On Sat, Mar 7, 2020 at 2:13 PM Marcel Fabian Krüger <t...@2krueger.de> 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:
>
> source/texk/web2c/luatexdir/lua/luanode.c | 9 +++------
> 1 file changed, 3 insertions(+), 6 deletions(-)
>
> diff --git a/source/texk/web2c/luatexdir/lua/luanode.c
> b/source/texk/web2c/luatexdir/lua/luanode.c
> index a0c97e37d..d4baab96d 100644
> --- a/source/texk/web2c/luatexdir/lua/luanode.c
> +++ b/source/texk/web2c/luatexdir/lua/luanode.c
> @@ -183,12 +183,9 @@ int lua_appendtovlist_callback(halfword box, int
> location, halfword prev_depth,
> if (lua_type(Luas,-1) == LUA_TNUMBER) {
> *next_depth = lua_roundnumber(Luas,-1);
> *prev_set = true;
> - if (lua_type(Luas, -2) != LUA_TNIL) {
> - p = check_isnode(Luas, -2);
> - *result = *p;
> - }
> - } else if (lua_type(Luas, -1) != LUA_TNIL) {
> - p = check_isnode(Luas, -1);
> + }
> + if (lua_type(Luas, -2) != LUA_TNIL) {
> + p = check_isnode(Luas, -2);
> *result = *p;
> }
> return 1;
> --
>
Thank you very much. Very likely I will see it this evening.
--
luigi
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