Adds cross-repository blob mounting behavior

Extends blob upload POST endpoint to support mount and from query
parameters as described in #634

Signed-off-by: Brian Bland <brian.bland@docker.com>

blobs.go

@@ -155,6 +155,10 @@ type BlobIngester interface {
 
 	// Resume attempts to resume a write to a blob, identified by an id.
 	Resume(ctx context.Context, id string) (BlobWriter, error)
+
+	// Mount adds a blob to this service from another source repository,
+	// identified by a digest.
+	Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (Descriptor, error)
 }
 
 // BlobWriter provides a handle for inserting data into a blob store.

docs/spec/api.md

@@ -707,6 +707,34 @@ registry server will dump all intermediate data. While uploads will time out
 if not completed, clients should issue this request if they encounter a fatal
 error but still have the ability to issue an http request.
 
+##### Cross Repository Blob Mount
+
+A blob may be mounted from another repository that the client has read access
+to, removing the need to upload a blob already known to the registry. To issue
+a blob mount instead of an upload, a POST request should be issued in the
+following format:
+
+```
+POST /v2/<name>/blobs/uploads/?mount=<digest>&from=<repository name>
+Content-Length: 0
+```
+
+If the blob is successfully mounted, the client will receive a `201 Created`
+response:
+
+```
+201 Created
+Location: /v2/<name>/blobs/<digest>
+Content-Length: 0
+Docker-Content-Digest: <digest>
+```
+
+The `Location` header will contain the registry URL to access the accepted
+layer file. The `Docker-Content-Digest` header returns the canonical digest of
+the uploaded blob which may differ from the provided digest. Most clients may
+ignore the value but if it is used, the client should verify the value against
+the uploaded blob data.
+
 ##### Errors
 
 If an 502, 503 or 504 error is received, the client should assume that the
@@ -1023,7 +1051,7 @@ A list of methods and URIs are covered in the table below:
 |------|----|------|-----------|
 | GET | `/v2/` | Base | Check that the endpoint implements Docker Registry API V2. |
 | GET | `/v2/<name>/tags/list` | Tags | Fetch the tags under the repository identified by `name`. |
-| GET | `/v2/<name>/manifests/<reference>` | Manifest | Fetch the manifest identified by `name` and `reference` where `reference` can be a tag or digest. |
+| GET | `/v2/<name>/manifests/<reference>` | Manifest | Fetch the manifest identified by `name` and `reference` where `reference` can be a tag or digest. A `HEAD` request can also be issued to this endpoint to obtain resource information without receiving all data. |
 | PUT | `/v2/<name>/manifests/<reference>` | Manifest | Put the manifest identified by `name` and `reference` where `reference` can be a tag or digest. |
 | DELETE | `/v2/<name>/manifests/<reference>` | Manifest | Delete the manifest identified by `name` and `reference`. Note that a manifest can _only_ be deleted by `digest`. |
 | GET | `/v2/<name>/blobs/<digest>` | Blob | Retrieve the blob from the registry identified by `digest`. A `HEAD` request can also be issued to this endpoint to obtain resource information without receiving all data. |
@@ -1500,7 +1528,7 @@ Create, update, delete and retrieve manifests.
 
 #### GET Manifest
 
-Fetch the manifest identified by `name` and `reference` where `reference` can be a tag or digest.
+Fetch the manifest identified by `name` and `reference` where `reference` can be a tag or digest. A `HEAD` request can also be issued to this endpoint to obtain resource information without receiving all data.
 
