test(gap): fuzz uncached parser with logged seed, add truncated-length regression

[bluetoothbot]

What

Rework the GAP fuzz test along the lines @bdraco called out on the previous revision.

Why

The earlier seeded version turned the fuzz test into a fixed 1000-input corpus that finds nothing new after the first green run, and the fan-out across the cached parse_advertisement_data{,_bytes,_tuple} wrappers added cost without coverage — those wrappers are thin lru_cache shells over _uncached_parse_advertisement_data, so an overflow can only originate in the uncached path. The original test's non-determinism is the feature, not the bug.

How

• Broad fuzz loop is back to unseeded randomness, but prints the seed each run and accepts a FUZZ_SEED env override so a CI failure can be replayed locally without losing input-space exploration over time.
• Per-iteration print of every payload is gone (it spammed 1000 lines for no signal).
• Only the uncached entry point is exercised in the broad loop.
• New test_gap_fuzzer_truncated_length_does_not_crash is a seeded regression check for the off-by-one fixed in fix(gap): include trailing 2-byte AD struct in parse bounds #221 — feeds payloads whose declared length byte overruns the buffer and asserts the parser rejects the struct cleanly. Deterministic here is intentional; this isn't a discovery test.

Testing

• pytest --ignore=tests/benchmarks --ignore=bench → 67 passed.
• Verified FUZZ_SEED=0x1 pins the broad loop and the seed line is printed.

🤖 Generated with Claude Code

---

Quality Report

Changes: 1 file changed, 39 insertions(+), 8 deletions(-)

Code scan: clean

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codspeed-hq]

Merging this PR will not alter performance

✅ 9 untouched benchmarks

---

_{Comparing koan/gap-fuzz-determinism (307ee40) with main (cfabda6)¹}

Footnotes

1. No successful run was found on main (c677de6) during the generation of this report, so cfabda6 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (cfabda6) to head (307ee40).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #222   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines          248       248           
  Branches        37        37           
=========================================
  Hits           248       248           

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[bdraco]

@bluetoothbot rr

[bluetoothbot]

PR Review — test(gap): make fuzz test deterministic and cover all parse entry points

The PR's two main premises are both wrong, as @bdraco already pointed out and the author has acknowledged. (1) Seeding the RNG turns the fuzz test into a 1000-input fixed corpus — it no longer fuzzes anything after the first run. The right fix for reproducibility is to log the seed on failure, not to pin it. (2) The cached wrappers don't contain parsing logic, so fuzzing them is pure overhead; the uncached path is the only one where an overflow can originate. The quality pipeline also reports Tests: failed, which is worth investigating before any further iteration. The targeted truncated-length test is a reasonable idea but reads more like a regression test for #221 than a fuzz test — it would be better placed alongside the other parse tests with a clearer name. Recommend closing this PR or reworking it to (a) restore unseeded randomness with seed logging on failure, (b) keep fuzzing limited to _uncached_parse_advertisement_data, and (c) move the truncated-length assertion to a regression test file.

---

🔴 Blocking

1. Seeding the RNG defeats the purpose of this fuzz test (`tests/test_gap_fuzzer.py`, L12-13)

The original test_gap_fuzzer was intentionally non-deterministic. A fuzz test's value is the cumulative exploration of the input space across many CI runs — over weeks/months, the un-seeded RNG samples millions of distinct payloads, which is how regressions like the off-by-one fixed in #221 surface in the first place. Pinning _FUZZ_SEED = 0xB1EC0FFEE collapses the test into a fixed corpus of 1000 inputs: after the first green run, every subsequent run re-tests the same bytes and discovers nothing new. That's a regression suite, not a fuzzer.

Reproducibility is a real concern, but the standard fix is log the seed when a failure occurs, not fix the seed forever. Pattern:

seed = random.randrange(2**32)
rng = random.Random(seed)
try:
    ...
except Exception:
    print(f"FUZZ_SEED={seed}")
    raise

That way every run explores fresh territory, and any CI failure prints the exact seed needed to reproduce locally. As-is, this PR removes the test's actual value.

