fix(json): Ensure that JSON.GET returns Nil response (#3470)

* fix conflicts

* Fix JSON nil response handling

Ensure that JSON.GET returns redis.Nil for missing keys/paths,
making it consistent with other Redis commands.

- Restore proper nil detection logic in JSONCmd.readReply
- Add comprehensive test coverage for JSON nil scenarios
- Handle both non-existent keys and non-existent paths consistently
- Distinguish between empty arrays and nil responses
- Add documentation for Val() and Expanded() methods

Original work and problem identification by Nic Gibson.
Enhanced implementation with comprehensive testing and fixes
for the broken nil detection logic.

Fixes #2987

* Fix JSON nil response handling - align with Redis behavior

- Non-existent keys return redis.Nil (consistent with other Redis commands)
- Non-existent paths in existing keys return empty array '[]'
- Fix broken test that was using wrong doc1 reference
- Add comprehensive test coverage for JSON nil scenarios

This aligns with official Redis JSON.GET behavior:
- Missing keys should return nil error like other Redis commands
- Missing paths should return empty JSON array, not error

* Fix JSONDel tests

---------

Co-authored-by: Nic Gibson <nic.gibson@redis.com>
Co-authored-by: Nedyalko Dyakov <1547186+ndyakov@users.noreply.github.com>

json.go

@@ -82,6 +82,7 @@ func (cmd *JSONCmd) SetVal(val string) {
 	cmd.val = val
 }
 
+// Val returns the result of the JSON.GET command as a string.
 func (cmd *JSONCmd) Val() string {
 	if len(cmd.val) == 0 && cmd.expanded != nil {
 		val, err := json.Marshal(cmd.expanded)
@@ -100,6 +101,7 @@ func (cmd *JSONCmd) Result() (string, error) {
 	return cmd.Val(), cmd.Err()
 }
 
+// Expanded returns the result of the JSON.GET command as unmarshalled JSON.
 func (cmd *JSONCmd) Expanded() (interface{}, error) {
 	if len(cmd.val) != 0 && cmd.expanded == nil {
 		err := json.Unmarshal([]byte(cmd.val), &cmd.expanded)
@@ -113,11 +115,17 @@ func (cmd *JSONCmd) Expanded() (interface{}, error) {
 
 func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 	// nil response from JSON.(M)GET (cmd.baseCmd.err will be "redis: nil")
+	// This happens when the key doesn't exist
 	if cmd.baseCmd.Err() == Nil {
 		cmd.val = ""
 		return Nil
 	}
 
+	// Handle other base command errors
+	if cmd.baseCmd.Err() != nil {
+		return cmd.baseCmd.Err()
+	}
+
 	if readType, err := rd.PeekReplyType(); err != nil {
 		return err
 	} else if readType == proto.RespArray {
@@ -127,6 +135,13 @@ func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 			return err
 		}
 
+		// Empty array means no results found for JSON path, but key exists
+		// This should return "[]", not an error
+		if size == 0 {
+			cmd.val = "[]"
+			return nil
+		}
+
 		expanded := make([]interface{}, size)
 
 		for i := 0; i < size; i++ {
@@ -141,6 +156,7 @@ func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 			return err
 		} else if str == "" || err == Nil {
 			cmd.val = ""
+			return Nil
 		} else {
 			cmd.val = str
 		}

json_test.go

@@ -142,6 +142,10 @@ var _ = Describe("JSON Commands", Label("json"), func() {
 				resArr, err := client.JSONArrIndex(ctx, "doc1", "$.store.book[?(@.price<10)].size", 20).Result()
 				Expect(err).NotTo(HaveOccurred())
 				Expect(resArr).To(Equal([]int64{1, 2}))
+
+				_, err = client.JSONGet(ctx, "this-key-does-not-exist", "$").Result()
+				Expect(err).To(HaveOccurred())
+				Expect(err).To(BeIdenticalTo(redis.Nil))
 			})
 
 			It("should JSONArrInsert", Label("json.arrinsert", "json"), func() {
@@ -402,8 +406,8 @@ var _ = Describe("JSON Commands", Label("json"), func() {
 				Expect(cmd2.Val()).To(Equal(int64(1)))
 
 				cmd3 := client.JSONGet(ctx, "del1", "$")
-				Expect(cmd3.Err()).NotTo(HaveOccurred())
-				Expect(cmd3.Val()).To(HaveLen(0))
+				Expect(cmd3.Err()).To(Equal(redis.Nil))
+				Expect(cmd3.Val()).To(Equal(""))
 			})
 
 			It("should JSONDel with $", Label("json.del", "json"), func() {
@@ -673,6 +677,58 @@ var _ = Describe("JSON Commands", Label("json"), func() {
 				Expect(cmd2.Val()[0]).To(Or(Equal([]interface{}{"boolean"}), Equal("boolean")))
 			})
 		})
+
+		Describe("JSON Nil Handling", func() {
+			It("should return redis.Nil for non-existent key", func() {
+				_, err := client.JSONGet(ctx, "non-existent-key", "$").Result()
+				Expect(err).To(Equal(redis.Nil))
+			})
+
+			It("should return empty array for non-existent path in existing key", func() {
+				err := client.JSONSet(ctx, "test-key", "$", `{"a": 1, "b": "hello"}`).Err()
+				Expect(err).NotTo(HaveOccurred())
+
+				// Non-existent path should return empty array, not error
+				val, err := client.JSONGet(ctx, "test-key", "$.nonexistent").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(val).To(Equal("[]"))
+			})
+
+			It("should distinguish empty array from non-existent path", func() {
+				err := client.JSONSet(ctx, "test-key", "$", `{"arr": [], "obj": {}}`).Err()
+				Expect(err).NotTo(HaveOccurred())
+
+				// Empty array should return the array
+				val, err := client.JSONGet(ctx, "test-key", "$.arr").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(val).To(Equal("[[]]"))
+
+				// Non-existent field should return empty array
+				val, err = client.JSONGet(ctx, "test-key", "$.missing").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(val).To(Equal("[]"))
+			})
+
+			It("should handle multiple paths with mixed results", func() {
+				err := client.JSONSet(ctx, "test-key", "$", `{"a": 1, "b": 2}`).Err()
+				Expect(err).NotTo(HaveOccurred())
+
+				// Path that exists
+				val, err := client.JSONGet(ctx, "test-key", "$.a").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(val).To(Equal("[1]"))
+
+				// Path that doesn't exist should return empty array
+				val, err = client.JSONGet(ctx, "test-key", "$.c").Result()
+				Expect(err).NotTo(HaveOccurred())
+				Expect(val).To(Equal("[]"))
+			})
+
+			AfterEach(func() {
+				// Clean up test keys
+				client.Del(ctx, "test-key", "non-existent-key")
+			})
+		})
 	}
 })