Add shared content label to namespaces

Adds shared content labels to namespaces allowing content to be shared
between namespaces if that namespace is specifically tagged as being
sharable by adding the `containerd.io/namespace/sharable` label to the
namespace.

Signed-off-by: Cody Roseborough <cdr@amazon.com>

metadata/buckets.go

@@ -15,7 +15,7 @@
 */
 
 // Package metadata stores all labels and object specific metadata by namespace.
-// This package also contains the main garbage collection logic  for cleaning up
+// This package also contains the main garbage collection logic for cleaning up
 // resources consistently and atomically. Resources used by backends will be
 // tracked in the metadata store to be exposed to consumers of this package.
 //
@@ -115,6 +115,7 @@
 package metadata
 
 import (
+	"github.com/containerd/containerd/labels"
 	digest "github.com/opencontainers/go-digest"
 	bolt "go.etcd.io/bbolt"
 )
@@ -182,6 +183,45 @@ func createBucketIfNotExists(tx *bolt.Tx, keys ...[]byte) (*bolt.Bucket, error)
 	return bkt, nil
 }
 
+func namespacesBucketPath() []byte {
+	return bucketKeyVersion
+}
+
+func getNamespacesBucket(tx *bolt.Tx) *bolt.Bucket {
+	return getBucket(tx, namespacesBucketPath())
+}
+
+// Given a namespace string and a bolt transaction
+// return true if the ns has the shared label in it.
+func hasSharedLabel(tx *bolt.Tx, ns string) bool {
+	labelsBkt := getNamespaceLabelsBucket(tx, ns)
+	if labelsBkt == nil {
+		return false
+	}
+	cur := labelsBkt.Cursor()
+	for k, v := cur.First(); k != nil; k, v = cur.Next() {
+		if string(k) == labels.LabelSharedNamespace && string(v) == "true" {
+			return true
+		}
+	}
+	return false
+}
+
+func getShareableBucket(tx *bolt.Tx, dgst digest.Digest) *bolt.Bucket {
+	var bkt *bolt.Bucket
+	nsbkt := getNamespacesBucket(tx)
+	cur := nsbkt.Cursor()
+	for k, _ := cur.First(); k != nil; k, _ = cur.Next() {
+		// If this bucket has shared label
+		// get the bucket and return it.
+		if hasSharedLabel(tx, string(k)) {
+			bkt = getBlobBucket(tx, string(k), dgst)
+			break
+		}
+	}
+	return bkt
+}
+
 func namespaceLabelsBucketPath(namespace string) [][]byte {
 	return [][]byte{bucketKeyVersion, []byte(namespace), bucketKeyObjectLabels}
 }