fix: race condition in auto-scaler during Caddy config reload

[nicolas-grekas]

Spotted by a failing job on #2287

scaleWorkerThread and scaleRegularThread read the global mainThread from goroutines started by initAutoScaling. During a Caddy reload, FrankenPHPApp.Start() calls Shutdown() then Init(), which overwrites mainThread. The scaler goroutines could read the stale global concurrently with the write.

The fix captures mainThread.done and mainThread.state locally in initAutoScaling and passes them as parameters through startUpscalingThreads → scaleWorkerThread / scaleRegularThread. No package-level globals for these values — each autoscaler generation uses its own captured references, eliminating the race even during concurrent reloads.

[dunglas]

Good catch!

[Copilot]

Pull request overview

Fixes a data race during Caddy config reloads by avoiding direct reads of the global mainThread from autoscaler goroutines.

Changes:

• Introduces scalingDone and scalingState globals intended to capture mainThread.done / mainThread.state at initAutoScaling time.
• Updates scaleWorkerThread and scaleRegularThread to use the captured references instead of mainThread.

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[nicolas-grekas]

Patch updated

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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[dunglas]

Thanks!