 
 
@@ -3313,6 +3341,204 @@ The error codes that may be included in the response body are enumerated below:
 
 
 
+##### Mount Blob
+
+```
+POST /v2/<name>/blobs/uploads/?mount=<digest>&from=<repository name>
+Host: <registry host>
+Authorization: <scheme> <token>
+Content-Length: 0
+```
+
+Mount a blob identified by the `mount` parameter from another repository.
+
+
+The following parameters should be specified on the request:
+
+|Name|Kind|Description|
+|----|----|-----------|
+|`Host`|header|Standard HTTP Host Header. Should be set to the registry host.|
+|`Authorization`|header|An RFC7235 compliant authorization header.|
+|`Content-Length`|header|The `Content-Length` header must be zero and the body must be empty.|
+|`name`|path|Name of the target repository.|
+|`mount`|query|Digest of blob to mount from the source repository.|
+|`from`|query|Name of the source repository.|
+
+
+
+
+###### On Success: Created
+
+```
+201 Created
+Location: <blob location>
+Content-Length: 0
+Docker-Upload-UUID: <uuid>
+```
+
+The blob has been mounted in the repository and is available at the provided location.
+
+The following headers will be returned with the response:
+
+|Name|Description|
+|----|-----------|
+|`Location`||
+|`Content-Length`|The `Content-Length` header must be zero and the body must be empty.|
+|`Docker-Upload-UUID`|Identifies the docker upload uuid for the current request.|
+
+
+
+
+###### On Failure: Invalid Name or Digest
+
+```
+400 Bad Request
+```
+
+
+
+
+
+The error codes that may be included in the response body are enumerated below:
+
+|Code|Message|Description|
+|----|-------|-----------|
+| `DIGEST_INVALID` | provided digest did not match uploaded content | When a blob is uploaded, the registry will check that the content matches the digest provided by the client. The error may include a detail structure with the key "digest", including the invalid digest string. This error may also be returned when a manifest includes an invalid layer digest. |
+| `NAME_INVALID` | invalid repository name | Invalid repository name encountered either during manifest validation or any API operation. |
+
+
+
+###### On Failure: Not allowed
+
+```
+405 Method Not Allowed
+```
+
+Blob mount is not allowed because the registry is configured as a pull-through cache or for some other reason
+
+
+
+The error codes that may be included in the response body are enumerated below:
+
+|Code|Message|Description|
+|----|-------|-----------|
+| `UNSUPPORTED` | The operation is unsupported. | The operation was unsupported due to a missing implementation or invalid set of parameters. |
+
+
+
+###### On Failure: Authentication Required
+
+```
+401 Unauthorized
+WWW-Authenticate: <scheme> realm="<realm>", ..."
+Content-Length: <length>
+Content-Type: application/json; charset=utf-8
+
+{
+	"errors:" [
+	    {
+            "code": <error code>,
+            "message": "<error message>",
+            "detail": ...
+        },
+        ...
+    ]
+}
+```
+
+The client is not authenticated.
+
+The following headers will be returned on the response:
+
+|Name|Description|
+|----|-----------|
+|`WWW-Authenticate`|An RFC7235 compliant authentication challenge header.|
+|`Content-Length`|Length of the JSON response body.|
+
+
+
+The error codes that may be included in the response body are enumerated below:
+
+|Code|Message|Description|
+|----|-------|-----------|
+| `UNAUTHORIZED` | authentication required | The access controller was unable to authenticate the client. Often this will be accompanied by a Www-Authenticate HTTP response header indicating how to authenticate. |
+
+
+
+###### On Failure: No Such Repository Error
+
+```
+404 Not Found
+Content-Length: <length>
+Content-Type: application/json; charset=utf-8
+
+{
+	"errors:" [
+	    {
+            "code": <error code>,
+            "message": "<error message>",
+            "detail": ...
+        },
+        ...
+    ]
+}
+```
+
+The repository is not known to the registry.
+
+The following headers will be returned on the response:
+
+|Name|Description|
+|----|-----------|
+|`Content-Length`|Length of the JSON response body.|
+
+
+
+The error codes that may be included in the response body are enumerated below:
+
+|Code|Message|Description|
+|----|-------|-----------|
+| `NAME_UNKNOWN` | repository name not known to registry | This is returned if the name used during an operation is unknown to the registry. |
+
+
+
+###### On Failure: Access Denied
+
+```
+403 Forbidden
+Content-Length: <length>
+Content-Type: application/json; charset=utf-8
+
+{
+	"errors:" [
+	    {
+            "code": <error code>,
+            "message": "<error message>",
+            "detail": ...
+        },
+        ...
+    ]
+}
+```
+
+The client does not have required access to the repository.
+
+The following headers will be returned on the response:
+
+|Name|Description|
+|----|-----------|
+|`Content-Length`|Length of the JSON response body.|
+
+
+
+The error codes that may be included in the response body are enumerated below:
+
+|Code|Message|Description|
+|----|-------|-----------|
+| `DENIED` | requested access to the resource is denied | The access controller denied access for the operation on a resource. |
+
+
+
 
