test: add fuzzing coverage for incoming packet parsing - #1829

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (77f8cf0) to head (2f17ae8).
⚠️ Report is 1 commits behind head on master.

@@            Coverage Diff             @@
##           master    #1829      +/-   ##
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+ Coverage   99.81%   99.86%   +0.05%     
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  Files          33       33              
  Lines        3839     3839              
  Branches      553      553              
==========================================
+ Hits         3832     3834       +2     
+ Misses          5        3       -2     
  Partials        2        2              

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Merging this PR will not alter performance

✅ 24 untouched benchmarks

_{Comparing fuzz_incoming (2f17ae8) with master (77f8cf0)¹}

Previous review — superseded by a newer review below.

PR Review — test: add fuzzing coverage for incoming packet parsing

All six findings from the previous round are addressed; what remains are two nits. Merge-ready.

The follow-up commit is a clean response rather than a minimal one: the Hypothesis profile registration moved into tests/conftest.py so it is session-wide config instead of a single module's import side effect; the availability guard is now importlib.util.find_spec("hypothesis"), which covers both PyPy and a non-dev install with one condition, and the accompanying comment's stated "why" now matches reality (I verified .github/workflows/ci.yml:60 pins pypy-3.10 and the lock ships only pp311 wheels). MAX_PACKET_SIZE is derived from const._MAX_MSG_ABSOLUTE with the slack explained, _parse now reprs and hashes every returned record, and test_corpus_parses_cleanly gives the suite the positive control it was missing — without it, a regression that made the parser reject everything would have left all six tests green. The poetry.lock version churn flagged last time is gone (lock-version stays at 2.1), and exceptiongroup already carries a python_version == "3.10" marker wide enough for Hypothesis's backport requirement, so the 3.10 matrix entry resolves.

I re-verified the two properties the suite leans on: DNSRecord.__eq__ compares wire fields only (_dns.py:283-292, 575-594) and excludes created, so the determinism assertion at line 70 is stable; and _seen_logs is capped by _mark_seen (_logger.py:62), while tests/test_logger.py and tests/test_protocol.py both .clear() it up front, so the fuzz run cannot leak state into them.

• _parse still never calls repr(incoming) — the one call that covers truncated, the counts, and every DNSQuestion repr — and always uses the bare DNSIncoming(data) form, while production passes source/scope_id/now (_listener.py:187).
• start = pos % (len(data) - 1) at line 103 breaks on any future corpus entry shorter than 2 bytes.
• Noted, not a finding: nothing in .github/workflows/ sets HYPOTHESIS_PROFILE=long, so CI replays the same derandomized 200 examples and the deep run stays manual — the description now says so explicitly, which is the author's call to make.

✅ Resolved since last review (6)

🟢 Suggestions

def _parse(data: bytes, scope_id: int | None = None) -> list[DNSRecord]:
    incoming = DNSIncoming(data, ("127.0.0.1", 5353), scope_id)
    ...
    repr(incoming)
    return answers

    answers = incoming.answers()
    assert isinstance(answers, list)
    for record in answers:
        repr(record)
        hash(record)
    return answers

    data = bytearray(packet)
    start = pos % (len(data) - 1)
    data[start] = 0xC0 | (target >> 8)
    data[start + 1] = target & 0xFF

Checklist

• No hardcoded secrets or credentials
• No unsafe deserialization / eval / exec
• Error handling — no silently swallowed exceptions introduced
• No unbounded collections (_seen_logs capped by _mark_seen; pre-existing tests clear it)
• Test isolation — Hypothesis profile now set session-wide in conftest, no cross-module global leakage
• Positive control present — corpus liveness pinned, suite cannot pass vacuously
• Edge case and downstream coverage of new tests — suggestion ＃1, suggestion ＃2
• Tests verify observable behavior, not source inspection
• Python-specific: no mutable defaults, no is vs == misuse
• Free-threaded 3.14t / PyPy matrix not broken (cp314t wheels locked; PyPy skipped via find_spec)
• Dependency lock is coherent (content-hash updated, exceptiongroup marker covers 3.10)
• Diff matches PR description, no scope creep

Automated review by Kōan (Claude) HEAD=2f17ae8 4 min 41s