Fix easy confirmed bugs from bugreport_2026-08-21_merged.md

[Copilot]

Summary

Fixes easy confirmed bugs from bugreport_2026-08-21_merged.md, with a regression test hardened per code review to derive its expected value independently rather than duplicating the implementation's formula.

Changes

• 0045b90 — Fix easy confirmed bugs from bugreport_2026-08-21_merged.md
• f5d599a — Address code review: derive expected value independently in radamsa test (uses _log2_ceil(9) instead of hardcoding the expected result)

Summary by Sourcery

Fix confirmed seed-generation and random-number reproducibility bugs, with regression coverage for the corrected behavior.

Bug Fixes:

• Prevent generic seed generation from failing when the configured maximum length is below four bytes.
• Preserve injected-RNG reproducibility for radamsa numeric mutation sign selection and grammar expansion.

Documentation:

• Mark the corresponding confirmed bug-report findings as partially or fully fixed and document their remaining scope.

Tests:

• Add regression coverage for short maximum seed lengths and injected-RNG use in radamsa mutation and grammar generation.

[sourcery-ai]

Reviewer's Guide

Fixes three confirmed reproducibility and seed-generation bugs: short maximum lengths no longer produce invalid random ranges, radamsa numeric sign selection honors injected RNGs, and grammar generation uses its injected RNG for deterministic output; documentation and independent regression coverage were added.

Sequence diagram for deterministic injected-RNG mutation

sequenceDiagram
participant Caller
participant Radamsa as radamsa_mutate_num
participant RNG as InjectedRNG
Caller->>Radamsa: radamsa_mutate_num(val, rng)
Radamsa->>RNG: randint(1, 128)
Radamsa->>RNG: random()
RNG-->>Radamsa: deterministic sign decision
Radamsa-->>Caller: mutated integer

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Sequence diagram for deterministic grammar generation

sequenceDiagram
participant Caller
participant Grammar
participant RNG as InjectedRNG
Caller->>Grammar: mutate(..., rng)
Grammar->>Grammar: _expand_rule(name, depth)
Grammar->>RNG: choice(alts)
Grammar->>Grammar: _expand_tokens(tokens, depth)
Grammar->>RNG: randint(lo, hi)
RNG-->>Grammar: deterministic choices
Grammar-->>Caller: generated bytes

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Flow diagram for safe short-length generic seed generation

flowchart TD
A["Generic seed generation"] --> B["Read f.max_len"]
B --> C["Compute lower bound min(4, f.max_len)"]
C --> D["Compute upper bound min(64, f.max_len)"]
D --> E["rng.randint(lower bound, upper bound)"]
E --> F["Create random buffer of selected length"]

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File-Level Changes

Change	Details	Files
Clamped generic seed lengths to remain valid when the configured maximum is below the normal minimum.	Use the smaller of 4 and the configured maximum as the lower bound. Added regression coverage for short maximum lengths and empty-corpus generic seed generation.	src/fuzzer_tool/services/seed_picker.py tests/test_regression_bugreport_easy_fixes.py
Made radamsa numeric mutations and grammar expansion consistently use injected RNG instances.	Route the radamsa scaling sign decision through the injected RNG. Store and use the grammar instance's RNG for alternative selection and repetition counts. Added deterministic regression tests for both RNG plumbing paths.	src/fuzzer_tool/core/mutations/generic.py src/fuzzer_tool/core/grammar.py tests/test_regression_bugreport_easy_fixes.py
Documented the confirmed bug fixes and linked them to their regression tests.	Marked the seed-picker and RNG findings as partially fixed with implementation locations and remaining scope. Recorded the new regression test coverage in the bug report.	docs/bugreport_2026-08-21_merged.md

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[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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[Copilot]

🟡 Changes recommended

The two new RNG regression tests can still pass nondeterministically on regression and should be hardened to fail deterministically (e.g., by patching module-global random.* to raise).

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Fixes a few confirmed “easy” bugreport findings affecting seed generation for small --max-len values and RNG determinism (in radamsa numeric mutation and grammar expansion), and adds regressions + notes in the merged bugreport doc.

Changes:

• Clamp generic seed length bounds to avoid randint() invalid ranges when max_len < 4.
• Ensure injected RNG is used for radamsa_mutate_num op==9 sign selection and for Grammar rule/token expansion.
• Add/extend regression tests and mark the corresponding bugreport items as fixed.

File summaries

File	Description
tests/test_regression_bugreport_easy_fixes.py	Adds regression coverage for the fixed RNG leaks and short max_len seed generation.
src/fuzzer_tool/services/seed_picker.py	Fixes invalid randint() bounds in generic seed generation when max_len is small.
src/fuzzer_tool/core/mutations/generic.py	Uses injected RNG for radamsa op==9 sign selection to preserve -s determinism.
src/fuzzer_tool/core/grammar.py	Uses self._rng (set by mutate()) instead of module-global random during expansion.
docs/bugreport_2026-08-21_merged.md	Marks specific findings as fixed and links to regression tests.

Review details

• Files reviewed: 5/5 changed files
• Comments generated: 2
• Review effort level: Lite

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[daedalus]

@copilot Fix the code for all comments in this review thread.

When a review comment includes a suggested change, apply the suggestion exactly.

Do not make changes beyond what is described in the linked review thread.