Lua 5.5 compatibility

[thegushi]

Lua 5.5 was released December 2025 and is now the current stable release. Lua 5.4 remains maintained but 5.3 and earlier are end-of-life. OpenDKIM currently blocks 5.5 at configure time (added in PR #264), but the C API changes required are modest.

Breaking C API changes and impact on OpenDKIM

Lua 5.5 change	OpenDKIM affected?
lua_newstate takes a new 3rd seed parameter	Yes — called in 5 places in opendkim-lua.c and once in miltertest.c. Compile error.
lua_dump changed stack behavior + extra writer call to signal end of dump	Yes — called 5 times in opendkim-lua.c. Could cause subtle runtime bugs.
LUA_GCINC/LUA_GCGEN replaced by LUA_GCPARAM	No — OpenDKIM does not touch GC params.
lua_resetthread deprecated	No — not used.
lua_setcstacklimit deprecated	No — not used.
lua_call nresults capped at 250	No — unlikely to hit in practice.
lua_pushvfstring now reports errors instead of raising them	No — not used.

Work needed

1. Add a 3rd NULL argument to all lua_newstate calls.
2. Audit lua_dump usage in opendkim-lua.c — the extra end-of-dump writer call must not corrupt script serialization.
3. Update configure.ac to detect Lua 5.5 and remove the LUA_VERSION_NUM >= 505 upper bound gate (added in PR configure.ac: Improve lua detection with/without pkg-config #264).

[thegushi]

Addressed across two PRs:

• Lua 5.5 C API compatibility #267 (merged): lua_newstate signature fix and initial dkimf_lua_writer guard
• Lua fixes for dkimf_lua_writer and lua_pop placement #268 (merged): corrects the writer guard to use buf == NULL rather than sz == 0, and fixes the lua_pop stack corruption in Lua >= 5.2 (incorporating Lua: Fix stack level after register odkim functions. #201)

[futatuki]

1. Add a 3rd NULL argument to all lua_newstate calls.

The 3rd parameter of lua_newstate is int (seed for hash), not a ponter. (The code in #267 itself is correct).

(edit) not a (signed) int, but a unsigned int. sorry.

[thegushi]

So, is any other work needed?

[futatuki]

It seems the seed for lua_newstate() is used for the randomness of the hash in the new state. In src/lauxlib.c in Lua 5.5.0, it uses luaL_makeseed(NULL) for a seed of lua_newstate(). So may we use it instead of '0' ?