btrfs: add initial implementation of btrfs snapshot driver

Signed-off-by: Stephen J Day <stephen.day@docker.com>

snapshot/btrfs.go

@@ -0,0 +1,71 @@
+package snapshot
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/docker/containerd"
+	"github.com/stevvooe/go-btrfs"
+)
+
+type Btrfs struct {
+	device string // maybe we can resolve it with path?
+	root   string // root provides paths for internal storage.
+}
+
+func NewBtrfs(device, root string) (*Btrfs, error) {
+	return &Btrfs{device: device, root: root}, nil
+}
+
+func (lm *Btrfs) Prepare(key, parent string) ([]containerd.Mount, error) {
+	active := filepath.Join(lm.root, "active")
+	if err := os.MkdirAll(active, 0755); err != nil {
+		return nil, err
+	}
+
+	dir := filepath.Join(active, hash(key))
+
+	fmt.Println("dir", dir)
+	if parent == "" {
+		// create new subvolume
+		// btrfs subvolume create /dir
+		if err := btrfs.SubvolCreate(dir); err != nil {
+			return nil, err
+		}
+	} else {
+		// btrfs subvolume snapshot /parent /subvol
+		if err := btrfs.SubvolSnapshot(dir, parent, false); err != nil {
+			return nil, err
+		}
+	}
+
+	// get the subvolume id back out for the mount
+	info, err := btrfs.SubvolInfo(dir)
+	if err != nil {
+		return nil, err
+	}
+
+	return []containerd.Mount{
+		{
+			Type:   "btrfs",
+			Source: lm.device, // device?
+			Target: key,
+
+			// NOTE(stevvooe): While it would be nice to use to uuids for
+			// mounts, they don't work reliably if the uuids are missing.
+			Options: []string{fmt.Sprintf("subvolid=%d", info.ID)},
+		},
+	}, nil
+}
+
+func (lm *Btrfs) Commit(name, key string) error {
+	dir := filepath.Join(lm.root, "active", hash(key))
+
+	fmt.Println("commit to", name)
+	if err := btrfs.SubvolSnapshot(name, dir, true); err != nil {
+		return err
+	}
+
+	return btrfs.SubvolDelete(dir)
+}

snapshot/btrfs_test.go

@@ -0,0 +1,87 @@
+package snapshot
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/docker/containerd"
+)
+
+func TestBtrfs(t *testing.T) {
+	// SORRY(stevvooe): This is where I mount a btrfs loopback. We can probably
+	// set this up as part of the test.
+	root, err := ioutil.TempDir("/tmp/snapshots", "TestBtrfsPrepare-")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(root)
+	// TODO(stevvooe): Cleanup subvolumes
+
+	sm, err := NewBtrfs("/dev/loop0", root)
+	if err != nil {
+		t.Fatal(err)
+	}
+	mounts, err := sm.Prepare(filepath.Join(root, "test"), "")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Log(mounts)
+
+	for _, mount := range mounts {
+		if mount.Type != "btrfs" {
+			t.Fatal("wrong mount type: %v != btrfs", mount.Type)
+		}
+
+		// assumes the first, maybe incorrect in the future
+		if !strings.HasPrefix(mount.Options[0], "subvolid=") {
+			t.Fatal("no subvolid option in %v", mount.Options)
+		}
+	}
+
+	if err := os.MkdirAll(mounts[0].Target, 0755); err != nil {
+		t.Fatal(err)
+	}
+	if err := containerd.MountAll(mounts...); err != nil {
+		t.Fatal(err)
+	}
+
+	// write in some data
+	if err := ioutil.WriteFile(filepath.Join(mounts[0].Target, "foo"), []byte("content"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	// TODO(stevvooe): We don't really make this with the driver, but that
+	// might prove annoying in practice.
+	if err := os.MkdirAll(filepath.Join(root, "snapshots"), 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := sm.Commit(filepath.Join(root, "snapshots/committed"), filepath.Join(root, "test")); err != nil {
+		t.Fatal(err)
+	}
+
+	mounts, err = sm.Prepare(filepath.Join(root, "test2"), filepath.Join(root, "snapshots/committed"))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if err := os.MkdirAll(filepath.Join(root, "test2"), 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := containerd.MountAll(mounts...); err != nil {
+		t.Fatal(err)
+	}
+
+	// TODO(stevvooe): Verify contents of "foo"
+	if err := ioutil.WriteFile(filepath.Join(mounts[0].Target, "bar"), []byte("content"), 0777); err != nil {
+		t.Fatal(err)
+	}
+
+	if err := sm.Commit(filepath.Join(root, "snapshots/committed2"), filepath.Join(root, "test2")); err != nil {
+		t.Fatal(err)
+	}
+}