mount: add mount.Lookup for ease of implementing snapshotters

`func Lookup(dir string) (Info, error)` returns the mount
info that corresponds to the dir

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

testutil/loopback_linux.go

@@ -0,0 +1,53 @@
+// +build linux
+
+package testutil
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+)
+
+// NewLoopback creates a loopback device, and returns its device name (/dev/loopX), and its clean-up function.
+func NewLoopback(t *testing.T, size int64) (string, func()) {
+	// create temporary file for the disk image
+	file, err := ioutil.TempFile("", "containerd-test-loopback")
+	if err != nil {
+		t.Fatalf("could not create temporary file for loopback: %v", err)
+	}
+
+	if err := file.Truncate(size); err != nil {
+		t.Fatal(err)
+	}
+	file.Close()
+
+	// create device
+	losetup := exec.Command("losetup", "--find", "--show", file.Name())
+	p, err := losetup.Output()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	deviceName := strings.TrimSpace(string(p))
+	t.Logf("Created loop device %s (using %s)", deviceName, file.Name())
+
+	cleanup := func() {
+		// detach device
+		t.Logf("Removing loop device %s", deviceName)
+		losetup := exec.Command("losetup", "--detach", deviceName)
+		err := losetup.Run()
+		if err != nil {
+			t.Error("Could not remove loop device", deviceName, err)
+		}
+
+		// remove file
+		t.Logf("Removing temporary file %s", file.Name())
+		if err = os.Remove(file.Name()); err != nil {
+			t.Error(err)
+		}
+	}
+
+	return deviceName, cleanup
+}