fix: atomic.Value should be a concrete type to avoid panics (#13740)

Go's atomic.Value does not support `nil` type,
concrete type is necessary to avoid any panics with
the current implementation.

Also remove boolean to turn-off tracking of freezeCount.

cmd/handler-api.go

@@ -228,7 +228,7 @@ func (t *apiConfig) getRequestsPool() (chan struct{}, time.Duration) {
 func maxClients(f http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		if val := globalServiceFreeze.Load(); val != nil {
-			if unlock, ok := val.(chan struct{}); ok {
+			if unlock, ok := val.(chan struct{}); ok && unlock != nil {
 				// Wait until unfrozen.
 				<-unlock
 			}

cmd/service.go

@@ -68,14 +68,10 @@ func restartProcess() error {
 	return syscall.Exec(argv0, os.Args, os.Environ())
 }
 
-// Keep track of number of freeze/unfreeze calls.
-const trackFreezeCount = true
-
 // freezeServices will freeze all incoming S3 API calls.
 // For each call, unfreezeServices must be called once.
 func freezeServices() {
 	// Use atomics for globalServiceFreeze, so we can read without locking.
-	if trackFreezeCount {
 	// We need a lock since we are need the 2 atomic values to remain in sync.
 	globalServiceFreezeMu.Lock()
 	// If multiple calls, first one creates channel.
@@ -84,16 +80,11 @@ func freezeServices() {
 		globalServiceFreeze.Store(make(chan struct{}))
 	}
 	globalServiceFreezeMu.Unlock()
-	} else {
-		// If multiple calls, first one creates channel.
-		globalServiceFreeze.CompareAndSwap(nil, make(chan struct{}))
-	}
 }
 
 // unfreezeServices will unfreeze all incoming S3 API calls.
 // For each call, unfreezeServices must be called once.
 func unfreezeServices() {
-	if trackFreezeCount {
 	// We need a lock since we need the 2 atomic values to remain in sync.
 	globalServiceFreezeMu.Lock()
 	// Close when we reach 0
@@ -101,23 +92,13 @@ func unfreezeServices() {
 	if globalServiceFreezeCnt <= 0 {
 		// Ensure we only close once.
 		if val := globalServiceFreeze.Load(); val != nil {
+			var _ch chan struct{}
 			if ch, ok := val.(chan struct{}); ok {
-					globalServiceFreeze.Store(nil)
+				globalServiceFreeze.Store(_ch)
 				close(ch)
 			}
 		}
 		globalServiceFreezeCnt = 0 // Don't risk going negative.
 	}
 	globalServiceFreezeMu.Unlock()
-	} else {
-		// If multiple calls, first one closes channel.
-		if val := globalServiceFreeze.Load(); val != nil {
-			if ch, ok := val.(chan struct{}); ok {
-				// Ensure we only close once.
-				if globalServiceFreeze.CompareAndSwap(val, nil) {
-					close(ch)
-				}
-			}
-		}
-	}
 }