 
 ### Blob Upload

docs/spec/api.md.tmpl

@@ -707,6 +707,34 @@ registry server will dump all intermediate data. While uploads will time out
 if not completed, clients should issue this request if they encounter a fatal
 error but still have the ability to issue an http request.
 
+##### Cross Repository Blob Mount
+
+A blob may be mounted from another repository that the client has read access
+to, removing the need to upload a blob already known to the registry. To issue
+a blob mount instead of an upload, a POST request should be issued in the
+following format:
+
+```
+POST /v2/<name>/blobs/uploads/?mount=<digest>&from=<repository name>
+Content-Length: 0
+```
+
+If the blob is successfully mounted, the client will receive a `201 Created`
+response:
+
+```
+201 Created
+Location: /v2/<name>/blobs/<digest>
+Content-Length: 0
+Docker-Content-Digest: <digest>
+```
+
+The `Location` header will contain the registry URL to access the accepted
+layer file. The `Docker-Content-Digest` header returns the canonical digest of
+the uploaded blob which may differ from the provided digest. Most clients may
+ignore the value but if it is used, the client should verify the value against
+the uploaded blob data.
+
 ##### Errors
 
 If an 502, 503 or 504 error is received, the client should assume that the

manifest/schema1/config_builder_test.go

@@ -50,6 +50,10 @@ func (bs *mockBlobService) Resume(ctx context.Context, id string) (distribution.
 	panic("not implemented")
 }
 
+func (bs *mockBlobService) Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (distribution.Descriptor, error) {
+	panic("not implemented")
+}
+
 func TestEmptyTar(t *testing.T) {
 	// Confirm that gzippedEmptyTar expands to 1024 NULL bytes.
 	var decompressed [2048]byte

manifest/schema2/builder_test.go

@@ -46,6 +46,10 @@ func (bs *mockBlobService) Resume(ctx context.Context, id string) (distribution.
 	panic("not implemented")
 }
 
+func (bs *mockBlobService) Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (distribution.Descriptor, error) {
+	panic("not implemented")
+}
+
 func TestBuilder(t *testing.T) {
 	imgJSON := []byte(`{
     "architecture": "amd64",

