Refactor Blob Service API

This PR refactors the blob service API to be oriented around blob descriptors.
Identified by digests, blobs become an abstract entity that can be read and
written using a descriptor as a handle. This allows blobs to take many forms,
such as a ReadSeekCloser or a simple byte buffer, allowing blob oriented
operations to better integrate with blob agnostic APIs (such as the `io`
package). The error definitions are now better organized to reflect conditions
that can only be seen when interacting with the blob API.

The main benefit of this is to separate the much smaller metadata from large
file storage. Many benefits also follow from this. Reading and writing has
been separated into discrete services. Backend implementation is also
simplified, by reducing the amount of metadata that needs to be picked up to
simply serve a read. This also improves cacheability.

"Opening" a blob simply consists of an access check (Stat) and a path
calculation. Caching is greatly simplified and we've made the mapping of
provisional to canonical hashes a first-class concept. BlobDescriptorService
and BlobProvider can be combined in different ways to achieve varying effects.

Recommend Review Approach
-------------------------

This is a very large patch. While apologies are in order, we are getting a
considerable amount of refactoring. Most changes follow from the changes to
the root package (distribution), so start there. From there, the main changes
are in storage. Looking at (*repository).Blobs will help to understand the how
the linkedBlobStore is wired. One can explore the internals within and also
branch out into understanding the changes to the caching layer. Following the
descriptions below will also help to guide you.

To reduce the chances for regressions, it was critical that major changes to
unit tests were avoided. Where possible, they are left untouched and where
not, the spirit is hopefully captured. Pay particular attention to where
behavior may have changed.

Storage
-------

The primary changes to the `storage` package, other than the interface
updates, were to merge the layerstore and blobstore. Blob access is now
layered even further. The first layer, blobStore, exposes a global
`BlobStatter` and `BlobProvider`. Operations here provide a fast path for most
read operations that don't take access control into account. The
`linkedBlobStore` layers on top of the `blobStore`, providing repository-
scoped blob link management in the backend. The `linkedBlobStore` implements
the full `BlobStore` suite, providing access-controlled, repository-local blob
writers. The abstraction between the two is slightly broken in that
`linkedBlobStore` is the only channel under which one can write into the global
blob store. The `linkedBlobStore` also provides flexibility in that it can act
over different link sets depending on configuration. This allows us to use the
same code for signature links, manifest links and blob links.  Eventually, we
will fully consolidate this storage.

The improved cache flow comes from the `linkedBlobStatter` component
of `linkedBlobStore`. Using a `cachedBlobStatter`, these combine together to
provide a simple cache hierarchy that should streamline access checks on read
and write operations, or at least provide a single path to optimize. The
metrics have been changed in a slightly incompatible way since the former
operations, Fetch and Exists, are no longer relevant.

The fileWriter and fileReader have been slightly modified to support the rest
of the changes. The most interesting is the removal of the `Stat` call from
`newFileReader`. This was the source of unnecessary round trips that were only
present to look up the size of the resulting reader. Now, one must simply pass
in the size, requiring the caller to decide whether or not the `Stat` call is
appropriate. In several cases, it turned out the caller already had the size
already. The `WriterAt` implementation has been removed from `fileWriter`,
since it is no longer required for `BlobWriter`, reducing the number of paths
which writes may take.

Cache
-----

Unfortunately, the `cache` package required a near full rewrite. It was pretty
mechanical in that the cache is oriented around the `BlobDescriptorService`
slightly modified to include the ability to set the values for individual
digests. While the implementation is oriented towards caching, it can act as a
primary store. Provisions are in place to have repository local metadata, in
addition to global metadata. Fallback is implemented as a part of the storage
package to maintain this flexibility.

One unfortunate side-effect is that caching is now repository-scoped, rather
than global. This should have little effect on performance but may increase
memory usage.

Handlers
--------

The `handlers` package has been updated to leverage the new API. For the most
part, the changes are superficial or mechanical based on the API changes. This
did expose a bug in the handling of provisional vs canonical digests that was
fixed in the unit tests.

Configuration
-------------

One user-facing change has been made to the configuration and is updated in
the associated documentation. The `layerinfo` cache parameter has been
deprecated by the `blobdescriptor` cache parameter. Both are equivalent and
configuration files should be backward compatible.

Notifications
-------------

Changes the `notification` package are simply to support the interface
changes.

