Hacktoberfest 2026：メンテナが10月に向けて印を付けた、オープンで初心者向けの issue。 Hacktoberfest の issue を見る

CFFI backend parity gaps: garbled `ZstdError` messages, `__exit__` ordering mismatch with C backend, minor parameter-validation drift

オープン
  • 何か問題がありますか？
#301 コメント 0 件 リアクション 0 件 担当者 0 名 GitHub で見る

メンテナーはふだん 10 日以内に返信

まだ誰も着手していません。

評価

難易度
4/5
見積もり時間
3〜5日
初心者へのやさしさ
52/100
issue の種類
バグ
明瞭さ
おおむね明確
活発さ
静か
技術スタック
c, python
領域
backend

調査の方向性

zstandard/backend_cffi.py で報告されている箇所から始め、対応する C-backend の実装と比較してください。可能であれば PyPy または CFFI のみを使用する環境で、報告されている 3 つのパリティの差異を検証してください。エラー表示が修正され、exit の順序が一致し、パラメーター検証の動作が C-backend と一致すれば完了です。

索引モデルが issue の本文から書いたものです。

説明

Summary

The pure-Python CFFI backend has several small-but-real parity gaps with the C extension backend. The most visible is that several ZstdError constructors are called with positional args as if %-formatting were going to happen, but it doesn't — so user-facing error messages look like garbled tuples. Two additional gaps affect resource-lifecycle invariants and parameter validation.

Impact

  • Severity: User-visible garbled error messages (main issue); resource-lifecycle invariant violation in __exit__ (secondary); small parameter-validation drift.
  • Reachability: Any user running the CFFI backend — typically PyPy, or environments where the C extension can't be built.
  • Version: 0.25.0 (commit 7a77a75).
  • Note: Report was produced without a live CFFI environment available; the below is based on static review of zstandard/backend_cffi.py. Happy to verify with a concrete reproducer if you set up a PyPy / CFFI-only test environment.

Gap 1: Tuple-args to ZstdError — garbled error messages

Pattern: raise ZstdError("... %s", error) passes two positional args to the exception constructor, making .args == ("... %s", error). No %-formatting happens. The rendered message looks like ('..., %s', <something>) instead of the intended interpolated string.

Sites: zstandard/backend_cffi.py:1531, 1575, 1682.

Fix: raise ZstdError("... %s" % error) or an f-string.

Gap 2: __exit__ ordering mismatch with the C backend

C backend's __exit__ calls close() first, then clears the compressor/decompressor field. CFFI backend sets _compressor = None first, then calls close() — which runs with the invariants already broken. Any reference to self._compressor inside close() observes None.

Sites: zstandard/backend_cffi.py:1413, 3161.

Fix: Reorder to match the C backend:

def __exit__(self, exc_type, exc_value, tb):
    self.close()                    # first
    self._compressor = None         # then clear
    return False

Gap 3: Minor parameter-validation drift

  • Off-by-one in level-validation error message. CFFI says "less than 22"; C says "less than 23". One of them has the boundary wrong (if the C backend's boundary is correct — ZSTD_maxCLevel() returns 22 — then both messages should say "less than 23" in the "strictly-less-than" formulation or "more than 22" in the symmetric one).
  • compressobj(size=-1) is accepted by CFFI but raises OverflowError in C — signed/unsigned mismatch somewhere in the CFFI argument handling.
  • CFFI backend does not declare Py_MOD_GIL_NOT_USED (doesn't apply — it's pure Python), but has no FT-story either; worth either documenting or gating free-threaded wheels to C-backend-only.

Suggested PR shape

All three gaps are one small PR (pure-Python fixes, likely ~20 lines of diff). If you'd prefer to keep C-side and CFFI-side PRs separate I can do that — otherwise one combined parity PR seems cleanest.

Methodology

Found via cext-review-toolkit — the parity-checker agent identifies places where two implementations of the same interface diverge. Gaps 1 and 2 were flagged via structural pattern matching against the C backend; Gap 3 via argument-validation diffing. The CFFI environment wasn't available during the analysis, so none of the above was live-reproduced; verification is recommended but the diffs are small and the patterns are unambiguous on inspection.

Discovery, root-cause analysis, and issue drafting were performed by Claude Code and reviewed by a human before filing.

Full report

Complete multi-agent analysis (48 FIX findings across 13 categories, plus a reproducer appendix): https://gist.github.com/devdanzin/b86039ac097141579590c1a0f3a43605

主要言語
C
スター
642
フォーク
118
平均マージ
9日 3時間
マージ済み PR（30日）
6

環境構築

はじめの一歩

  1. issue を最後まで読み、次にプロジェクトのコントリビューションガイドを読みます。
  2. 着手することを issue にコメントします — 二人が同じ作業をするのを防げます。
  3. リポジトリをフォークし、ブランチを切って変更します。
  4. issue 番号を参照したプルリクエストを送ります。

indygreg/python-zstandard のほかの issue

indygreg/python-zstandard の issue をすべて見る

似ている issue

C の issue をもっと見る

新しい issue をメールで受け取る

初心者向けの GitHub issue を短くまとめたダイジェスト。