diffoscope: broken at runtime — missing native libarchive dependency (no libarchive package exists) #523

Summary

diffoscope installs and its binary is on PATH, but it cannot start: the Python libarchive-c binding it ships loads the native libarchive shared library via ctypes, and that library is not in the closure.

$ diffoscope --version
Traceback (most recent call last):
  File "/usr/lib/python3.14/site-packages/diffoscope/main.py", line 753, in main
    import libarchive  # noqa
  ...
  File "/usr/lib/python3.14/ctypes/__init__.py", line 491, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: /usr/bin/python3.14: undefined symbol: archive_version_number

Missing or incomplete libarchive module. Try installing your system's 'libarchive' package.

Reproduced in a min session (min 0.5.0-rc2.dev.48.g9f56446f) with diffoscope in the task's packages.

Cause

packages/diffoscope/build.ncl declares:

runtime_deps = [
  python,
],

libarchive-c is a pure-Python ctypes binding — it needs libarchive.so at runtime, which nothing provides. There is also no libarchive package in pkgs at all, so this needs a new package before the dep can be added.

Note the ELF-based --from-build / DT_NEEDED runtime-dep derivation can't catch this class: the dependency is a ctypes.CDLL() lookup at import time, not a DT_NEEDED entry on any binary in the output. Python/ctypes packages need their native libs declared by hand.

Suggested fix

1. Add a libarchive package to pkgs.
2. Add it to diffoscope's runtime_deps.
3. Guard it with a smoke test that actually invokes the tool (diffoscope --version) rather than checking the binary exists — presence and function differ here, which is exactly how this stayed hidden.

Why it matters

diffoscope is the canonical reproducibility-diff tool and is the "second opinion" instrument for inbox#253 — 100% byte-reproducible pkgs. It being silently non-functional means the deep-dive step of that work isn't available in a session.

🤖 Filed via Claude Code