Implements garbage collection subcommand

- Includes a change in the command to run the registry. The registry
  server itself is now started up as a subcommand.
- Includes changes to the high level interfaces to support enumeration
  of various registry objects.

Signed-off-by: Andrew T Nguyen <andrew.nguyen@docker.com>

docs/garbagecollect.go

@@ -0,0 +1,150 @@
+package registry
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/docker/distribution"
+	"github.com/docker/distribution/context"
+	"github.com/docker/distribution/digest"
+	"github.com/docker/distribution/manifest/schema1"
+	"github.com/docker/distribution/manifest/schema2"
+	"github.com/docker/distribution/reference"
+	"github.com/docker/distribution/registry/storage"
+	"github.com/docker/distribution/registry/storage/driver"
+	"github.com/docker/distribution/registry/storage/driver/factory"
+
+	"github.com/spf13/cobra"
+)
+
+func markAndSweep(storageDriver driver.StorageDriver) error {
+	ctx := context.Background()
+
+	// Construct a registry
+	registry, err := storage.NewRegistry(ctx, storageDriver)
+	if err != nil {
+		return fmt.Errorf("failed to construct registry: %v", err)
+	}
+
+	repositoryEnumerator, ok := registry.(distribution.RepositoryEnumerator)
+	if !ok {
+		return fmt.Errorf("coercion error: unable to convert Namespace to RepositoryEnumerator")
+	}
+
+	// mark
+	markSet := make(map[digest.Digest]struct{})
+	err = repositoryEnumerator.Enumerate(ctx, func(repoName string) error {
+		var err error
+		named, err := reference.ParseNamed(repoName)
+		if err != nil {
+			return fmt.Errorf("failed to parse repo name %s: %v", repoName, err)
+		}
+		repository, err := registry.Repository(ctx, named)
+		if err != nil {
+			return fmt.Errorf("failed to construct repository: %v", err)
+		}
+
+		manifestService, err := repository.Manifests(ctx)
+		if err != nil {
+			return fmt.Errorf("failed to construct manifest service: %v", err)
+		}
+
+		manifestEnumerator, ok := manifestService.(distribution.ManifestEnumerator)
+		if !ok {
+			return fmt.Errorf("coercion error: unable to convert ManifestService into ManifestEnumerator")
+		}
+
+		err = manifestEnumerator.Enumerate(ctx, func(dgst digest.Digest) error {
+			// Mark the manifest's blob
+			markSet[dgst] = struct{}{}
+
+			manifest, err := manifestService.Get(ctx, dgst)
+			if err != nil {
+				return fmt.Errorf("failed to retrieve manifest for digest %v: %v", dgst, err)
+			}
+
+			descriptors := manifest.References()
+			for _, descriptor := range descriptors {
+				markSet[descriptor.Digest] = struct{}{}
+			}
+
+			switch manifest.(type) {
+			case *schema1.SignedManifest:
+				signaturesGetter, ok := manifestService.(distribution.SignaturesGetter)
+				if !ok {
+					return fmt.Errorf("coercion error: unable to convert ManifestSErvice into SignaturesGetter")
+				}
+				signatures, err := signaturesGetter.GetSignatures(ctx, dgst)
+				if err != nil {
+					return fmt.Errorf("failed to get signatures for signed manifest: %v", err)
+				}
+				for _, signatureDigest := range signatures {
+					markSet[signatureDigest] = struct{}{}
+				}
+				break
+			case *schema2.DeserializedManifest:
+				config := manifest.(*schema2.DeserializedManifest).Config
+				markSet[config.Digest] = struct{}{}
+				break
+			}
+
+			return nil
+		})
+
+		return err
+	})
+
+	if err != nil {
+		return fmt.Errorf("failed to mark: %v\n", err)
+	}
+
+	// sweep
+	blobService := registry.Blobs()
+	deleteSet := make(map[digest.Digest]struct{})
+	err = blobService.Enumerate(ctx, func(dgst digest.Digest) error {
+		// check if digest is in markSet. If not, delete it!
+		if _, ok := markSet[dgst]; !ok {
+			deleteSet[dgst] = struct{}{}
+		}
+		return nil
+	})
+
+	// Construct vacuum
+	vacuum := storage.NewVacuum(ctx, storageDriver)
+	for dgst := range deleteSet {
+		err = vacuum.RemoveBlob(string(dgst))
+		if err != nil {
+			return fmt.Errorf("failed to delete blob %s: %v\n", dgst, err)
+		}
+	}
+
+	return err
+}
+
+// GCCmd is the cobra command that corresponds to the garbage-collect subcommand
+var GCCmd = &cobra.Command{
+	Use:   "garbage-collect <config>",
+	Short: "`garbage-collects` deletes layers not referenced by any manifests",
+	Long:  "`garbage-collects` deletes layers not referenced by any manifests",
+	Run: func(cmd *cobra.Command, args []string) {
+
+		config, err := resolveConfiguration(args)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "configuration error: %v\n", err)
+			cmd.Usage()
+			os.Exit(1)
+		}
+
+		driver, err := factory.Create(config.Storage.Type(), config.Storage.Parameters())
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "failed to construct %s driver: %v", config.Storage.Type(), err)
+			os.Exit(1)
+		}
+
+		err = markAndSweep(driver)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "failed to garbage collect: %v", err)
+			os.Exit(1)
+		}
+	},
+}