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>

mount/lookup_test/dummy.go

@@ -0,0 +1,3 @@
+package lookuptest
+
+// FIXME: without this dummy file, `make build` fails with "no buildable Go source files" error

mount/lookup_test/lookup_linux_test.go

@@ -0,0 +1,59 @@
+// +build linux
+
+// FIXME: we can't put this test to the mount package:
+// import cycle not allowed in test
+// package github.com/containerd/containerd/mount (test)
+//         imports github.com/containerd/containerd/testutil
+//         imports github.com/containerd/containerd/mount
+//
+// NOTE: we can't have this as lookup_test (compilation fails)
+package lookuptest
+
+import (
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strings"
+	"testing"
+
+	"github.com/containerd/containerd/mount"
+	"github.com/containerd/containerd/testutil"
+	"github.com/stretchr/testify/assert"
+)
+
+func testLookup(t *testing.T, fsType string) {
+	testutil.RequiresRoot(t)
+	mnt, err := ioutil.TempDir("", "containerd-mountinfo-test-lookup")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(mnt)
+
+	deviceName, cleanupDevice := testutil.NewLoopback(t, 100<<20) // 100 MB
+	if out, err := exec.Command("mkfs", "-t", fsType, deviceName).CombinedOutput(); err != nil {
+		// not fatal
+		t.Skipf("could not mkfs (%s) %s: %v (out: %q)", fsType, deviceName, err, string(out))
+	}
+	if out, err := exec.Command("mount", deviceName, mnt).CombinedOutput(); err != nil {
+		// not fatal
+		t.Skipf("could not mount %s: %v (out: %q)", deviceName, err, string(out))
+	}
+	defer func() {
+		testutil.Unmount(t, mnt)
+		cleanupDevice()
+	}()
+	assert.True(t, strings.HasPrefix(deviceName, "/dev/loop"))
+	info, err := mount.Lookup(mnt)
+	if err != nil {
+		t.Fatal(err)
+	}
+	assert.Equal(t, fsType, info.FSType)
+}
+
+func TestLookupWithExt4(t *testing.T) {
+	testLookup(t, "ext4")
+}
+
+func TestLookupWithXFS(t *testing.T) {
+	testLookup(t, "xfs")
+}

mount/lookup_unix.go

@@ -0,0 +1,37 @@
+// +build !windows
+
+package mount
+
+import (
+	"fmt"
+	"syscall"
+
+	"github.com/pkg/errors"
+)
+
+// Lookup returns the mount info corresponds to the path.
+func Lookup(dir string) (Info, error) {
+	var dirStat syscall.Stat_t
+	if err := syscall.Stat(dir, &dirStat); err != nil {
+		return Info{}, errors.Wrapf(err, "failed to access %q", dir)
+	}
+
+	mounts, err := Self()
+	if err != nil {
+		return Info{}, err
+	}
+	for _, m := range mounts {
+		// Note that m.{Major, Minor} are generally unreliable for our purpose here
+		// https://www.spinics.net/lists/linux-btrfs/msg58908.html
+		var st syscall.Stat_t
+		if err := syscall.Stat(m.Mountpoint, &st); err != nil {
+			// may fail; ignore err
+			continue
+		}
+		if st.Dev == dirStat.Dev {
+			return m, nil
+		}
+	}
+
+	return Info{}, fmt.Errorf("failed to find the mount info for %q", dir)
+}

mount/lookup_unsupported.go

@@ -0,0 +1,13 @@
+// +build windows
+
+package mount
+
+import (
+	"fmt"
+	"runtime"
+)
+
+// Lookup returns the mount info corresponds to the path.
+func Lookup(dir string) (Info, error) {
+	return Info{}, fmt.Errorf("mount.Lookup is not implemented on %s/%s", runtime.GOOS, runtime.GOARCH)
+}