Merge pull request #1833 from giuseppe/remove-exec-polling

exec: remove polling for PID file

libpod/container_api.go

@@ -328,6 +328,11 @@ func (c *Container) Exec(tty, privileged bool, env, cmd []string, user string) e
 	if err != nil {
 		return errors.Wrapf(err, "error exec %s", c.ID())
 	}
+	chWait := make(chan error)
+	go func() {
+		chWait <- execCmd.Wait()
+	}()
+	defer close(chWait)
 
 	pidFile := c.execPidPath(sessionID)
 	// 60 second seems a reasonable time to wait
@@ -336,18 +341,12 @@ func (c *Container) Exec(tty, privileged bool, env, cmd []string, user string) e
 	const pidWaitTimeout = 60000
 
 	// Wait until the runtime makes the pidfile
-	// TODO: If runtime errors before the PID file is created, we have to
-	// wait for timeout here
-	if err := WaitForFile(pidFile, pidWaitTimeout*time.Millisecond); err != nil {
-		logrus.Debugf("Timed out waiting for pidfile from runtime for container %s exec", c.ID())
-
-		// Check if an error occurred in the process before we made a pidfile
-		// TODO: Wait() here is a poor choice - is there a way to see if
-		// a process has finished, instead of waiting for it to finish?
-		if err := execCmd.Wait(); err != nil {
+	exited, err := WaitForFile(pidFile, chWait, pidWaitTimeout*time.Millisecond)
+	if err != nil {
+		if exited {
+			// If the runtime exited, propagate the error we got from the process.
 			return err
 		}
-
 		return errors.Wrapf(err, "timed out waiting for runtime to create pidfile for exec session in container %s", c.ID())
 	}
 
@@ -389,7 +388,10 @@ func (c *Container) Exec(tty, privileged bool, env, cmd []string, user string) e
 		locked = false
 	}
 
-	waitErr := execCmd.Wait()
+	var waitErr error
+	if !exited {
+		waitErr = <-chWait
+	}
 
 	// Lock again
 	if !c.batched {

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"
 )
@@ -90,31 +91,64 @@ func MountExists(specMounts []spec.Mount, dest string) bool {
 }
 
 // WaitForFile waits until a file has been created or the given timeout has occurred
-func WaitForFile(path string, timeout time.Duration) error {
+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)
 			}
 		}
 	}()
 
 	select {
+	case e := <-chWait:
+		return true, e
 	case <-done:
-		return nil
+		return false, nil
 	case <-time.After(timeout):
 		close(chControl)
-		return errors.Wrapf(ErrInternal, "timed out waiting for file %s", path)
+		return false, errors.Wrapf(ErrInternal, "timed out waiting for file %s", path)
 	}
 }