functions with identical description can cause problems

[Udi-Fogiel]

Brief outline of the bug

When adding several functions with the same descriptions,
there is no warning or error, so if a user does not do any checking
he may add a function with description that is already exists
in the callback, then he may accidentally remove the wrong
function.

What is the expected usage here? Use a prefix as
a name space? check for the existence of such
description before adding the function?
or should there be a test in ltluatex?

Minimal example showing the bug

\directlua{
luatexbase.create_callback("foo", "data", false)
luatexbase.add_to_callback("foo", function() print("bar") return true end, "foo")
luatexbase.add_to_callback("foo", function() print("baz") return true end, "foo")
for _, desc in ipairs(luatexbase.callback_descriptions("foo")) do print(desc) end
print("================================")
luatexbase.remove_from_callback("foo", "foo")
for _, desc in ipairs(luatexbase.callback_descriptions("foo")) do print(desc) end
print("================================")
luatexbase.call_callback("foo")
}

\stop

foo
foo
================================
foo
================================
baz

Log file (required) and possibly PDF file

This is LuaHBTeX, Version 1.18.0 (TeX Live 2024)  (format=lualatex 2025.1.1)  12 JAN 2025 15:34
 restricted system commands enabled.
**lualineno
(./lualineno.tex
LaTeX2e <2024-11-01> patch level 1
L3 programming layer <2024-12-09>
Lua module: luaotfload 2024-02-14 v3.28 Lua based OpenType font support
Lua module: lualibs 2023-07-13 v2.76 ConTeXt Lua standard libraries.
Lua module: lualibs-extended 2023-07-13 v2.76 ConTeXt Lua libraries -- extended 
collection.
luaotfload | conf : Root cache directory is "/home/udico/.texlive2024/texmf-var/
luatex-cache/generic/names".
luaotfload | init : Loading fontloader "fontloader-2023-12-28.lua" from kpse-res
olved path "/usr/local/texlive/2024/texmf-dist/tex/luatex/luaotfload/fontloader-
2023-12-28.lua".
Lua-only attribute luaotfload@noligature = 1
luaotfload | init : Context OpenType loader version 3.134
Inserting `luaotfload.node_processor' in `pre_linebreak_filter'.
Inserting `luaotfload.node_processor' in `hpack_filter'.
Inserting `luaotfload.glyph_stream' in `glyph_stream_provider'.
Inserting `luaotfload.define_font' in `define_font'.
Lua-only attribute luaotfload_color_attribute = 2
luaotfload | conf : Root cache directory is "/home/udico/.texlive2024/texmf-var/
luatex-cache/generic/names".
Inserting `luaotfload.harf.strip_prefix' in `find_opentype_file'.
Inserting `luaotfload.harf.strip_prefix' in `find_truetype_file'.
Removing  `luaotfload.glyph_stream' from `glyph_stream_provider'.
Inserting `luaotfload.harf.glyphstream' in `glyph_stream_provider'.
Inserting `luaotfload.harf.finalize_vlist' in `post_linebreak_filter'.
Inserting `luaotfload.harf.finalize_hlist' in `hpack_filter'.
Inserting `luaotfload.cleanup_files' in `wrapup_run'.
Inserting `luaotfload.harf.finalize_unicode' in `finish_pdffile'.
Inserting `luaotfload.glyphinfo' in `glyph_info'.
Lua-only attribute luaotfload.letterspace_done = 3
Inserting `luaotfload.aux.set_sscale_dimens' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.set_font_index' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.patch_cambria_domh' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.fixup_fontdata' in `luaotfload.patch_font_unsafe'.
Inserting `luaotfload.aux.set_capheight' in `luaotfload.patch_font'.
Inserting `luaotfload.aux.set_xheight' in `luaotfload.patch_font'.
Inserting `luaotfload.rewrite_fontname' in `luaotfload.patch_font'.
Inserting `tracingstacklevels' in `input_level_string'.
Inserting `foo' in `foo'.
Inserting `foo' in `foo'.
Removing  `foo' from `foo'.)

Here is how much of LuaTeX's memory you used:
 16 strings out of 475554
 100000,552014 words of node,token memory allocated
 339 words of node memory still in use:
   2 hlist, 1 vlist, 2 glue, 3 kern, 1 glyph, 3 attribute, 39 glue_spec, 3 attri
bute_list, 1 write nodes
   avail lists: 2:7,3:2,4:1,5:2,7:1,9:1
 23099 multiletter control sequences out of 65536+600000
 14 fonts using 591679 bytes
 12i,1n,13p,92b,15s stack positions out of 10000i,1000n,20000p,200000b,200000s

warning  (pdf backend): no pages of output.

PDF statistics: 0 PDF objects out of 1000 (max. 8388607)
 0 named destinations out of 1000 (max. 131072)
 1 words of extra memory for PDF output out of 10000 (max. 100000000)

[davidcarlisle]

certainly the description despite its name, is essentially the unique identifier for the function, so adding two with the same is user error. I suppose the question is whether add_to_callback should error if there is already a function with that description in the specified callback, or whether this should be left to the documentation (which could be clearer on this point)

[Udi-Fogiel]

Yes, that what I was thinking. Initially I thought there was some convention in LaTeX seeing luaofload, luacolor and babel using the package name as a prefix, but not all packages are doing so...