xinfo-groups: support nil lag in XINFO GROUPS

command.go

@@ -2189,6 +2189,8 @@ func (cmd *XInfoGroupsCmd) readReply(rd *proto.Reader) error {
 				// to the group's consumers, or a NULL(Nil) when that number can't be determined.
 				if err != nil && err != Nil {
 					return err
+				} else if err == Nil {
+					group.Lag = -1
 				}
 			default:
 				return fmt.Errorf("redis: unexpected key %q in XINFO GROUPS reply", key)

commands_test.go

@@ -6772,6 +6772,36 @@ var _ = Describe("Commands", func() {
 				}))
 			})
 
+			It("should return -1 for nil lag in XINFO GROUPS", func() {
+				_, err := client.XAdd(ctx, &redis.XAddArgs{Stream: "s", ID: "0-1", Values: []string{"foo", "1"}}).Result()
+				Expect(err).NotTo(HaveOccurred())
+
+				client.XAdd(ctx, &redis.XAddArgs{Stream: "s", ID: "0-2", Values: []string{"foo", "2"}})
+				Expect(err).NotTo(HaveOccurred())
+				client.XAdd(ctx, &redis.XAddArgs{Stream: "s", ID: "0-3", Values: []string{"foo", "3"}})
+				Expect(err).NotTo(HaveOccurred())
+
+				err = client.XGroupCreate(ctx, "s", "g", "0").Err()
+				Expect(err).NotTo(HaveOccurred())
+				err = client.XReadGroup(ctx, &redis.XReadGroupArgs{Group: "g", Consumer: "c", Streams: []string{"s", ">"}, Count: 1, Block: -1, NoAck: false}).Err()
+				Expect(err).NotTo(HaveOccurred())
+
+				client.XDel(ctx, "s", "0-2")
+
+				res, err := client.XInfoGroups(ctx, "s").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(res).To(Equal([]redis.XInfoGroup{
+					{
+						Name:            "g",
+						Consumers:       1,
+						Pending:         1,
+						LastDeliveredID: "0-1",
+						EntriesRead:     1,
+						Lag:             -1, // nil lag from Redis is reported as -1
+					},
+				}))
+			})
+
 			It("should XINFO CONSUMERS", func() {
 				res, err := client.XInfoConsumers(ctx, "stream", "group1").Result()
 				Expect(err).NotTo(HaveOccurred())