fix: deadlock if /change is called during /restart

[toddbaert]

If the /change endpoint was called while flagd was restarting (due to the /restart endpoint) the filewatcher was closed (because the allFlags.json was deleted). Due to this, the waitgroup never stopped waiting.

In the Java e2e suite, it looks like this:

Error:  Failures: 
Error:    RunRpcTest.Flag change event 1 expectation failed.
Expected status code <200> but was <500>.

Error:    RunRpcTest.Flag change event with caching 1 expectation failed.
Expected status code <200> but was <500>.

Error:    RunRpcTest.Use enriched context on RPC connection will not cache the value 1 expectation failed.
Expected status code <200> but was <500>.

https://github.com/open-feature/java-sdk-contrib/actions/runs/20754500347/job/59592648835?pr=1624

This change removes the PauseFileWatcher and ContinueFileWatcher funcs, and instead just allows to the filewatcher to be safely restarted, preventing this deadlock. I ran this against the Java suite locally and it fixed the intermittent failure caused by the deadlock

[gemini-code-assist]

Summary of Changes

Hello @toddbaert, 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 resolves a critical deadlock scenario that could arise from the interaction between flag change operations and system restarts. By refactoring the file watcher mechanism to allow for a safe and controlled restart, the changes eliminate a race condition where the file watcher's waitgroup would never complete. This significantly enhances the system's robustness and prevents service disruptions during dynamic configuration updates.

Highlights

• Deadlock Fix: Addresses a deadlock condition that occurred when the /change endpoint was called during a /restart operation, specifically when the filewatcher was closed prematurely.
• File Watcher Refactor: Removes the PauseFileWatcher and ContinueFileWatcher functions, replacing them with a single RestartFileWatcher function that safely closes and reinitializes the file watcher.
• Improved Stability: Prevents intermittent 500 errors observed in the Java e2e suite, ensuring the system remains responsive during concurrent flag changes and restarts.

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[gemini-code-assist]

Code Review

This pull request addresses a deadlock issue by replacing the pause/continue logic for the file watcher with a more robust restart mechanism. This is a solid approach to fixing the underlying problem. However, the new implementation introduces a couple of issues: a potential data race in RestartFileWatcher and an ignored error return value. My review includes suggestions to resolve these concerns.