Enforce LIFO ordering for shutdown handlers

This allows us to run both the Libpod and Server handlers at the
same time without unregistering one.

Also, pass the signal that killed us into the handlers, in case
they want to use it to determine what to do (e.g. what exit code
to set).

Signed-off-by: Matthew Heon <mheon@redhat.com>

libpod/runtime.go

@@ -183,7 +183,7 @@ func newRuntimeFromConfig(ctx context.Context, conf *config.Config, options ...R
 		return nil, err
 	}
 
-	if err := shutdown.Register("libpod", func() error {
+	if err := shutdown.Register("libpod", func(sig os.Signal) error {
 		os.Exit(1)
 		return nil
 	}); err != nil {

libpod/shutdown/handler.go

@@ -11,17 +11,20 @@ import (
 )
 
 var (
-	stopped         bool
-	sigChan         chan os.Signal
-	cancelChan      chan bool
-	handlers        map[string]func() error
+	stopped    bool
+	sigChan    chan os.Signal
+	cancelChan chan bool
+	// Definitions of all on-shutdown handlers
+	handlers map[string]func(os.Signal) error
+	// Ordering that on-shutdown handlers will be invoked.
+	handlerOrder    []string
 	shutdownInhibit sync.RWMutex
 )
 
 // Start begins handling SIGTERM and SIGINT and will run the given on-signal
 // handlers when one is called. This can be cancelled by calling Stop().
 func Start() error {
-	if sigChan != nil && !stopped {
+	if sigChan != nil {
 		// Already running, do nothing.
 		return nil
 	}
@@ -43,9 +46,14 @@ func Start() error {
 		case sig := <-sigChan:
 			logrus.Infof("Received shutdown signal %v, terminating!", sig)
 			shutdownInhibit.Lock()
-			for name, handler := range handlers {
+			for _, name := range handlerOrder {
+				handler, ok := handlers[name]
+				if !ok {
+					logrus.Errorf("Shutdown handler %s definition not found!", name)
+					continue
+				}
 				logrus.Infof("Invoking shutdown handler %s", name)
-				if err := handler(); err != nil {
+				if err := handler(sig); err != nil {
 					logrus.Errorf("Error running shutdown handler %s: %v", name, err)
 				}
 			}
@@ -82,10 +90,11 @@ func Uninhibit() {
 }
 
 // Register registers a function that will be executed when Podman is terminated
-// by a signal.
-func Register(name string, handler func() error) error {
+// by a signal. Handlers are invoked LIFO - the last handler registered is the
+// first run.
+func Register(name string, handler func(os.Signal) error) error {
 	if handlers == nil {
-		handlers = make(map[string]func() error)
+		handlers = make(map[string]func(os.Signal) error)
 	}
 
 	if _, ok := handlers[name]; ok {
@@ -93,6 +102,7 @@ func Register(name string, handler func() error) error {
 	}
 
 	handlers[name] = handler
+	handlerOrder = append([]string{name}, handlerOrder...)
 
 	return nil
 }
@@ -100,14 +110,22 @@ func Register(name string, handler func() error) error {
 // Unregister un-registers a given shutdown handler.
 func Unregister(name string) error {
 	if handlers == nil {
-		handlers = make(map[string]func() error)
+		return nil
 	}
 
 	if _, ok := handlers[name]; !ok {
-		return errors.Errorf("no handler with name %s found", name)
+		return nil
 	}
 
 	delete(handlers, name)
 
+	newOrder := []string{}
+	for _, checkName := range handlerOrder {
+		if checkName != name {
+			newOrder = append(newOrder, checkName)
+		}
+	}
+	handlerOrder = newOrder
+
 	return nil
 }

pkg/api/server/server.go

@@ -185,14 +185,11 @@ func (s *APIServer) Serve() error {
 	if err := shutdown.Start(); err != nil {
 		return err
 	}
-	if err := shutdown.Register("server", func() error {
+	if err := shutdown.Register("server", func(sig os.Signal) error {
 		return s.Shutdown()
 	}); err != nil {
 		return err
 	}
-	// Unregister the libpod handler, which just calls exit(1).
-	// Ignore errors if it doesn't exist.
-	_ = shutdown.Unregister("libpod")
 
 	errChan := make(chan error, 1)