metadata: define content sharing policy

This changeset modifies the metadata store to allow one to set a
"content sharing policy" that defines how blobs are shared between
namespaces in the content store.

The default mode "shared" will make blobs available in all namespaces
once it is pulled into any namespace.  The blob will be pulled into
the namespace if a writer is opened with the "Expected" digest that
is already present in the backend.

The alternative mode, "isolated" requires that clients prove they have
access to the content by providing all of the content to the ingest
before the blob is added to the namespace.

Both modes share backing data, while "shared" will reduce total
bandwidth across namespaces, at the cost of allowing access to any
blob just by knowing its digest.

Note: Most functional codes and changelog of this commit originate from
Stephen J Day <stephen.day@docker.com>, see
40455aade8

Fixes #1713 Fixes #2865

Signed-off-by: Eric Lin <linxiulei@gmail.com>

metadata/db.go

@@ -46,6 +46,19 @@ const (
 	dbVersion = 3
 )
 
+// DBOpt configures how we set up the DB
+type DBOpt func(*dbOptions)
+
+// WithPolicyIsolated isolates contents between namespaces
+func WithPolicyIsolated(o *dbOptions) {
+	o.shared = false
+}
+
+// dbOptions configure db options.
+type dbOptions struct {
+	shared bool
+}
+
 // DB represents a metadata database backed by a bolt
 // database. The database is fully namespaced and stores
 // image, container, namespace, snapshot, and content data
@@ -72,19 +85,28 @@ type DB struct {
 	// mutationCallbacks are called after each mutation with the flag
 	// set indicating whether any dirty flags are set
 	mutationCallbacks []func(bool)
+
+	dbopts dbOptions
 }
 
 // NewDB creates a new metadata database using the provided
 // bolt database, content store, and snapshotters.
-func NewDB(db *bolt.DB, cs content.Store, ss map[string]snapshots.Snapshotter) *DB {
+func NewDB(db *bolt.DB, cs content.Store, ss map[string]snapshots.Snapshotter, opts ...DBOpt) *DB {
 	m := &DB{
 		db:      db,
 		ss:      make(map[string]*snapshotter, len(ss)),
 		dirtySS: map[string]struct{}{},
+		dbopts: dbOptions{
+			shared: true,
+		},
+	}
+
+	for _, opt := range opts {
+		opt(&m.dbopts)
 	}
 
 	// Initialize data stores
-	m.cs = newContentStore(m, cs)
+	m.cs = newContentStore(m, m.dbopts.shared, cs)
 	for name, sn := range ss {
 		m.ss[name] = newSnapshotter(m, name, sn)
 	}