (lua_type(Luas, -2) == LUA_TNIL) {
         *result = null;
     } else {
         normal_warning("append to vlist","error: node or nil expected");
     }
     if (lua_type(Luas, -1) == LUA_TNUMBER) {
         *next_depth = lmt_roundnumber(Luas, -1);
         *prev_set = 1;
     }


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