Add init and get mounts to rootfs service

Update rootfs package to add mount initializer function.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

rootfs/init.go

@@ -0,0 +1,98 @@
+package rootfs
+
+import (
+	"context"
+	"fmt"
+	"io/ioutil"
+	"os"
+
+	"github.com/docker/containerd"
+	"github.com/docker/containerd/log"
+	"github.com/docker/containerd/snapshot"
+	digest "github.com/opencontainers/go-digest"
+	"github.com/pkg/errors"
+)
+
+var (
+	initializers = map[string]initializerFunc{}
+)
+
+type initializerFunc func(string) error
+
+func InitRootFS(ctx context.Context, name string, parent digest.Digest, readonly bool, snapshotter snapshot.Snapshotter, mounter Mounter) ([]containerd.Mount, error) {
+	_, err := snapshotter.Stat(ctx, name)
+	if err == nil {
+		return nil, errors.Errorf("rootfs already exists")
+	}
+	// TODO: check if should return error
+
+	parentS := parent.String()
+
+	initName := defaultInitializer
+	initFn := initializers[initName]
+	if initFn != nil {
+		parentS, err = createInitLayer(ctx, parentS, initName, initFn, snapshotter, mounter)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	if readonly {
+		return snapshotter.View(ctx, name, parentS)
+	}
+
+	return snapshotter.Prepare(ctx, name, parentS)
+}
+
+func GetRootFS(ctx context.Context, name string, snapshotter snapshot.Snapshotter) ([]containerd.Mount, error) {
+	return snapshotter.Mounts(ctx, name)
+}
+
+func createInitLayer(ctx context.Context, parent, initName string, initFn func(string) error, snapshotter snapshot.Snapshotter, mounter Mounter) (string, error) {
+	initS := fmt.Sprintf("%s %s", parent, initName)
+	if _, err := snapshotter.Stat(ctx, initS); err == nil {
+		return initS, nil
+	}
+	// TODO: check if should return error
+
+	// Create tempdir
+	td, err := ioutil.TempDir("", "create-init-")
+	if err != nil {
+		return "", err
+	}
+	defer os.RemoveAll(td)
+
+	mounts, err := snapshotter.Prepare(ctx, td, parent)
+	if err != nil {
+		return "", err
+	}
+	defer func() {
+		if err != nil {
+			// TODO: once implemented uncomment
+			//if rerr := snapshotter.Remove(ctx, td); rerr != nil {
+			//	log.G(ctx).Errorf("Failed to remove snapshot %s: %v", td, merr)
+			//}
+		}
+	}()
+
+	if err = mounter.Mount(td, mounts...); err != nil {
+		return "", err
+	}
+
+	if err = initFn(td); err != nil {
+		if merr := mounter.Unmount(td); merr != nil {
+			log.G(ctx).Errorf("Failed to unmount %s: %v", td, merr)
+		}
+		return "", err
+	}
+
+	if err = mounter.Unmount(td); err != nil {
+		return "", err
+	}
+
+	if err := snapshotter.Commit(ctx, initS, td); err != nil {
+		return "", err
+	}
+
+	return initS, nil
+}