registry/api/v2/descriptors.go

@@ -1041,6 +1041,70 @@ var routeDescriptors = []RouteDescriptor{
 							deniedResponseDescriptor,
 						},
 					},
+					{
+						Name:        "Mount Blob",
+						Description: "Mount a blob identified by the `mount` parameter from another repository.",
+						Headers: []ParameterDescriptor{
+							hostHeader,
+							authHeader,
+							contentLengthZeroHeader,
+						},
+						PathParameters: []ParameterDescriptor{
+							nameParameterDescriptor,
+						},
+						QueryParameters: []ParameterDescriptor{
+							{
+								Name:        "mount",
+								Type:        "query",
+								Format:      "<digest>",
+								Regexp:      digest.DigestRegexp,
+								Description: `Digest of blob to mount from the source repository.`,
+							},
+							{
+								Name:        "from",
+								Type:        "query",
+								Format:      "<repository name>",
+								Regexp:      reference.NameRegexp,
+								Description: `Name of the source repository.`,
+							},
+						},
+						Successes: []ResponseDescriptor{
+							{
+								Description: "The blob has been mounted in the repository and is available at the provided location.",
+								StatusCode:  http.StatusCreated,
+								Headers: []ParameterDescriptor{
+									{
+										Name:   "Location",
+										Type:   "url",
+										Format: "<blob location>",
+									},
+									contentLengthZeroHeader,
+									dockerUploadUUIDHeader,
+								},
+							},
+						},
+						Failures: []ResponseDescriptor{
+							{
+								Name:       "Invalid Name or Digest",
+								StatusCode: http.StatusBadRequest,
+								ErrorCodes: []errcode.ErrorCode{
+									ErrorCodeDigestInvalid,
+									ErrorCodeNameInvalid,
+								},
+							},
+							{
+								Name:        "Not allowed",
+								Description: "Blob mount is not allowed because the registry is configured as a pull-through cache or for some other reason",
+								StatusCode:  http.StatusMethodNotAllowed,
+								ErrorCodes: []errcode.ErrorCode{
+									errcode.ErrorCodeUnsupported,
+								},
+							},
+							unauthorizedResponseDescriptor,
+							repositoryNotFoundResponseDescriptor,
+							deniedResponseDescriptor,
+						},
+					},
 				},
 			},
 		},

registry/client/repository.go

@@ -10,6 +10,7 @@ import (
 	"net/http"
 	"net/url"
 	"strconv"
+	"sync"
 	"time"
 
 	"github.com/docker/distribution"
@@ -499,6 +500,9 @@ type blobs struct {
 
 	statter distribution.BlobDescriptorService
 	distribution.BlobDeleter
+
+	cacheLock        sync.Mutex
+	cachedBlobUpload distribution.BlobWriter
 }
 
 func sanitizeLocation(location, base string) (string, error) {
@@ -573,7 +577,20 @@ func (bs *blobs) Put(ctx context.Context, mediaType string, p []byte) (distribut
 }
 
 func (bs *blobs) Create(ctx context.Context) (distribution.BlobWriter, error) {
+	bs.cacheLock.Lock()
+	if bs.cachedBlobUpload != nil {
+		upload := bs.cachedBlobUpload
+		bs.cachedBlobUpload = nil
+		bs.cacheLock.Unlock()
+
+		return upload, nil
+	}
+	bs.cacheLock.Unlock()
+
 	u, err := bs.ub.BuildBlobUploadURL(bs.name)
+	if err != nil {
+		return nil, err
+	}
 
 	resp, err := bs.client.Post(u, "", nil)
 	if err != nil {
@@ -604,6 +621,45 @@ func (bs *blobs) Resume(ctx context.Context, id string) (distribution.BlobWriter
 	panic("not implemented")
 }
 
+func (bs *blobs) Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (distribution.Descriptor, error) {
+	u, err := bs.ub.BuildBlobUploadURL(bs.name, url.Values{"from": {sourceRepo}, "mount": {dgst.String()}})
+	if err != nil {
+		return distribution.Descriptor{}, err
+	}
+
+	resp, err := bs.client.Post(u, "", nil)
+	if err != nil {
+		return distribution.Descriptor{}, err
+	}
+	defer resp.Body.Close()
+
+	switch resp.StatusCode {
+	case http.StatusCreated:
+		return bs.Stat(ctx, dgst)
+	case http.StatusAccepted:
+		// Triggered a blob upload (legacy behavior), so cache the creation info
+		uuid := resp.Header.Get("Docker-Upload-UUID")
+		location, err := sanitizeLocation(resp.Header.Get("Location"), u)
+		if err != nil {
+			return distribution.Descriptor{}, err
+		}
+
+		bs.cacheLock.Lock()
+		bs.cachedBlobUpload = &httpBlobUpload{
+			statter:   bs.statter,
+			client:    bs.client,
+			uuid:      uuid,
+			startedAt: time.Now(),
+			location:  location,
+		}
+		bs.cacheLock.Unlock()
+
+		return distribution.Descriptor{}, HandleErrorResponse(resp)
+	default:
+		return distribution.Descriptor{}, HandleErrorResponse(resp)
+	}
+}
+
 func (bs *blobs) Delete(ctx context.Context, dgst digest.Digest) error {
 	return bs.statter.Clear(ctx, dgst)
 }