Context
-------

A small change has been made to the tracing log-level. Traces have been moved
from "info" to "debug" level to reduce output when not needed.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

registry.go

@@ -1,13 +1,9 @@
 package distribution
 
 import (
-	"io"
-	"net/http"
-	"time"
-
+	"github.com/docker/distribution/context"
 	"github.com/docker/distribution/digest"
 	"github.com/docker/distribution/manifest"
-	"golang.org/x/net/context"
 )
 
 // Scope defines the set of items that match a namespace.
@@ -49,8 +45,12 @@ type Repository interface {
 	// Manifests returns a reference to this repository's manifest service.
 	Manifests() ManifestService
 
-	// Layers returns a reference to this repository's layers service.
-	Layers() LayerService
+	// Blobs returns a reference to this repository's blob service.
+	Blobs(ctx context.Context) BlobStore
+
+	// TODO(stevvooe): The above BlobStore return can probably be relaxed to
+	// be a BlobService for use with clients. This will allow such
+	// implementations to avoid implementing ServeBlob.
 
 	// Signatures returns a reference to this repository's signatures service.
 	Signatures() SignatureService
@@ -100,70 +100,6 @@ type ManifestService interface {
 	//       really be concerned with the storage format.
 }
 
-// LayerService provides operations on layer files in a backend storage.
-type LayerService interface {
-	// Exists returns true if the layer exists.
-	Exists(digest digest.Digest) (bool, error)
-
-	// Fetch the layer identifed by TarSum.
-	Fetch(digest digest.Digest) (Layer, error)
-
-	// Upload begins a layer upload to repository identified by name,
-	// returning a handle.
-	Upload() (LayerUpload, error)
-
-	// Resume continues an in progress layer upload, returning a handle to the
-	// upload. The caller should seek to the latest desired upload location
-	// before proceeding.
-	Resume(uuid string) (LayerUpload, error)
-}
-
-// Layer provides a readable and seekable layer object. Typically,
-// implementations are *not* goroutine safe.
-type Layer interface {
-	// http.ServeContent requires an efficient implementation of
-	// ReadSeeker.Seek(0, os.SEEK_END).
-	io.ReadSeeker
-	io.Closer
-
-	// Digest returns the unique digest of the blob.
-	Digest() digest.Digest
-
-	// Length returns the length in bytes of the blob.
-	Length() int64
-
-	// CreatedAt returns the time this layer was created.
-	CreatedAt() time.Time
-
-	// Handler returns an HTTP handler which serves the layer content, whether
-	// by providing a redirect directly to the content, or by serving the
-	// content itself.
-	Handler(r *http.Request) (http.Handler, error)
-}
-
-// LayerUpload provides a handle for working with in-progress uploads.
-// Instances can be obtained from the LayerService.Upload and
-// LayerService.Resume.
-type LayerUpload interface {
-	io.WriteSeeker
-	io.ReaderFrom
-	io.Closer
-
-	// UUID returns the identifier for this upload.
-	UUID() string
-
-	// StartedAt returns the time this layer upload was started.
-	StartedAt() time.Time
-
-	// Finish marks the upload as completed, returning a valid handle to the
-	// uploaded layer. The digest is validated against the contents of the
-	// uploaded layer.
-	Finish(digest digest.Digest) (Layer, error)
-
-	// Cancel the layer upload process.
-	Cancel() error
-}
-
 // SignatureService provides operations on signatures.
 type SignatureService interface {
 	// Get retrieves all of the signature blobs for the specified digest.
@@ -172,24 +108,3 @@ type SignatureService interface {
 	// Put stores the signature for the provided digest.
 	Put(dgst digest.Digest, signatures ...[]byte) error
 }
-
-// Descriptor describes targeted content. Used in conjunction with a blob
-// store, a descriptor can be used to fetch, store and target any kind of
-// blob. The struct also describes the wire protocol format. Fields should
-// only be added but never changed.
-type Descriptor struct {
-	// MediaType describe the type of the content. All text based formats are
-	// encoded as utf-8.
-	MediaType string `json:"mediaType,omitempty"`
-
-	// Length in bytes of content.
-	Length int64 `json:"length,omitempty"`
-
-	// Digest uniquely identifies the content. A byte stream can be verified
-	// against against this digest.
-	Digest digest.Digest `json:"digest,omitempty"`
-
-	// NOTE: Before adding a field here, please ensure that all
-	// other options have been exhausted. Much of the type relationships
-	// depend on the simplicity of this type.
-}