snapshots: separate implementations into packages

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

snapshot/btrfs/btrfs.go

@@ -0,0 +1,75 @@
+package btrfs
+
+import (
+	"crypto/sha256"
+	"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))
+
+	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)
+}
+
+func hash(k string) string {
+	return fmt.Sprintf("%x", sha256.Sum224([]byte(k)))
+}