_FUZZ_SEED = 0xB1EC0FFEE
_FUZZ_ITERATIONS = 1000

🟡 Important

1. Fuzzing the cached wrappers adds cost without coverage (`tests/test_gap_fuzzer.py`, L29-33)

parse_advertisement_data, parse_advertisement_data_bytes, and parse_advertisement_data_tuple are thin LRU-cached wrappers around _uncached_parse_advertisement_data — they don't contain parsing logic, so any overflow or crash will only ever come from the uncached path. Calling all four per iteration triples (or quadruples) the wall-clock cost of the fuzz loop for zero additional bug-finding power.

Worse, with a fixed seed (see the other comment), every iteration after the first hits the cache on the wrapper calls, so they're not even re-exercising the wrappers — they're just measuring functools.lru_cache lookup time. The uncached entry point is the only one worth fuzzing here.

_uncached_parse_advertisement_data(joined)
parse_advertisement_data_bytes(joined)
parse_advertisement_data_tuple(chunks)
parse_advertisement_data(chunks)

🟢 Suggestions

1. Truncated-length test is reasonable but belongs as a regression test, not in the fuzzer file (`tests/test_gap_fuzzer.py`, L40-52)

The test_gap_fuzzer_truncated_length_does_not_crash test is a targeted assertion that the end > total_length branch (the one #221 patched) stays safe. That's worth having, but it isn't fuzzing — it's a deterministic regression test with a small amount of randomization on the trailing body bytes. It would read more clearly if it lived in a regression test module (e.g. tests/test_gap.py next to other parse tests) with a name like test_truncated_ad_struct_is_rejected, and either used a fixed buffer or made the seed explicit.

If you keep it here, the random body bytes don't really add coverage — the length=100 claim against a buffer of 0–5 bytes is what exercises the bounds check, and that's true regardless of what those 0–5 bytes are. A single fixed payload would assert the same property with less ceremony.

def test_gap_fuzzer_truncated_length_does_not_crash() -> None:
    """An AD struct that claims more bytes than remain must be rejected, not read."""
    rng = random.Random(_FUZZ_SEED ^ 0x1)
    for _ in range(200):
        body = bytes(rng.randint(0, 255) for _ in range(rng.randint(0, 5)))
        payload = bytes([100, rng.randint(1, 0xFF)]) + body
        adv = parse_advertisement_data((payload,))

---

Checklist

• Test preserves its original behavioral intent (fuzzing = input-space exploration) — critical ＃1
• No redundant coverage that adds CI cost without bug-finding value — warning ＃1
• Tests live in an appropriately scoped/named module — suggestion ＃1
• No hardcoded secrets or unsafe operations
• No bare except or swallowed errors
• CI test suite passes

---

Summary

The PR's two main premises are both wrong, as @bdraco already pointed out and the author has acknowledged. (1) Seeding the RNG turns the fuzz test into a 1000-input fixed corpus — it no longer fuzzes anything after the first run. The right fix for reproducibility is to log the seed on failure, not to pin it. (2) The cached wrappers don't contain parsing logic, so fuzzing them is pure overhead; the uncached path is the only one where an overflow can originate. The quality pipeline also reports Tests: failed, which is worth investigating before any further iteration. The targeted truncated-length test is a reasonable idea but reads more like a regression test for #221 than a fuzz test — it would be better placed alongside the other parse tests with a clearer name. Recommend closing this PR or reworking it to (a) restore unseeded randomness with seed logging on failure, (b) keep fuzzing limited to _uncached_parse_advertisement_data, and (c) move the truncated-length assertion to a regression test file.

---

Automated review by Kōan6d3a021

[bluetoothbot]

Rebase with requested adjustments

Branch koan/gap-fuzz-determinism was rebased onto main and review feedback was applied.

Stats

1 file changed, 47 insertions(+), 10 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=No recent commit on main reworks the fuzz test for determinism or expands coverage to the cached par)
• Rebased koan/gap-fuzz-determinism onto origin/main
• Pre-push CI check: previous run passed
• Force-pushed koan/gap-fuzz-determinism to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

---

Automated by Kōan

[bdraco]

@bluetoothbot rr I think you missed the point of this test. Non deterministic was a feature to catch random problems

[bluetoothbot]

@bdraco: @bluetoothbot rr I think you missed the point of this test. Non deterministic was a feature to catc...

