Add a shutdown handler package

We need a unified package for handling signals that shut down
Libpod and Podman. We need to be able to do different things on
receiving such a signal (`system service` wants to shut down the
service gracefully, while most other commands just want to exit)
and we need to be able to inhibit this shutdown signal while we
are waiting for some critical operations (e.g. creating a
container) to finish. This takes the first step by defining the
package that will handle this.

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

libpod/shutdown/handler.go

@@ -0,0 +1,105 @@
+package shutdown
+
+import (
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
+
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+)
+
+var (
+	stopped         bool
+	sigChan         chan os.Signal
+	cancelChan      chan bool
+	handlers        map[string]func() error
+	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 {
+		// Already running, do nothing.
+		return nil
+	}
+
+	sigChan = make(chan os.Signal, 1)
+	cancelChan = make(chan bool, 1)
+	stopped = false
+
+	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		select {
+		case <-cancelChan:
+			signal.Stop(sigChan)
+			close(sigChan)
+			close(cancelChan)
+			stopped = true
+			return
+		case sig := <-sigChan:
+			logrus.Infof("Received shutdown signal %v, terminating!", sig)
+			shutdownInhibit.Lock()
+			for name, handler := range handlers {
+				logrus.Infof("Invoking shutdown handler %s", name)
+				if err := handler(); err != nil {
+					logrus.Errorf("Error running shutdown handler %s: %v", name, err)
+				}
+			}
+			shutdownInhibit.Unlock()
+			return
+		}
+	}()
+
+	return nil
+}
+
+// Stop the shutdown signal handler.
+func Stop() error {
+	if cancelChan == nil {
+		return errors.New("shutdown signal handler has not yet been started")
+	}
+	if stopped {
+		return nil
+	}
+
+	cancelChan <- true
+
+	return nil
+}
+
+// Temporarily inhibit signals from shutting down Libpod.
+func Inhibit() {
+	shutdownInhibit.RLock()
+}
+
+// Stop inhibiting signals from shutting down Libpod.
+func Uninhibit() {
+	shutdownInhibit.RUnlock()
+}
+
+// Register registers a function that will be executed when Podman is terminated
+// by a signal.
+func Register(name string, handler func() error) error {
+	if _, ok := handlers[name]; ok {
+		return errors.Errorf("handler with name %s already exists", name)
+	}
+
+	handlers[name] = handler
+
+	return nil
+}
+
+// Unregister un-registers a given shutdown handler.
+func Unregister(name string) error {
+	if _, ok := handlers[name]; !ok {
+		return errors.Errorf("no handler with name %s found", name)
+	}
+
+	delete(handlers, name)
+
+	return nil
+}