formally require Coze to error on duplicates; change ErrJSONDuplicate…

… from variable into type; key unmarhal errors on duplicate

README.md

@@ -32,12 +32,12 @@ readability.
 4. Provide defined cipher suites.
 
 ### Coze Fields
-Coze JSON fields are case sensitive and unique.  Coze defines standard fields
-and applications may include additional fields as desired.  All fields are
+Coze defines standard fields and applications may include additional fields as
+desired.  Coze JSON fields are case sensitive and unique.  All fields are
 optional, but omitting standard fields may limit compatibility. Binary values
 are encoded as [RFC 4648 base 64 URI canonical with padding truncated][RFC4648]
 (b64ut). The Coze objects `pay`, `key`, and `coze` have respective standard
-fields.
+fields.  Unmarshalling JSON with duplicate fields must error.
 
 #### All Coze Standard Fields
 ![Coze Standard Fields](docs/img/coze_standard_fields.png)

coze.go

@@ -396,15 +396,14 @@ func checkDuplicate(d *json.Decoder) error {
 	case '{':
 		keys := make(map[string]bool)
 		for d.More() {
-			// Get field key.
-			t, err := d.Token()
+			t, err := d.Token() // