util: use fsnotify to wait for file

prefer a fsnotify watcher to polling the file, we take advantage of
inotify on Linux and react more promptly to the PID file being
created.

If the watcher cannot be created, then fallback to the old polling
mechanism.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/util.go

@@ -13,6 +13,7 @@ import (
 	"github.com/containers/image/signature"
 	"github.com/containers/image/types"
 	"github.com/containers/libpod/pkg/util"
+	"github.com/fsnotify/fsnotify"
 	spec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/pkg/errors"
 )
@@ -93,18 +94,49 @@ func MountExists(specMounts []spec.Mount, dest string) bool {
 func WaitForFile(path string, chWait chan error, timeout time.Duration) (bool, error) {
 	done := make(chan struct{})
 	chControl := make(chan struct{})
+
+	var inotifyEvents chan fsnotify.Event
+	var timer chan struct{}
+	watcher, err := fsnotify.NewWatcher()
+	if err == nil {
+		if err := watcher.Add(filepath.Dir(path)); err == nil {
+			inotifyEvents = watcher.Events
+		}
+		defer watcher.Close()
+	}
+	if inotifyEvents == nil {
+		// If for any reason we fail to create the inotify
+		// watcher, fallback to polling the file
+		timer = make(chan struct{})
+		go func() {
+			select {
+			case <-chControl:
+				close(timer)
+				return
+			default:
+				time.Sleep(25 * time.Millisecond)
+				timer <- struct{}{}
+			}
+		}()
+	}
+
 	go func() {
 		for {
 			select {
 			case <-chControl:
 				return
-			default:
+			case <-timer:
+				_, err := os.Stat(path)
+				if err == nil {
+					close(done)
+					return
+				}
+			case <-inotifyEvents:
 				_, err := os.Stat(path)
 				if err == nil {
 					close(done)
 					return
 				}
-				time.Sleep(25 * time.Millisecond)
 			}
 		}
 	}()