refactor(flagd): replace Thread.sleep with ScheduledExecutorService in SyncStreamQueueSource

[aepfli]

Closes #1659
Supersedes #1660

Changes

Replaces the blocking Thread.sleep(maxBackoffMs) throttle in SyncStreamQueueSource.observeSyncStream() with a non-blocking ScheduledExecutorService.

Key changes to SyncStreamQueueSource

• Add a single-threaded ScheduledExecutorService field (flagd-sync-retry-scheduler, daemon thread)
• init(): submit initial observeSyncStream() run to the scheduler instead of spawning a raw daemon thread
• observeSyncStream(): when throttle is needed, schedule the next invocation via retryScheduler.schedule() and return (non-blocking) instead of sleeping
• shutdown(): call retryScheduler.shutdownNow() before shutting down the channel connector
• Handle RejectedExecutionException on schedule() (race condition between throttle and shutdown)
• Restore Thread.currentThread().interrupt() on InterruptedException

Why

Thread.sleep blocks the thread for the entire backoff duration, preventing clean shutdown and wasting a thread. The scheduler approach is non-blocking and integrates cleanly with the existing shutdown logic.

[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 refactors the SyncStreamQueueSource to improve thread management and enable cleaner shutdowns. By replacing a blocking Thread.sleep with a ScheduledExecutorService, the system avoids tying up threads during backoff periods, leading to more efficient resource utilization and better responsiveness to shutdown requests.

Highlights

• Replaced Thread.sleep: The blocking Thread.sleep(maxBackoffMs) throttle in SyncStreamQueueSource.observeSyncStream() was replaced with a non-blocking ScheduledExecutorService.
• ScheduledExecutorService: A single-threaded ScheduledExecutorService (flagd-sync-retry-scheduler, daemon thread) was added to manage retry delays.
• Initialization: The init() method now submits the initial observeSyncStream() run to the scheduler instead of spawning a raw daemon thread.
• Throttling Logic: When throttling is needed, the next invocation is scheduled via retryScheduler.schedule() and the method returns immediately, making it non-blocking.
• Shutdown: The shutdown() method now calls retryScheduler.shutdownNow() to properly terminate the scheduler.
• Error Handling: RejectedExecutionException is now handled during scheduling, and Thread.currentThread().interrupt() is restored on InterruptedException.

Changelog

• providers/flagd/src/main/java/dev/openfeature/contrib/providers/flagd/resolver/process/storage/connector/sync/SyncStreamQueueSource.java
  • Added necessary imports for Executors, RejectedExecutionException, and ScheduledExecutorService.
  • Introduced a ScheduledExecutorService for managing retry delays.
  • Modified the init method to use the scheduler for initial stream observation.
  • Ensured the ScheduledExecutorService is shut down cleanly.
  • Updated the observeSyncStream method to use scheduled retries instead of blocking Thread.sleep.
  • Added handling for RejectedExecutionException during scheduling.
  • Restored the interrupt status of the current thread on InterruptedException.

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[gemini-code-assist]

Code Review

This pull request refactors SyncStreamQueueSource to replace a blocking Thread.sleep with a non-blocking ScheduledExecutorService for handling retry backoff. This is a great improvement for resource management and enables a cleaner shutdown process. The implementation is solid, including handling for RejectedExecutionException during shutdown and correctly managing the thread's interrupted status. I have one suggestion to make the shutdown sequence even more robust by ensuring the scheduler has terminated before shutting down other components.