btrfs: auto-detect device

No longer need to set `plugins.snapshot-btrfs.device` manually

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

snapshot/btrfs/btrfs.go

@@ -7,51 +7,69 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/Sirupsen/logrus"
 	"github.com/containerd/btrfs"
 	"github.com/containerd/containerd"
 	"github.com/containerd/containerd/log"
+	"github.com/containerd/containerd/mountinfo"
 	"github.com/containerd/containerd/plugin"
 	"github.com/containerd/containerd/snapshot"
 	"github.com/containerd/containerd/snapshot/storage"
 	"github.com/pkg/errors"
 )
 
-type btrfsConfig struct {
-	Device string `toml:"device"`
-}
-
 func init() {
 	plugin.Register("snapshot-btrfs", &plugin.Registration{
-		Type:   plugin.SnapshotPlugin,
-		Config: &btrfsConfig{},
+		Type: plugin.SnapshotPlugin,
 		Init: func(ic *plugin.InitContext) (interface{}, error) {
 			root := filepath.Join(ic.Root, "snapshot", "btrfs")
-			conf := ic.Config.(*btrfsConfig)
-			if conf.Device == "" {
-				// TODO: check device for root
-				return nil, errors.Errorf("btrfs requires \"device\" configuration")
-			}
-			return NewSnapshotter(conf.Device, root)
+			return NewSnapshotter(root)
 		},
 	})
 }
 
 type snapshotter struct {
-	device string // maybe we can resolve it with path?
+	device string // device of the root
 	root   string // root provides paths for internal storage.
 	ms     *storage.MetaStore
 }
 
+func getBtrfsDevice(root string, mounts []*mountinfo.Info) (string, error) {
+	device := ""
+	deviceMountpoint := ""
+	for _, info := range mounts {
+		if (info.Root == "/" || info.Root == "") && strings.HasPrefix(root, info.Mountpoint) {
+			if info.Fstype == "btrfs" && len(info.Mountpoint) > len(deviceMountpoint) {
+				device = info.Source
+				deviceMountpoint = info.Mountpoint
+			}
+			if root == info.Mountpoint && info.Fstype != "btrfs" {
+				return "", fmt.Errorf("%s needs to be btrfs, but seems %s", root, info.Fstype)
+			}
+		}
+	}
+	if device == "" {
+		// TODO: automatically mount loopback device here?
+		return "", fmt.Errorf("%s is not mounted as btrfs", root)
+	}
+	return device, nil
+}
+
 // NewSnapshotter returns a Snapshotter using btrfs. Uses the provided
-// device and root directory for snapshots and stores the metadata in
+// root directory for snapshots and stores the metadata in
 // a file in the provided root.
-func NewSnapshotter(device, root string) (snapshot.Snapshotter, error) {
-	if err := os.MkdirAll(root, 0700); err != nil {
+// root needs to be a mount point of btrfs.
+func NewSnapshotter(root string) (snapshot.Snapshotter, error) {
+	mounts, err := mountinfo.GetMounts()
+	if err != nil {
+		return nil, err
+	}
+	device, err := getBtrfsDevice(root, mounts)
+	if err != nil {
 		return nil, err
 	}
-
 	var (
 		active    = filepath.Join(root, "active")
 		snapshots = filepath.Join(root, "snapshots")
@@ -193,7 +211,7 @@ func (b *snapshotter) mounts(dir string) ([]containerd.Mount, error) {
 	return []containerd.Mount{
 		{
 			Type:   "btrfs",
-			Source: b.device, // device?
+			Source: b.device,
 			// NOTE(stevvooe): While it would be nice to use to uuids for
 			// mounts, they don't work reliably if the uuids are missing.
 			Options: options,

