build: fix data race in RotatingLogWriter during startup

[Euler-B]

The RotatingLogWriter.Write() method called rotator.Write() directly while rotator.Run() executed concurrently in a background goroutine. Both accessed the rotator's internal size field without synchronization, triggering a data race detected by -race.

The fix writes to the io.PipeWriter instead of calling rotator.Write() directly. The pipe is designed for concurrent use and feeds data to Run() through its reader end. A sync.RWMutex is introduced to protect the pipe field from concurrent access between Write, InitLogRotator, and Close. Additionally, Close() now closes the pipe first (setting it to nil for idempotency), waits for the Run goroutine to finish via sync.WaitGroup, then closes the underlying rotator to ensure a clean shutdown.

Fixes #10867

Steps to Test

1. Build lnd with race detector:

go build -race -o lnd-race ./cmd/lnd

2. Start btcd in simnet mode (minimal config)

3. Start lnd with race detector in simnet mode

4. Verify no data race warnings appear during startup

5. Run unit tests with race detector:

go test -race ./build/...

7. Run linter:

make lint

Pull Request Checklist

Testing

• Your PR passes all CI checks.
• Tests covering the positive and negative (error paths) are included.
• Bug fixes contain tests triggering the bug to prevent regressions.

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• Commits follow the Ideal Git Commit Structure.
• Any new logging statements use an appropriate subsystem and logging level.
• Any new lncli commands have appropriate tags in the comments for the rpc in the proto file.
• There is a change description in the release notes, or [skip ci] in the commit message for small changes.

📝 Please see our Contribution Guidelines for further guidance.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a data race condition identified during the startup of the log rotator in lnd. By introducing an io.Pipe to handle log data, the writer is now decoupled from the rotator's internal state, ensuring thread-safe operations. Additionally, the shutdown sequence has been improved to ensure all background processes complete before the rotator is closed.

Highlights

• Data Race Resolution: Replaced direct calls to the rotator's Write method with an io.PipeWriter to decouple the writer from the background rotator goroutine, eliminating the concurrent access to the internal size field.
• Graceful Shutdown: Updated the Close method to properly signal the pipe to close, wait for the rotator's background goroutine to finish using a sync.WaitGroup, and then safely close the underlying rotator.

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[gemini-code-assist]

Code Review

This pull request updates the log rotator to use a sync.WaitGroup to ensure the rotator's background goroutine completes before the rotator is closed, and redirects writes to the pipe instead of the rotator directly. The review feedback highlights critical data races on the pipe field when accessed concurrently across InitLogRotator, Write, and Close. It is recommended to introduce a sync.RWMutex to synchronize access to the pipe field, protecting it from concurrent reads and writes and ensuring safe, idempotent closure.

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[github-actions]

PR Severity: MEDIUM

Automated classification | 1 file | 24 lines changed

Medium (1 file):

• build/logrotator.go - Build infrastructure / log rotation utility; falls under Other Go files not categorized above

Analysis:

This PR modifies a single file in the build/ package (build/logrotator.go), which is build infrastructure and log rotation support code. This path does not fall into any CRITICAL or HIGH severity category, nor is it documentation, scripts, or test-only code. It is therefore classified as MEDIUM.

With only 1 non-test, non-generated file changed and 24 total lines modified, no severity bump triggers apply.

To override, add a severity-override-{critical,high,medium,low} label.
<!-- pr-severity-bot -->

[lightninglabs-deploy]

@Euler-B, remember to re-request review from reviewers when ready