perf(mobile): preserve Android native outputs during clean prebuild

[bompus]

Problem

mobile-native-client runs a clean Expo prebuild before building an Android development client. Deleting the generated Android directory also deletes .gradle and app/.cxx, so repeated builds lose Gradle task history and the app's native compilation outputs even when their inputs are unchanged.

Change

Move those two directories aside on the same filesystem before Android prebuild, then restore them before Gradle runs. Skip caches behind symlinked directories and refuse restoration through a symlinked parent. Clean prebuild still regenerates native source, autolinking and JS bundle outputs. The existing tracked-native-file guard remains in place, and iOS follows its existing path.

Restore saved outputs after prebuild failure or interruption too. If recovery fails or a regenerated cache directory conflicts, retain the saved files and print their recovery path. This also reports the path when a pending interruption suppresses the returned error. If recovery also fails after a prebuild or save failure, the helper returns the original cause and prints the recovery instructions.

Scope and approval

This is a focused fix for cache loss in the existing native-client command. It preserves outputs that Gradle/Ninja invalidate against their inputs; it adds no build mode, dependency or product setting. I am submitting it under the focused obvious-bug exception without a prior issue or maintainer approval.

Verification

• Eleven filesystem tests pass, covering successful regeneration, prebuild failure, partial save failure, conflicting directories, restore failure and interruption. One test exercises all six regeneration/completion/interruption orders with both restoration outcomes. Symlink tests verify that caches stay at their original targets and recovery does not write through a regenerated symlinked parent.
• Removing cache preservation makes the successful-regeneration test fail with a missing cache file. Removing recovery reporting makes the interruption/recovery-failure test fail.
• Focused formatting and lint checks pass. vp exec tsc -p scripts/tsconfig.json --noEmit passes.
• On Linux, at f0ebcf6, the production helper completed two real expo prebuild --clean --platform android --no-install runs followed by ARM64 :app:assembleDebug builds. Both APKs passed signature verification. A further clean prebuild and debug build passed after rebasing onto the current upstream base.

Before-and-after timing

A controlled comparison of clean Expo prebuild plus ARM64 debug Gradle build on 4cff81a reduced mean elapsed time from 67.7 s to 46.2 s, saving 21.6 s (31.8%).

Build stage	Measured runs (s)	Mean (s)	Sample standard deviation (s)
Before: clean prebuild discards native outputs	71.166, 65.872, 66.185	67.741	2.970
After: same prebuild through prebuildAndroid	48.697, 44.199, 45.647	46.181	2.296

Both versions used the same source, checkout, environment and warm shared Gradle cache. Each received three warm-ups and an excluded pilot before six measured runs in ABBAAB order. An earlier sequence was discarded after another job briefly overlapped. The reported sequence had no competing heavy job or swap activity. Hyperfine timed the commands; APK signature and content checks ran outside the timed interval.

Environment: Ubuntu/WSL, Ryzen 9 9950X3D with 16 exposed vCPUs, 47 GiB RAM, Node 24.21.0, Java 17 and Android build tools 37.0.0. Both versions used APP_VARIANT=development, NODE_ENV=development, a 12 GiB memory cap and nice -n 10. The prebuild command was expo prebuild --clean --platform android --no-install; the Gradle command was:

./gradlew :app:assembleDebug -PreactNativeArchitectures=arm64-v8a \
  --build-cache --parallel --max-workers=2 --no-daemon \
  -Dorg.gradle.jvmargs='-Xmx3072m -XX:MaxMetaspaceSize=768m' \
  -Pkotlin.compiler.execution.strategy=in-process

All six reported builds succeeded, and their APKs passed signature verification. All 3,242 non-signature ZIP entries had identical content hashes across both versions. Generated project/build outputs were regenerated each time.

This is development evidence for warm repeated debug builds of one unchanged app revision. It excludes dependency installation, warm-ups, fingerprint discovery and device installation, and does not establish cold-build or cross-revision performance. macOS/Windows execution and cancellation of an actual Expo subprocess were not checked; interruption tests use Effect fibers with deterministic barriers.

