Simplify mount cleanup on failure by using defer

This avoids someone adding a new error path and forgetting to call the cleanup
function.

We prefer to use an explicit flag to gate the clean rather than relying on `err
!= nil` so we don't have to rely on people never accidentally shadowing the
`err` as seen by the closure.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

linux/shim/init.go

@@ -55,6 +55,8 @@ type initProcess struct {
 }
 
 func newInitProcess(context context.Context, path, namespace string, r *shimapi.CreateTaskRequest) (*initProcess, error) {
+	var success bool
+
 	if err := identifiers.Validate(r.ID); err != nil {
 		return nil, errors.Wrapf(err, "invalid task id")
 	}
@@ -72,11 +74,14 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 	// what was actually done rather than what should have been
 	// done.
 	nrRootMounts := 0
-	cleanupMounts := func() {
+	defer func() {
+		if success {
+			return
+		}
 		if err2 := mount.UnmountN(rootfs, 0, nrRootMounts); err2 != nil {
 			log.G(context).WithError(err2).Warn("Failed to cleanup rootfs mount")
 		}
-	}
+	}()
 	for _, rm := range r.Rootfs {
 		m := &mount.Mount{
 			Type:    rm.Type,
@@ -84,7 +89,6 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 			Options: rm.Options,
 		}
 		if err := m.Mount(rootfs); err != nil {
-			cleanupMounts()
 			return nil, errors.Wrapf(err, "failed to mount rootfs component %v", m)
 		}
 		nrRootMounts++
@@ -116,14 +120,12 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 	)
 	if r.Terminal {
 		if socket, err = runc.NewConsoleSocket(filepath.Join(path, "pty.sock")); err != nil {
-			cleanupMounts()
 			return nil, errors.Wrap(err, "failed to create OCI runtime console socket")
 		}
 		defer os.Remove(socket.Path())
 	} else {
 		// TODO: get uid/gid
 		if io, err = runc.NewPipeIO(0, 0); err != nil {
-			cleanupMounts()
 			return nil, errors.Wrap(err, "failed to create OCI runtime io pipes")
 		}
 		p.io = io
@@ -143,7 +145,6 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 			NoSubreaper: true,
 		}
 		if _, err := p.runtime.Restore(context, r.ID, r.Bundle, opts); err != nil {
-			cleanupMounts()
 			return nil, p.runtimeError(err, "OCI runtime restore failed")
 		}
 	} else {
@@ -157,7 +158,6 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 			opts.ConsoleSocket = socket
 		}
 		if err := p.runtime.Create(context, r.ID, r.Bundle, opts); err != nil {
-			cleanupMounts()
 			return nil, p.runtimeError(err, "OCI runtime create failed")
 		}
 	}
@@ -173,17 +173,14 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 	if socket != nil {
 		console, err := socket.ReceiveMaster()
 		if err != nil {
-			cleanupMounts()
 			return nil, errors.Wrap(err, "failed to retrieve console master")
 		}
 		p.console = console
 		if err := copyConsole(context, console, r.Stdin, r.Stdout, r.Stderr, &p.WaitGroup, &copyWaitGroup); err != nil {
-			cleanupMounts()
 			return nil, errors.Wrap(err, "failed to start console copy")
 		}
 	} else {
 		if err := copyPipes(context, io, r.Stdin, r.Stdout, r.Stderr, &p.WaitGroup, &copyWaitGroup); err != nil {
-			cleanupMounts()
 			return nil, errors.Wrap(err, "failed to start io pipe copy")
 		}
 	}
@@ -191,10 +188,10 @@ func newInitProcess(context context.Context, path, namespace string, r *shimapi.
 	copyWaitGroup.Wait()
 	pid, err := runc.ReadPidFile(pidFile)
 	if err != nil {
-		cleanupMounts()
 		return nil, errors.Wrap(err, "failed to retrieve OCI runtime container pid")
 	}
 	p.pid = pid
+	success = true
 	return p, nil
 }