registry/client/repository_test.go

@@ -466,6 +466,61 @@ func TestBlobUploadMonolithic(t *testing.T) {
 	}
 }
 
+func TestBlobMount(t *testing.T) {
+	dgst, content := newRandomBlob(1024)
+	var m testutil.RequestResponseMap
+	repo := "test.example.com/uploadrepo"
+	sourceRepo := "test.example.com/sourcerepo"
+	m = append(m, testutil.RequestResponseMapping{
+		Request: testutil.Request{
+			Method:      "POST",
+			Route:       "/v2/" + repo + "/blobs/uploads/",
+			QueryParams: map[string][]string{"from": {sourceRepo}, "mount": {dgst.String()}},
+		},
+		Response: testutil.Response{
+			StatusCode: http.StatusCreated,
+			Headers: http.Header(map[string][]string{
+				"Content-Length":        {"0"},
+				"Location":              {"/v2/" + repo + "/blobs/" + dgst.String()},
+				"Docker-Content-Digest": {dgst.String()},
+			}),
+		},
+	})
+	m = append(m, testutil.RequestResponseMapping{
+		Request: testutil.Request{
+			Method: "HEAD",
+			Route:  "/v2/" + repo + "/blobs/" + dgst.String(),
+		},
+		Response: testutil.Response{
+			StatusCode: http.StatusOK,
+			Headers: http.Header(map[string][]string{
+				"Content-Length": {fmt.Sprint(len(content))},
+				"Last-Modified":  {time.Now().Add(-1 * time.Second).Format(time.ANSIC)},
+			}),
+		},
+	})
+
+	e, c := testServer(m)
+	defer c()
+
+	ctx := context.Background()
+	r, err := NewRepository(ctx, repo, e, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	l := r.Blobs(ctx)
+
+	stat, err := l.Mount(ctx, sourceRepo, dgst)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if stat.Digest != dgst {
+		t.Fatalf("Unexpected digest: %s, expected %s", stat.Digest, dgst)
+	}
+}
+
 func newRandomSchemaV1Manifest(name, tag string, blobCount int) (*schema1.SignedManifest, digest.Digest, []byte) {
 	blobs := make([]schema1.FSLayer, blobCount)
 	history := make([]schema1.History, blobCount)

registry/handlers/app.go

@@ -710,6 +710,11 @@ func (app *App) authorized(w http.ResponseWriter, r *http.Request, context *Cont
 
 	if repo != "" {
 		accessRecords = appendAccessRecords(accessRecords, r.Method, repo)
+		if fromRepo := r.FormValue("from"); fromRepo != "" {
+			// mounting a blob from one repository to another requires pull (GET)
+			// access to the source repository.
+			accessRecords = appendAccessRecords(accessRecords, "GET", fromRepo)
+		}
 	} else {
 		// Only allow the name not to be set on the base route.
 		if app.nameRequired(r) {

registry/handlers/blobupload.go

@@ -116,8 +116,16 @@ type blobUploadHandler struct {
 }
 
 // StartBlobUpload begins the blob upload process and allocates a server-side
-// blob writer session.
+// blob writer session, optionally mounting the blob from a separate repository.
 func (buh *blobUploadHandler) StartBlobUpload(w http.ResponseWriter, r *http.Request) {
+	fromRepo := r.FormValue("from")
+	mountDigest := r.FormValue("mount")
+
+	if mountDigest != "" && fromRepo != "" {
+		buh.mountBlob(w, fromRepo, mountDigest)
+		return
+	}
+
 	blobs := buh.Repository.Blobs(buh)
 	upload, err := blobs.Create(buh)
 