GPT-6.1 Sol, in Codex.

[macroscopeapp]

Approvability

Verdict: Would Approve

Macroscope's review found this PR approvable — The change is confined to the developer-facing Android native-client prebuild workflow and adds extensive filesystem recovery tests, without affecting product defaults or customer request paths. An unresolved Medium-severity finding identifies a directory-symlink cache edge case that remains a separate blocking correctness risk.

Not approved because:

• 1 blocking correctness issue found at or above your repo's Minimum Blocking Severity

^{Adjust the Minimum Blocking Severity for this repo — including turning it Off — in Settings. You can add or adjust custom eligibility rules. Learn more.}

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📒 Files selected for processing (2)

• scripts/mobile-native-client.test.ts
• scripts/mobile-native-client.ts

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• scripts/mobile-native-client.test.ts
• scripts/mobile-native-client.ts

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📝 Walkthrough

Walkthrough

Android prebuild now saves eligible Gradle and C++ build directories before regenerating native inputs, then restores them. The tests cover symlinks, regeneration and recovery failures, and interruption. iOS continues to run prebuild directly.

Changes

Android prebuild cache preservation

Layer / File(s)	Summary
Preserve and recover Android build outputs scripts/mobile-native-client.ts, scripts/mobile-native-client.test.ts	Android prebuild saves eligible .gradle and app/.cxx directories, regenerates native inputs, and attempts to restore the saved directories. Tests cover symlink handling, failure paths, and interruption. The build flow still runs iOS prebuild directly.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 4cff8

No actionable issue remains for this change after normal checks.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 91708

The change is confined to local Android build-cache handling and preserves existing build safeguards. No new remote exposure or privilege expansion is demonstrated. Recovery is well bounded for normal failures and managed interruption, but concurrent execution and abrupt process termination remain outside the verified recovery model.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The introduced file operations affect the repository-derived mobile Android directory and sibling recovery directories under the invoking process's filesystem authority. Existing build subprocesses inherit the host environment and install a development client on the selected device; no additional service, tenant or credential authority is demonstrated.

Trust Boundaries and Controls

• observed — Saving skips symlinked cache locations. Restoration checks the Android directory, cache parent and destination, refusing symlinks and existing destinations rather than overwriting them. Tests confirm external symlink targets remain unchanged and a regenerated symlinked parent leaves saved outputs recoverable. These are point-in-time checks, not atomic protection against concurrent path replacement.

Resilience and Maintainability Implications

• inferred — The helper does not serialize independent invocations or replay earlier recovery directories. Its masked cleanup supports managed interruption, but does not establish recovery after an operating-system kill or exclusive ownership during overlapping commands. These are bounded guarantees, not verified PR-introduced vulnerabilities.

Hardening Proposals

• proposed — If overlapping native-client commands must be supported, serialize regeneration and building per mobile workspace so cache ownership and path checks remain stable across the complete operation.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly identifies the performance change: preserving Android native outputs during clean prebuilds.
Description check	✅ Passed	The description covers the problem, change, scope rationale, and detailed verification results. It also states limitations and provides concrete performance measurements.

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• Create a new PR

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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @scripts/mobile-native-client.ts:
- Around line 354-366: Capture the cache recovery outcome with Effect.exit
instead of letting its failure immediately abort cleanup. In the recovery flow,
remove saved only after successful recovery; then propagate result.cause first
when result failed, and propagate the recovery cause only when result succeeded
but recovery failed.

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• scripts/mobile-native-client.ts

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[maria-rcks]

Note

Written by claude-opus-5-5 on behalf of Maria

Hi! We are cleaning up open PRs, and this one does not say which model or harness was used to create it. If this change is really important, we recommend rebuilding the PR with a newer model and noting the model and harness in the PR description.

[bompus]

Replaced by #18073, which names the model and harness. The code is unchanged.