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