bug: race condition when checking if NetworkAvailabilityListener is null on ConnectivityManager.NetworkCallback()

[doromaraujo]

This PR addresses the racing condition issue where a thread may set NetworkChangeDetector's listener to null while another is trying to call its methods.

Summary by CodeRabbit

• Bug Fixes
  • Improved thread safety in network change detection to prevent potential race conditions during network state updates.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

Walkthrough

The NetworkChangeDetector listener field is made volatile and accessed via local copies within callbacks to prevent race conditions and ensure thread-safe visibility across concurrent threads during network availability and loss events.

Changes

Cohort / File(s)	Summary
Thread Safety Enhancement tool/src/main/java/io/netbird/client/tool/networks/NetworkChangeDetector.java	Made listener field volatile and refactored onAvailable and onLost callbacks to use local references, preventing unsynchronized reads and potential race conditions during network state callbacks.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

• Verify volatile modifier properly enforces memory visibility semantics
• Confirm local copy pattern is correctly applied in both callback methods to eliminate TOCTOU race conditions
• Validate that null-check logic remains functionally equivalent after refactoring

Poem

🐰 A listener volatile, now truly free,
No race conditions haunting thee,
With local copies, the threads align,
Thread safety sealed, by design divine!
The networks flow without a care,
Safe and sound, with threads to spare! ✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: fixing a race condition in null-checks for NetworkAvailabilityListener within ConnectivityManager.NetworkCallback callbacks.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch bug/on-network-available-called-on-null-listener

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📜 Recent review details

Configuration used: CodeRabbit UI

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between bf4ece6 and e4fa7e5.

📒 Files selected for processing (1)

• tool/src/main/java/io/netbird/client/tool/networks/NetworkChangeDetector.java (2 hunks)

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (2)

• GitHub Check: build-debug
• GitHub Check: Analyze (java-kotlin)

🔇 Additional comments (3)

tool/src/main/java/io/netbird/client/tool/networks/NetworkChangeDetector.java (3)

16-16: Correct use of volatile for thread-safe visibility.

Marking the field volatile ensures that updates from subscribe()/unsubscribe() are immediately visible to the callback threads. This is essential for the local copy pattern used in the callbacks.

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40-42: Correct local copy pattern to eliminate TOCTOU race.

Copying the volatile field to a local variable before the null check ensures that the same non-null reference is used throughout the callback, preventing the race where listener could be set to null between the check and the method invocation.

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47-49: Consistent application of the fix in onLost.

Same correct pattern applied here, ensuring both callbacks are protected from the race condition.

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