Log when external signal is received

PiperOrigin-RevId: 220204591
Change-Id: I21a9c6f5c12a376d18da5d10c1871837c4f49ad2

runsc/boot/controller.go

@@ -445,6 +445,18 @@ const (
 	DeliverToForegroundProcessGroup
 )
 
+func (s SignalDeliveryMode) String() string {
+	switch s {
+	case DeliverToProcess:
+		return "Process"
+	case DeliverToAllProcesses:
+		return "All"
+	case DeliverToForegroundProcessGroup:
+		return "Foreground Process Group"
+	}
+	return fmt.Sprintf("unknown signal delivery mode: %d", s)
+}
+
 // SignalArgs are arguments to the Signal method.
 type SignalArgs struct {
 	// CID is the container ID.

runsc/boot/loader.go

@@ -336,6 +336,7 @@ func New(args Args) (*Loader, error) {
 			// properly.
 			deliveryMode = DeliverToForegroundProcessGroup
 		}
+		log.Infof("Received external signal %d, mode: %v", sig, deliveryMode)
 		if err := l.signal(args.ID, 0, int32(sig), deliveryMode); err != nil {
 			log.Warningf("error sending signal %v to container %q: %v", sig, args.ID, err)
 		}