flate: Writer L7-9 with dictionary can emit the dictionary to stream

[klauspost]

Backport of golang/go#80538

Adds dictionary fuzz check. Status:

fuzz: elapsed: 2h12m45s, execs: 98125523 (22332/sec), new interesting: 152 (total: 3769)
fuzz: elapsed: 2h12m48s, execs: 98203921 (26130/sec), new interesting: 152 (total: 3769)

Summary by CodeRabbit

• Bug Fixes

  • Corrected block positioning after dictionary data is loaded, improving compression stream generation and round-trip reliability.

• Tests

  • Added coverage for dictionary-based compression involving non-compressed blocks.
  • Expanded fuzz testing across compression levels, reset states, and matching or non-matching dictionaries.
  • Added an option to run dictionary-focused fuzz tests with a capped input size.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Team

Run ID: 5316ca75-c33b-4e2c-afbc-cfda1a94e712

📥 Commits

Reviewing files that changed from the base of the PR and between f93f23a and 010181b.

📒 Files selected for processing (3)

• flate/deflate.go
• flate/fuzz_test.go
• flate/writer_test.go

Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review.

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📝 Walkthrough

Walkthrough

The change updates fillWindow block tracking after dictionary data is copied. It adds dictionary-focused fuzzing and regression coverage for non-compressed blocks across compression levels and reset states.

Changes

Dictionary encoding validation

Layer / File(s)	Summary
Window boundary tracking flate/deflate.go	fillWindow sets d.blockStart to the updated d.windowEnd after dictionary data is copied.
Dictionary regression coverage flate/fuzz_test.go, flate/writer_test.go	Dictionary fuzzing covers two dictionaries, all compression levels, and both reset states. Tests verify non-compressed block round trips and complete stream consumption.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to 01018

This localized change corrects dictionary handling in flate and adds regression and fuzz coverage; no actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the main change: flate writers at levels 7–9 can emit a configured dictionary to the stream.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

• Fix all pre-merge checks with AI

✨ Finishing Touches 💡 1

📝 Generate docstrings 💡

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch flate-dict

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