feat(storage): add ComponentCount as part of ObjectAttrs

storage/integration_test.go

@@ -1662,6 +1662,9 @@ func TestIntegration_Compose(t *testing.T) {
 		if _, err := c.Run(ctx); err != nil {
 			t.Fatalf("ComposeFrom error: %v", err)
 		}
+		if c.ComponentCount != int64(len(objects)) {
+			t.Fatalf("ComponentCount of dst Object: %v is incorrect", c.ComponentCount)
+		}
 		checkCompose(compDst, "application/octet-stream")
 
 		// It should also work if we do.
@@ -1671,6 +1674,9 @@ func TestIntegration_Compose(t *testing.T) {
 		if _, err := c.Run(ctx); err != nil {
 			t.Fatalf("ComposeFrom error: %v", err)
 		}
+		if c.ComponentCount != int64(len(objects)) {
+			t.Fatalf("ComponentCount of dst Object: %v is incorrect", c.ComponentCount)
+		}
 		checkCompose(compDst, "text/json")
 	})
 }

storage/storage.go

@@ -1315,6 +1315,27 @@ type ObjectAttrs struct {
 	// later value but not to an earlier one. For more information see
 	// https://cloud.google.com/storage/docs/metadata#custom-time .
 	CustomTime time.Time
+
+	// As of 2015-06-03, the official GCS documentation for this
+	// property (https://goo.gl/GwD5Dq) says this:
+	//
+	//     Newly uploaded objects have a component count of 1, and composing a
+	//     sequence of objects creates an object whose component count is equal
+	//     to the sum of component counts in the sequence.
+	//
+	// However, in Google-internal bug 21572928 it was clarified that this
+	// doesn't match the actual implementation, which can be documented as:
+	//
+	//     Newly uploaded objects do not have a component count. Composing a
+	//     sequence of objects creates an object whose component count is equal
+	//     to the sum of the component counts of the objects in the sequence,
+	//     where objects that do not have a component count are treated as having
+	//     a component count of 1.
+	//
+	// This is a much less elegant and convenient rule, so this package emulates
+	// the officially documented behavior above. That is, it synthesizes a
+	// component count of 1 for objects that do not have a component count.
+	ComponentCount int64
 }
 
 // convertTime converts a time in RFC3339 format to time.Time.
@@ -1385,6 +1406,7 @@ func newObject(o *raw.Object) *ObjectAttrs {
 		Updated:                 convertTime(o.Updated),
 		Etag:                    o.Etag,
 		CustomTime:              convertTime(o.CustomTime),
+		ComponentCount:          o.ComponentCount,
 	}
 }
 
@@ -1419,6 +1441,7 @@ func newObjectFromProto(o *storagepb.Object) *ObjectAttrs {
 		Deleted:           convertProtoTime(o.GetDeleteTime()),
 		Updated:           convertProtoTime(o.GetUpdateTime()),
 		CustomTime:        convertProtoTime(o.GetCustomTime()),
+		ComponentCount:    int64(o.ComponentCount),
 	}
 }
 

storage/storage_test.go

@@ -1747,6 +1747,7 @@ func TestRawObjectToObjectAttrs(t *testing.T) {
 				TimeCreated:             "2019-03-31T19:32:10Z",
 				TimeDeleted:             "2019-03-31T19:33:39Z",
 				TemporaryHold:           true,
+				ComponentCount:          2,
 			},
 			want: &ObjectAttrs{
 				Bucket:                  "Test",
@@ -1763,6 +1764,7 @@ func TestRawObjectToObjectAttrs(t *testing.T) {
 				RetentionExpirationTime: time.Date(2019, 3, 31, 19, 33, 36, 0, time.UTC),
 				Size:                    1 << 20,
 				TemporaryHold:           true,
+				ComponentCount:          2,
 			},
 		},
 	}
@@ -1833,6 +1835,7 @@ func TestProtoObjectToObjectAttrs(t *testing.T) {
 				CreateTime:          timestamppb.New(now),
 				DeleteTime:          timestamppb.New(now),
 				TemporaryHold:       true,
+				ComponentCount:      2,
 			},
 			want: &ObjectAttrs{
 				Bucket:                  "Test",
@@ -1848,6 +1851,7 @@ func TestProtoObjectToObjectAttrs(t *testing.T) {
 				RetentionExpirationTime: now,
 				Size:                    1 << 20,
 				TemporaryHold:           true,
+				ComponentCount:          2,
 			},
 		},
 	}