From fc3047322a527347072ce98ba183cbc8cb49231d Mon Sep 17 00:00:00 2001
From: Giuseppe Scrivano <gscrivan@redhat.com>
Date: Tue, 20 Nov 2018 11:19:40 +0100
Subject: 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 | 36 ++++++++++++++++++++++++++++++++++--
 1 file changed, 34 insertions(+), 2 deletions(-)

diff --git a/libpod/util.go b/libpod/util.go
index 25e78bd01..aa3494529 100644
--- a/libpod/util.go
+++ b/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)
 			}
 		}
 	}()
-- 
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