snapshot/btrfs/btrfs_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 
 	"github.com/containerd/containerd"
+	"github.com/containerd/containerd/mountinfo"
 	"github.com/containerd/containerd/snapshot"
 	"github.com/containerd/containerd/snapshot/testsuite"
 	"github.com/containerd/containerd/testutil"
@@ -24,7 +25,7 @@ const (
 func boltSnapshotter(t *testing.T) func(context.Context, string) (snapshot.Snapshotter, func(), error) {
 	return func(ctx context.Context, root string) (snapshot.Snapshotter, func(), error) {
 		device := setupBtrfsLoopbackDevice(t, root)
-		snapshotter, err := NewSnapshotter(device.deviceName, root)
+		snapshotter, err := NewSnapshotter(root)
 		if err != nil {
 			t.Fatal(err)
 		}
@@ -215,3 +216,64 @@ func (device *testDevice) remove(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+func TestGetBtrfsDevice(t *testing.T) {
+	testCases := []struct {
+		expectedDevice string
+		expectedError  string
+		root           string
+		mounts         []*mountinfo.Info
+	}{
+		{
+			expectedDevice: "/dev/loop0",
+			root:           "/var/lib/containerd/snapshot/btrfs",
+			mounts: []*mountinfo.Info{
+				&mountinfo.Info{Root: "/", Mountpoint: "/", Fstype: "ext4", Source: "/dev/sda1"},
+				&mountinfo.Info{Root: "/", Mountpoint: "/var/lib/containerd/snapshot/btrfs", Fstype: "btrfs", Source: "/dev/loop0"},
+			},
+		},
+		{
+			expectedError: "/var/lib/containerd/snapshot/btrfs is not mounted as btrfs",
+			root:          "/var/lib/containerd/snapshot/btrfs",
+			mounts: []*mountinfo.Info{
+				&mountinfo.Info{Root: "/", Mountpoint: "/", Fstype: "ext4", Source: "/dev/sda1"},
+			},
+		},
+		{
+			expectedDevice: "/dev/sda1",
+			root:           "/var/lib/containerd/snapshot/btrfs",
+			mounts: []*mountinfo.Info{
+				&mountinfo.Info{Root: "/", Mountpoint: "/", Fstype: "btrfs", Source: "/dev/sda1"},
+			},
+		},
+		{
+			expectedDevice: "/dev/sda2",
+			root:           "/var/lib/containerd/snapshot/btrfs",
+			mounts: []*mountinfo.Info{
+				&mountinfo.Info{Root: "/", Mountpoint: "/", Fstype: "btrfs", Source: "/dev/sda1"},
+				&mountinfo.Info{Root: "/", Mountpoint: "/var/lib/containerd/snapshot/btrfs", Fstype: "btrfs", Source: "/dev/sda2"},
+			},
+		},
+		{
+			expectedDevice: "/dev/sda2",
+			root:           "/var/lib/containerd/snapshot/btrfs",
+			mounts: []*mountinfo.Info{
+				&mountinfo.Info{Root: "/", Mountpoint: "/var/lib/containerd/snapshot/btrfs", Fstype: "btrfs", Source: "/dev/sda2"},
+				&mountinfo.Info{Root: "/", Mountpoint: "/var/lib/foooooooooooooooooooo/baaaaaaaaaaaaaaaaaaaar", Fstype: "btrfs", Source: "/dev/sda3"}, // mountpoint length longer than /var/lib/containerd/snapshot/btrfs
+				&mountinfo.Info{Root: "/", Mountpoint: "/", Fstype: "btrfs", Source: "/dev/sda1"},
+			},
+		},
+	}
+	for i, tc := range testCases {
+		device, err := getBtrfsDevice(tc.root, tc.mounts)
+		if err != nil && tc.expectedError == "" {
+			t.Fatalf("%d: expected nil, got %v", i, err)
+		}
+		if err != nil && !strings.Contains(err.Error(), tc.expectedError) {
+			t.Fatalf("%d: expected %s, got %v", i, tc.expectedError, err)
+		}
+		if err == nil && device != tc.expectedDevice {
+			t.Fatalf("%d: expected %s, got %s", i, tc.expectedDevice, device)
+		}
+	}
+}