@@ -254,18 +262,10 @@ func (buh *blobUploadHandler) PutBlobUploadComplete(w http.ResponseWriter, r *ht
 
 		return
 	}
-
+	if err := buh.writeBlobCreatedHeaders(w, desc); err != nil {
-	// Build our canonical blob url
-	blobURL, err := buh.urlBuilder.BuildBlobURL(buh.Repository.Name(), desc.Digest)
-	if err != nil {
 		buh.Errors = append(buh.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
 		return
 	}
-
-	w.Header().Set("Location", blobURL)
-	w.Header().Set("Content-Length", "0")
-	w.Header().Set("Docker-Content-Digest", desc.Digest.String())
-	w.WriteHeader(http.StatusCreated)
 }
 
 // CancelBlobUpload cancels an in-progress upload of a blob.
@@ -335,3 +335,45 @@ func (buh *blobUploadHandler) blobUploadResponse(w http.ResponseWriter, r *http.
 
 	return nil
 }
+
+// mountBlob attempts to mount a blob from another repository by its digest. If
+// successful, the blob is linked into the blob store and 201 Created is
+// returned with the canonical url of the blob.
+func (buh *blobUploadHandler) mountBlob(w http.ResponseWriter, fromRepo, mountDigest string) {
+	dgst, err := digest.ParseDigest(mountDigest)
+	if err != nil {
+		buh.Errors = append(buh.Errors, v2.ErrorCodeDigestInvalid.WithDetail(err))
+		return
+	}
+
+	blobs := buh.Repository.Blobs(buh)
+	desc, err := blobs.Mount(buh, fromRepo, dgst)
+	if err != nil {
+		if err == distribution.ErrBlobUnknown {
+			buh.Errors = append(buh.Errors, v2.ErrorCodeBlobUnknown.WithDetail(dgst))
+		} else {
+			buh.Errors = append(buh.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
+		}
+		return
+	}
+	if err := buh.writeBlobCreatedHeaders(w, desc); err != nil {
+		buh.Errors = append(buh.Errors, errcode.ErrorCodeUnknown.WithDetail(err))
+		return
+	}
+}
+
+// writeBlobCreatedHeaders writes the standard headers describing a newly
+// created blob. A 201 Created is written as well as the canonical URL and
+// blob digest.
+func (buh *blobUploadHandler) writeBlobCreatedHeaders(w http.ResponseWriter, desc distribution.Descriptor) error {
+	blobURL, err := buh.urlBuilder.BuildBlobURL(buh.Repository.Name(), desc.Digest)
+	if err != nil {
+		return err
+	}
+
+	w.Header().Set("Location", blobURL)
+	w.Header().Set("Content-Length", "0")
+	w.Header().Set("Docker-Content-Digest", desc.Digest.String())
+	w.WriteHeader(http.StatusCreated)
+	return nil
+}

registry/proxy/proxyblobstore.go

@@ -169,6 +169,10 @@ func (pbs *proxyBlobStore) Resume(ctx context.Context, id string) (distribution.
 	return nil, distribution.ErrUnsupported
 }
 
+func (pbs *proxyBlobStore) Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (distribution.Descriptor, error) {
+	return distribution.Descriptor{}, distribution.ErrUnsupported
+}
+
 func (pbs *proxyBlobStore) Open(ctx context.Context, dgst digest.Digest) (distribution.ReadSeekCloser, error) {
 	return nil, distribution.ErrUnsupported
 }

registry/proxy/proxyblobstore_test.go

@@ -58,6 +58,14 @@ func (sbs statsBlobStore) Resume(ctx context.Context, id string) (distribution.B
 	return sbs.blobs.Resume(ctx, id)
 }
 