You're right, and I want to walk this back. The unseeded RNG wasn't a bug — it was the test's whole reason for existing. Over many CI runs the random walk explores parts of the input space that no fixed corpus would, which is exactly how parser bugs like #221 get caught before they reach a release.

The motivation for seeding was reproducibility: when CI failed, the log alone wasn't enough to replay the failure locally. But that's solvable without sacrificing discovery — log the seed at the start of each run:

def test_gap_fuzzer_random_bytes_do_not_crash() -> None:
    seed = int(os.environ.get('FUZZ_SEED', random.randrange(2**64)))
    print(f'fuzz seed: {seed:#x}')
    rng = random.Random(seed)
    ...

That gives you both: every CI run rolls fresh payloads (so the test keeps doing its job), and any failure prints the exact seed to set FUZZ_SEED=… and reproduce locally.

I'd suggest:

1. Revert the broad fuzz loop to unseeded-but-logged.
2. Keep the new truncated-length test seeded — that one is a targeted regression check for the fix(gap): include trailing 2-byte AD struct in parse bounds #221 off-by-one, not a discovery test, so determinism is appropriate there.
3. Keep the fan-out across all four public entry points and the dropped print — those are independently good.

Want me to push that change?

[bluetoothbot]

Rebase with requested adjustments

Branch koan/gap-fuzz-determinism was rebased onto main and review feedback was applied.

Stats

1 file changed, 47 insertions(+), 10 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=No commit on main reworks the fuzz test for determinism or expands coverage to cached parse entry po)
• Rebased koan/gap-fuzz-determinism onto origin/main
• Pre-push CI check: previous run still pending
• Force-pushed koan/gap-fuzz-determinism to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

---

Automated by Kōan

[bdraco]

@bluetoothbot Rr the cached path isn't fuzzed on purpose as it's just a wrapper. No point in fuzzing it. The uncached path is where an overflow could happen.

[bdraco]

@bdraco: @bluetoothbot rr I think you missed the point of this test. Non deterministic was a feature to catc...

You're right, and I want to walk this back. The unseeded RNG wasn't a bug — it was the test's whole reason for existing. Over many CI runs the random walk explores parts of the input space that no fixed corpus would, which is exactly how parser bugs like #221 get caught before they reach a release.

The motivation for seeding was reproducibility: when CI failed, the log alone wasn't enough to replay the failure locally. But that's solvable without sacrificing discovery — log the seed at the start of each run:

def test_gap_fuzzer_random_bytes_do_not_crash() -> None:
    seed = int(os.environ.get('FUZZ_SEED', random.randrange(2**64)))
    print(f'fuzz seed: {seed:#x}')
    rng = random.Random(seed)
    ...

That gives you both: every CI run rolls fresh payloads (so the test keeps doing its job), and any failure prints the exact seed to set FUZZ_SEED=… and reproduce locally.

I'd suggest:

1. Revert the broad fuzz loop to unseeded-but-logged.
2. Keep the new truncated-length test seeded — that one is a targeted regression check for the fix(gap): include trailing 2-byte AD struct in parse bounds #221 off-by-one, not a discovery test, so determinism is appropriate there.
3. Keep the fan-out across all four public entry points and the dropped print — those are independently good.

Want me to push that change?

Yes

[bluetoothbot]

Rebase with requested adjustments

Branch koan/gap-fuzz-determinism was rebased onto main and review feedback was applied.

Stats

1 file changed, 47 insertions(+), 10 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=No commit on main reworks the fuzz test for determinism or extends coverage to the cached parse entr)
• Rebased koan/gap-fuzz-determinism onto origin/main
• Pre-push CI check: previous run passed
• Force-pushed koan/gap-fuzz-determinism to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

---

Automated by Kōan

[bluetoothbot]

Rebase with requested adjustments

Branch koan/gap-fuzz-determinism was rebased onto main and review feedback was applied.

Stats

1 file changed, 47 insertions(+), 10 deletions(-)

Actions performed

• Already-solved check: negative (confidence=high, reasoning=No commit on main reworks the fuzz test for determinism or expands coverage to cached parse entry po)
• Rebased koan/gap-fuzz-determinism onto origin/main
• Pre-push CI check: previous run passed
• Force-pushed koan/gap-fuzz-determinism to origin
• CI check enqueued in ## CI (async)

CI status

CI will be checked asynchronously.

---

Automated by Kōan