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;
--
Best regards,
Marcel
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