+func (sbs statsBlobStore) Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (distribution.Descriptor, error) {
+	sbsMu.Lock()
+	sbs.stats["mount"]++
+	sbsMu.Unlock()
+
+	return sbs.blobs.Mount(ctx, sourceRepo, dgst)
+}
+
 func (sbs statsBlobStore) Open(ctx context.Context, dgst digest.Digest) (distribution.ReadSeekCloser, error) {
 	sbsMu.Lock()
 	sbs.stats["open"]++

registry/storage/blob_test.go

@@ -310,6 +310,154 @@ func TestSimpleBlobRead(t *testing.T) {
 	}
 }
 
+// TestBlobMount covers the blob mount process, exercising common
+// error paths that might be seen during a mount.
+func TestBlobMount(t *testing.T) {
+	randomDataReader, dgst, err := testutil.CreateRandomTarFile()
+	if err != nil {
+		t.Fatalf("error creating random reader: %v", err)
+	}
+
+	ctx := context.Background()
+	imageName := "foo/bar"
+	sourceImageName := "foo/source"
+	driver := inmemory.New()
+	registry, err := NewRegistry(ctx, driver, BlobDescriptorCacheProvider(memory.NewInMemoryBlobDescriptorCacheProvider()), EnableDelete, EnableRedirect)
+	if err != nil {
+		t.Fatalf("error creating registry: %v", err)
+	}
+
+	repository, err := registry.Repository(ctx, imageName)
+	if err != nil {
+		t.Fatalf("unexpected error getting repo: %v", err)
+	}
+	sourceRepository, err := registry.Repository(ctx, sourceImageName)
+	if err != nil {
+		t.Fatalf("unexpected error getting repo: %v", err)
+	}
+
+	sbs := sourceRepository.Blobs(ctx)
+
+	blobUpload, err := sbs.Create(ctx)
+
+	if err != nil {
+		t.Fatalf("unexpected error starting layer upload: %s", err)
+	}
+
+	// Get the size of our random tarfile
+	randomDataSize, err := seekerSize(randomDataReader)
+	if err != nil {
+		t.Fatalf("error getting seeker size of random data: %v", err)
+	}
+
+	nn, err := io.Copy(blobUpload, randomDataReader)
+	if err != nil {
+		t.Fatalf("unexpected error uploading layer data: %v", err)
+	}
+
+	desc, err := blobUpload.Commit(ctx, distribution.Descriptor{Digest: dgst})
+	if err != nil {
+		t.Fatalf("unexpected error finishing layer upload: %v", err)
+	}
+
+	// Test for existence.
+	statDesc, err := sbs.Stat(ctx, desc.Digest)
+	if err != nil {
+		t.Fatalf("unexpected error checking for existence: %v, %#v", err, sbs)
+	}
+
+	if statDesc != desc {
+		t.Fatalf("descriptors not equal: %v != %v", statDesc, desc)
+	}
+
+	bs := repository.Blobs(ctx)
+	// Test destination for existence.
+	statDesc, err = bs.Stat(ctx, desc.Digest)
+	if err == nil {
+		t.Fatalf("unexpected non-error stating unmounted blob: %v", desc)
+	}
+
+	mountDesc, err := bs.Mount(ctx, sourceRepository.Name(), desc.Digest)
+	if err != nil {
+		t.Fatalf("unexpected error mounting layer: %v", err)
+	}
+
+	if mountDesc != desc {
+		t.Fatalf("descriptors not equal: %v != %v", mountDesc, desc)
+	}
+
+	// Test for existence.
+	statDesc, err = bs.Stat(ctx, desc.Digest)
+	if err != nil {
+		t.Fatalf("unexpected error checking for existence: %v, %#v", err, bs)
+	}
+
+	if statDesc != desc {
+		t.Fatalf("descriptors not equal: %v != %v", statDesc, desc)
+	}
+
+	rc, err := bs.Open(ctx, desc.Digest)
+	if err != nil {
+		t.Fatalf("unexpected error opening blob for read: %v", err)
+	}
+	defer rc.Close()
+
+	h := sha256.New()
+	nn, err = io.Copy(h, rc)
+	if err != nil {
+		t.Fatalf("error reading layer: %v", err)
+	}
+
+	if nn != randomDataSize {
+		t.Fatalf("incorrect read length")
+	}
+
+	if digest.NewDigest("sha256", h) != dgst {
+		t.Fatalf("unexpected digest from uploaded layer: %q != %q", digest.NewDigest("sha256", h), dgst)
+	}
+
+	// Delete the blob from the source repo
+	err = sbs.Delete(ctx, desc.Digest)
+	if err != nil {
+		t.Fatalf("Unexpected error deleting blob")
+	}
+
+	d, err := bs.Stat(ctx, desc.Digest)
+	if err != nil {
+		t.Fatalf("unexpected error stating blob deleted from source repository: %v", err)
+	}
+
+	d, err = sbs.Stat(ctx, desc.Digest)
+	if err == nil {
+		t.Fatalf("unexpected non-error stating deleted blob: %v", d)
+	}
+
+	switch err {
+	case distribution.ErrBlobUnknown:
+		break
+	default:
+		t.Errorf("Unexpected error type stat-ing deleted manifest: %#v", err)
+	}
+
+	// Delete the blob from the dest repo
+	err = bs.Delete(ctx, desc.Digest)
+	if err != nil {
+		t.Fatalf("Unexpected error deleting blob")
+	}
+
+	d, err = bs.Stat(ctx, desc.Digest)
+	if err == nil {
+		t.Fatalf("unexpected non-error stating deleted blob: %v", d)
+	}
+
+	switch err {
+	case distribution.ErrBlobUnknown:
+		break
+	default:
+		t.Errorf("Unexpected error type stat-ing deleted manifest: %#v", err)
+	}
+}
+
 // TestLayerUploadZeroLength uploads zero-length
 func TestLayerUploadZeroLength(t *testing.T) {
 	ctx := context.Background()

