fix(server): release oversized pull request diff cache entries

[juliusmarminge]

Large pull-request diff slices remained strongly referenced by both the normal diff cache and its stale fallback after the request finished. Those caches admitted up to 128 results without accounting for patch size.

Skip retention when a patch exceeds 512 KiB of estimated UTF-16 storage. Return the complete patch and pagination metadata unchanged, retain caching for small results, and guard invalidation against removing a replacement entry. Each persistent cache is consequently limited to 64 MiB of patch text. Repeated server requests for oversized slices fetch them again.

Before/after evidence against this PR's main base: an isolated GC check through the actual service read 16 synthetic diff results totaling 8 MiB. The base retained all 16 after GC (54 MiB heap); this revision retained zero (46 MiB heap). This demonstrates this retention defect; it does not establish the retaining root of the separately observed 3.86 GiB backend crash.

Validation: 165 focused PullRequestService tests passed, including large regular, paginated, and commit slices and small replacements; server package typecheck, scoped lint, formatting, and diff checks passed. The shared service applies to all source-control providers and web, desktop, and mobile connections without a contract change.

Prepared with Codex.

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Summary by CodeRabbit

• Bug Fixes
  • Improved diff caching behavior for very large responses.
  • Large diffs are still returned when requested but are no longer retained in the cache.
  • Smaller diff responses continue to be cached and reused.
  • Updated viewed-file handling to preserve cached diffs when file marks are invalidated.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	+2 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+7 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	−5 B (−0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	56.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	−4 B (−0.0%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	−4 B (−0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	0 B (0.0%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: e74c668 · PR result: 04bbab1 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[macroscopeapp]

Approvability

Verdict: Approved at 04bbab1

Macroscope's review found this PR approvable — This is a small, isolated cache-retention fix that preserves complete diff responses and existing small-diff caching while preventing oversized patches from remaining in either cache. Focused tests cover regular, paginated, and commit diff paths, including replacement behavior.

^{You can add or adjust custom eligibility rules. Learn more.}

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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📥 Commits

Reviewing files that changed from the base of the PR and between e74c668 and 04bbab1.

📒 Files selected for processing (2)

• apps/server/src/pullRequest/PullRequestService.test.ts
• apps/server/src/pullRequest/PullRequestService.ts

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📝 Walkthrough

Walkthrough

The pull request limits cached diffs to 512 KiB of UTF-16 storage. Oversized diffs are returned but not retained. Matching oversized cached values are invalidated, and tests cover repeated large responses and later small cached responses.

Changes

Diff cache handling

Layer / File(s)	Summary
Cache size policy apps/server/src/pullRequest/PullRequestService.ts	The service permits caching only diffs within the 512 KiB UTF-16 limit and skips oversized values during diff recording.
Oversized read behavior apps/server/src/pullRequest/PullRequestService.ts, apps/server/src/pullRequest/PullRequestService.test.ts	Diff reads invalidate matching oversized cached values before stale-diff handling. Tests cover repeated large responses, later small-response reuse, and cached diffs remaining after file-mark invalidation.

Priority: ⬇️ Low

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix

Suggested reviewers: maria-rcks

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: releasing oversized pull-request diff cache entries.
Description check	✅ Passed	The description explains the problem, the caching change, expected behavior, validation, and scope. It does not use the template headings or checklist, but it provides the required information and is …
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…
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.

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