containerd/rootfs/apply.go

/*
   Copyright The containerd Authors.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package rootfs

import (
	"context"
	"crypto/rand"
	"encoding/base64"
	"fmt"
	"time"

	"github.com/containerd/containerd/diff"
	"github.com/containerd/containerd/errdefs"
	"github.com/containerd/containerd/log"
	"github.com/containerd/containerd/snapshots"
	"github.com/opencontainers/go-digest"
	"github.com/opencontainers/image-spec/identity"
	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
	"github.com/pkg/errors"
)

// Layer represents the descriptors for a layer diff. These descriptions
// include the descriptor for the uncompressed tar diff as well as a blob
// used to transport that tar. The blob descriptor may or may not describe
// a compressed object.
type Layer struct {
	Diff ocispec.Descriptor
	Blob ocispec.Descriptor
}

// ApplyLayers applies all the layers using the given snapshotter and applier.
// The returned result is a chain id digest representing all the applied layers.
// Layers are applied in order they are given, making the first layer the
// bottom-most layer in the layer chain.
func ApplyLayers(ctx context.Context, layers []Layer, sn snapshots.Snapshotter, a diff.Applier) (digest.Digest, error) {
	var chain []digest.Digest
	for _, layer := range layers {
		if _, err := ApplyLayer(ctx, layer, chain, sn, a); err != nil {
			// TODO: possibly wait and retry if extraction of same chain id was in progress
			return "", err
		}

		chain = append(chain, layer.Diff.Digest)
	}
	return identity.ChainID(chain), nil
}

// ApplyLayer applies a single layer on top of the given provided layer chain,
// using the provided snapshotter and applier. If the layer was unpacked true
// is returned, if the layer already exists false is returned.
func ApplyLayer(ctx context.Context, layer Layer, chain []digest.Digest, sn snapshots.Snapshotter, a diff.Applier, opts ...snapshots.Opt) (bool, error) {
	var (
		parent  = identity.ChainID(chain)
		chainID = identity.ChainID(append(chain, layer.Diff.Digest))
		diff    ocispec.Descriptor
	)

	_, err := sn.Stat(ctx, chainID.String())
	if err == nil {
		log.G(ctx).Debugf("Extraction not needed, layer snapshot %s exists", chainID)
		return false, nil
	} else if !errdefs.IsNotFound(err) {
		return false, errors.Wrapf(err, "failed to stat snapshot %s", chainID)
	}

	key := fmt.Sprintf("extract-%s %s", uniquePart(), chainID)

	// Prepare snapshot with from parent, label as root
	mounts, err := sn.Prepare(ctx, key, parent.String(), opts...)
	if err != nil {
		//TODO: If is snapshot exists error, retry
		return false, errors.Wrapf(err, "failed to prepare extraction snapshot %q", key)
	}
	defer func() {
		if err != nil {
			log.G(ctx).WithError(err).WithField("key", key).Infof("Apply failure, attempting cleanup")
			if rerr := sn.Remove(ctx, key); rerr != nil {
				log.G(ctx).WithError(rerr).Warnf("Extraction snapshot %q removal failed", key)
			}
		}
	}()

	diff, err = a.Apply(ctx, layer.Blob, mounts)
	if err != nil {
		return false, errors.Wrapf(err, "failed to extract layer %s", layer.Diff.Digest)
	}
	if diff.Digest != layer.Diff.Digest {
		err = errors.Errorf("wrong diff id calculated on extraction %q", diff.Digest)
		return false, err
	}

	if err = sn.Commit(ctx, chainID.String(), key, opts...); err != nil {
		if !errdefs.IsAlreadyExists(err) {
			return false, errors.Wrapf(err, "failed to commit snapshot %s", key)
		}

		// Destination already exists, cleanup key and return without error
		err = nil
		if err := sn.Remove(ctx, key); err != nil {
			return false, errors.Wrapf(err, "failed to cleanup aborted apply %s", key)
		}
		return false, nil
	}

	return true, nil
}

func uniquePart() string {
	t := time.Now()
	var b [3]byte
	// Ignore read failures, just decreases uniqueness
	rand.Read(b[:])
	return fmt.Sprintf("%d-%s", t.Nanosecond(), base64.URLEncoding.EncodeToString(b[:]))
}