Add collectible resources to metadata gc

Adds a registration function to metadata which allows plugins to
register resources to be garbage collected. These resources allow
defining resources types which are ephemeral and stored outside the
metadata plugin without extending it. The garbage collection of these
resources will not fail the metadata gc process if their removal fails.
These resources may be referenced by existing metadata store resources
but may not be used to reference metadata store resources for the purpose
of preventing garbage collection.

Signed-off-by: Derek McGowan <derek@mcg.dev>

metadata/db_test.go

@@ -406,9 +406,45 @@ func TestMetadataCollector(t *testing.T) {
 					Type: "snapshots/native",
 				},
 			}, false, "containerd.io/gc.flat", time.Now().String()),
+
+			// Test Collectible Resource
+			blob(bytesFor(11), false, "containerd.io/gc.ref.test", "test1"),
+			blob(bytesFor(12), true, "containerd.io/gc.ref.test", "test2"),
+			lease("lease-3", []leases.Resource{
+				{
+					ID:   digestFor(11).String(),
+					Type: "content",
+				},
+			}, false),
+		}
+
+		testResource = gc.ResourceType(0x10)
+
+		remaining = []gc.Node{
+			gcnode(testResource, "test", "test1"),
+			gcnode(testResource, "test", "test3"),
+			gcnode(testResource, "test", "test4"),
+		}
+
+		collector = &testCollector{
+			all: []gc.Node{
+				gcnode(testResource, "random", "test1"),
+				gcnode(testResource, "test", "test1"),
+				gcnode(testResource, "test", "test2"),
+				gcnode(testResource, "test", "test3"),
+				gcnode(testResource, "test", "test4"),
+			},
+			active: []gc.Node{
+				gcnode(testResource, "test", "test4"),
+			},
+			leased: map[string][]gc.Node{
+				"lease-3": {
+					gcnode(testResource, "test", "test3"),
+				},
+			},
 		}
-		remaining []gc.Node
 	)
+	mdb.RegisterCollectibleResource(testResource, collector)
 
 	if err := mdb.Update(func(tx *bolt.Tx) error {
 		for _, obj := range objects {
@@ -436,7 +472,8 @@ func TestMetadataCollector(t *testing.T) {
 			actual = append(actual, node)
 			return nil
 		}
-		return scanAll(ctx, tx, scanFn)
+		cc := startGCContext(ctx, mdb.collectors)
+		return cc.scanAll(ctx, tx, scanFn)
 	}); err != nil {
 		t.Fatal(err)
 	}