diff: rename differ to comparer

Remove combined interface and split implementations.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

diff/apply/apply.go

@@ -0,0 +1,112 @@
+package apply
+
+import (
+	"context"
+	"io"
+	"io/ioutil"
+	"time"
+
+	"github.com/containerd/containerd/archive"
+	"github.com/containerd/containerd/archive/compression"
+	"github.com/containerd/containerd/content"
+	"github.com/containerd/containerd/diff"
+	"github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/log"
+	"github.com/containerd/containerd/mount"
+	digest "github.com/opencontainers/go-digest"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
+)
+
+// NewFileSystemApplier returns an applier which simply mounts
+// and applies diff onto the mounted filesystem.
+func NewFileSystemApplier(cs content.Store) diff.Applier {
+	return &fsApplier{
+		store: cs,
+	}
+}
+
+type fsApplier struct {
+	store content.Store
+}
+
+var emptyDesc = ocispec.Descriptor{}
+
+// Apply applies the content associated with the provided digests onto the
+// provided mounts. Archive content will be extracted and decompressed if
+// necessary.
+func (s *fsApplier) Apply(ctx context.Context, desc ocispec.Descriptor, mounts []mount.Mount) (d ocispec.Descriptor, err error) {
+	t1 := time.Now()
+	defer func() {
+		if err == nil {
+			log.G(ctx).WithFields(logrus.Fields{
+				"d":     time.Now().Sub(t1),
+				"dgst":  desc.Digest,
+				"size":  desc.Size,
+				"media": desc.MediaType,
+			}).Debugf("diff applied")
+		}
+	}()
+
+	isCompressed, err := images.IsCompressedDiff(ctx, desc.MediaType)
+	if err != nil {
+		return emptyDesc, errors.Wrapf(errdefs.ErrNotImplemented, "unsupported diff media type: %v", desc.MediaType)
+	}
+
+	var ocidesc ocispec.Descriptor
+	if err := mount.WithTempMount(ctx, mounts, func(root string) error {
+		ra, err := s.store.ReaderAt(ctx, desc.Digest)
+		if err != nil {
+			return errors.Wrap(err, "failed to get reader from content store")
+		}
+		defer ra.Close()
+
+		r := content.NewReader(ra)
+		if isCompressed {
+			ds, err := compression.DecompressStream(r)
+			if err != nil {
+				return err
+			}
+			defer ds.Close()
+			r = ds
+		}
+
+		digester := digest.Canonical.Digester()
+		rc := &readCounter{
+			r: io.TeeReader(r, digester.Hash()),
+		}
+
+		if _, err := archive.Apply(ctx, root, rc); err != nil {
+			return err
+		}
+
+		// Read any trailing data
+		if _, err := io.Copy(ioutil.Discard, rc); err != nil {
+			return err
+		}
+
+		ocidesc = ocispec.Descriptor{
+			MediaType: ocispec.MediaTypeImageLayer,
+			Size:      rc.c,
+			Digest:    digester.Digest(),
+		}
+		return nil
+
+	}); err != nil {
+		return emptyDesc, err
+	}
+	return ocidesc, nil
+}
+
+type readCounter struct {
+	r io.Reader
+	c int64
+}
+
+func (rc *readCounter) Read(p []byte) (n int, err error) {
+	n, err = rc.r.Read(p)
+	rc.c += int64(n)
+	return
+}