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

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.

Fix: capture mainThread.done and mainThread.state into package-level
variables at initAutoScaling time. Scaler goroutines use the captured
references instead of the global, eliminating the race.

Author: nicolas-grekas

scaling.go

@@ -32,6 +32,10 @@ var (
 	scaleChan         chan *frankenPHPContext
 	autoScaledThreads = []*phpThread{}
 	scalingMu         = new(sync.RWMutex)
+	// scalingDone and scalingState are captured at initAutoScaling time
+	// to avoid reading the global mainThread from scaler goroutines
+	scalingDone  chan struct{}
+	scalingState *state.ThreadState
 )
 
 func initAutoScaling(mainThread *phpMainThread) {
@@ -44,6 +48,8 @@ func initAutoScaling(mainThread *phpMainThread) {
 	scaleChan = make(chan *frankenPHPContext)
 	maxScaledThreads := mainThread.maxThreads - mainThread.numThreads
 	autoScaledThreads = make([]*phpThread, 0, maxScaledThreads)
+	scalingDone = mainThread.done
+	scalingState = mainThread.state
 	scalingMu.Unlock()
 
 	go startUpscalingThreads(maxScaledThreads, scaleChan, mainThread.done)
@@ -83,14 +89,14 @@ func addWorkerThread(worker *worker) (*phpThread, error) {
 // scaleWorkerThread adds a worker PHP thread automatically
 func scaleWorkerThread(worker *worker) {
 	// probe CPU usage before acquiring the lock (avoids holding lock during 120ms sleep)
-	if !cpu.ProbeCPUs(cpuProbeTime, maxCpuUsageForScaling, mainThread.done) {
+	if !cpu.ProbeCPUs(cpuProbeTime, maxCpuUsageForScaling, scalingDone) {
 		return
 	}
 
 	scalingMu.Lock()
 	defer scalingMu.Unlock()
 
-	if !mainThread.state.Is(state.Ready) {
+	if !scalingState.Is(state.Ready) {
 		return
 	}
 
@@ -113,14 +119,14 @@ func scaleWorkerThread(worker *worker) {
 // scaleRegularThread adds a regular PHP thread automatically
 func scaleRegularThread() {
 	// probe CPU usage before acquiring the lock (avoids holding lock during 120ms sleep)
-	if !cpu.ProbeCPUs(cpuProbeTime, maxCpuUsageForScaling, mainThread.done) {
+	if !cpu.ProbeCPUs(cpuProbeTime, maxCpuUsageForScaling, scalingDone) {
 		return
 	}
 
 	scalingMu.Lock()
 	defer scalingMu.Unlock()
 
-	if !mainThread.state.Is(state.Ready) {
+	if !scalingState.Is(state.Ready) {
 		return
 	}