fix: make file update atomic, wait for /start

[toddbaert]

Fixes a a few small reliability issues with the launchpad:

• /start now polls the health check instead of console messages which don't provide guarantees the ports are up
• make json updates atomic

[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 significantly enhances the reliability of the launchpad by addressing two key areas: ensuring data integrity during file modifications and improving the robustness of service startup. By introducing atomic file write operations, the system is better protected against partial or corrupted configuration files. Furthermore, the transition to an HTTP health check for flagd's readiness provides a more accurate and reliable signal of its operational status, reducing potential race conditions or false positives during service initialization.

Highlights

• Atomic File Writes: Implemented an atomicWriteFile function to ensure that file operations, particularly for configuration files, are atomic. This prevents data corruption in case of system crashes during a write operation. This new function is now used in ToggleChangingFlag and CombineJSONFiles.
• Improved flagd Startup Reliability: Replaced the previous method of checking flagd's readiness (parsing log output for a 'listening at' string) with a more robust HTTP health check. The system now polls the http://localhost:8014/readyz endpoint until it returns a 200 OK status, ensuring flagd is fully operational before proceeding.

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[toddbaert]

The log output we were looking at doesn't actually mean the listening services are running - that statement is printed before the servers are guaranteed to be up.

The healthcheck actually waits until all services are up, so let's use that.

[gemini-code-assist]

Code Review

This pull request introduces some good reliability improvements, namely making file writes atomic and implementing a health check poll to wait for flagd to start. My review includes a high-severity fix for the new atomicWriteFile function to make it more robust by handling more error cases and preventing temporary file leaks. I've also added a medium-severity suggestion to improve maintainability by extracting hardcoded values into constants.

[toddbaert]

This atomically writes the file, so there's never any change of half-written data. We do something similar in OFO.