registry/storage/linkedblobstore.go

@@ -20,6 +20,7 @@ type linkPathFunc func(name string, dgst digest.Digest) (string, error)
 // that grant access to the global blob store.
 type linkedBlobStore struct {
 	*blobStore
+	registry               *registry
 	blobServer             distribution.BlobServer
 	blobAccessController   distribution.BlobDescriptorService
 	repository             distribution.Repository
@@ -185,6 +186,28 @@ func (lbs *linkedBlobStore) Delete(ctx context.Context, dgst digest.Digest) erro
 	return nil
 }
 
+func (lbs *linkedBlobStore) Mount(ctx context.Context, sourceRepo string, dgst digest.Digest) (distribution.Descriptor, error) {
+	repo, err := lbs.registry.Repository(ctx, sourceRepo)
+	if err != nil {
+		return distribution.Descriptor{}, err
+	}
+	stat, err := repo.Blobs(ctx).Stat(ctx, dgst)
+	if err != nil {
+		return distribution.Descriptor{}, err
+	}
+
+	desc := distribution.Descriptor{
+		Size: stat.Size,
+
+		// NOTE(stevvooe): The central blob store firewalls media types from
+		// other users. The caller should look this up and override the value
+		// for the specific repository.
+		MediaType: "application/octet-stream",
+		Digest:    dgst,
+	}
+	return desc, lbs.linkBlob(ctx, desc)
+}
+
 // newBlobUpload allocates a new upload controller with the given state.
 func (lbs *linkedBlobStore) newBlobUpload(ctx context.Context, uuid, path string, startedAt time.Time) (distribution.BlobWriter, error) {
 	fw, err := newFileWriter(ctx, lbs.driver, path)

registry/storage/registry.go

@@ -233,6 +233,7 @@ func (repo *repository) Blobs(ctx context.Context) distribution.BlobStore {
 	}
 
 	return &linkedBlobStore{
+		registry:             repo.registry,
 		blobStore:            repo.blobStore,
 		blobServer:           repo.blobServer,
 		blobAccessController: statter,