Treat sandboxes as root gc resources and scan referenced objects

Signed-off-by: Henry Wang <henwang@amazon.com>

metadata/gc.go

@@ -430,6 +430,20 @@ func (c *gcContext) scanRoots(ctx context.Context, tx *bolt.Tx, nc chan<- gc.Nod
 			}
 		}
 
+		bbkt := nbkt.Bucket(bucketKeyObjectSandboxes)
+		if bbkt != nil {
+			if err := bbkt.ForEach(func(k, v []byte) error {
+				if v != nil {
+					return nil
+				}
+
+				sbbkt := bbkt.Bucket(k)
+				return c.sendLabelRefs(ns, sbbkt, fn)
+			}); err != nil {
+				return err
+			}
+		}
+
 		c.active(ns, fn)
 	}
 	return cerr

metadata/gc_test.go

@@ -101,6 +101,9 @@ func TestGCRoots(t *testing.T) {
 		addLeaseContent("ns3", "l1", dgst(1)),
 		addLeaseSnapshot("ns3", "l1", "overlay", "sn1"),
 		addLeaseIngest("ns3", "l1", "ingest-1"),
+
+		addSandbox("ns3", "sandbox1", nil),
+		addSandbox("ns4", "sandbox1", labelmap(string(labelGCSnapRef)+"overlay", "sn1")),
 	}
 
 	expected := []gc.Node{
@@ -136,6 +139,7 @@ func TestGCRoots(t *testing.T) {
 		gcnode(ResourceIngest, "ns3", "ingest-1"),
 		gcnode(resourceContentFlat, "ns3", dgst(1).String()),
 		gcnode(resourceSnapshotFlat, "ns3", "overlay/sn1"),
+		gcnode(ResourceSnapshot, "ns4", "overlay/sn1"),
 	}
 
 	if err := db.Update(func(tx *bolt.Tx) error {
@@ -760,6 +764,16 @@ func addContainer(ns, name, snapshotter, snapshot string, labels map[string]stri
 	}
 }
 
+func addSandbox(ns, name string, labels map[string]string) alterFunc {
+	return func(bkt *bolt.Bucket) error {
+		sbkt, err := createBuckets(bkt, ns, string(bucketKeyObjectSandboxes), name)
+		if err != nil {
+			return err
+		}
+		return boltutil.WriteLabels(sbkt, labels)
+	}
+}
+
 func createBuckets(bkt *bolt.Bucket, names ...string) (*bolt.Bucket, error) {
 	for _, name := range names {
 		nbkt, err := bkt.CreateBucketIfNotExists([]byte(name))