lib: stop downloading images if the caller already has this manifest

This commit allows a user of the library to provide a list of manifest
hashes, presumably generated from annotations left on images by prior
invocations of docker2aci, for images it already has. If docker2aci
fetches a manifest and the hash for it matches one of the elements in
the user provided list, docker2aci will stop working, and return with no
error and no produced ACIs.

lib/docker2aci.go

@@ -42,10 +42,11 @@ import (
 // CommonConfig represents the shared configuration options for converting
 // Docker images.
 type CommonConfig struct {
-	Squash      bool               // squash the layers in one file
-	OutputDir   string             // where to put the resulting ACI
-	TmpDir      string             // directory to use for temporary files
-	Compression common.Compression // which compression to use for the resulting file(s)
+	Squash                bool               // squash the layers in one file
+	OutputDir             string             // where to put the resulting ACI
+	TmpDir                string             // directory to use for temporary files
+	Compression           common.Compression // which compression to use for the resulting file(s)
+	CurrentManifestHashes []string           // any manifest hashes the caller already has
 
 	Info  log.Logger
 	Debug log.Logger
@@ -145,6 +146,11 @@ func (c *converter) convert() ([]string, error) {
 	if err != nil {
 		return nil, err
 	}
+	for _, h := range c.config.CurrentManifestHashes {
+		if manhash == h {
+			return nil, nil
+		}
+	}
 
 	layersOutputDir := c.config.OutputDir
 	if c.config.Squash {

lib/internal/backend/repository/repository2.go

@@ -15,8 +15,6 @@
 package repository
 
 import (
-	"crypto/sha256"
-	"encoding/hex"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -37,6 +35,7 @@ import (
 	"github.com/appc/docker2aci/lib/internal/util"
 	"github.com/appc/spec/schema"
 	"github.com/coreos/pkg/progressutil"
+	godigest "github.com/opencontainers/go-digest"
 )
 
 const (
@@ -334,9 +333,7 @@ func (rb *RepositoryBackend) getManifestV21(dockerURL *common.ParsedDockerURL, r
 		return nil, "", err
 	}
 
-	s := sha256.New()
-	s.Write(manblob)
-	manhash := hex.EncodeToString(s.Sum(nil))
+	manhash := godigest.FromBytes(manblob)
 
 	if manifest.Name != dockerURL.ImageName {
 		return nil, "", fmt.Errorf("name doesn't match what was requested, expected: %s, downloaded: %s", dockerURL.ImageName, manifest.Name)
@@ -363,7 +360,7 @@ func (rb *RepositoryBackend) getManifestV21(dockerURL *common.ParsedDockerURL, r
 
 	rb.imageManifests[*dockerURL] = *manifest
 
-	return layers, manhash, nil
+	return layers, string(manhash), nil
 }
 
 func (rb *RepositoryBackend) getManifestV22(dockerURL *common.ParsedDockerURL, res *http.Response) ([]string, string, error) {
@@ -379,9 +376,7 @@ func (rb *RepositoryBackend) getManifestV22(dockerURL *common.ParsedDockerURL, r
 		return nil, "", err
 	}
 
-	s := sha256.New()
-	s.Write(manblob)
-	manhash := hex.EncodeToString(s.Sum(nil))
+	manhash := godigest.FromBytes(manblob)
 
 	//TODO: verify signature here
 
@@ -398,7 +393,7 @@ func (rb *RepositoryBackend) getManifestV22(dockerURL *common.ParsedDockerURL, r
 
 	rb.imageV2Manifests[*dockerURL] = manifest
 
-	return layers, manhash, nil
+	return layers, string(manhash), nil
 }
 
 func (rb *RepositoryBackend) getConfigV22(dockerURL *common.ParsedDockerURL, configDigest string) error {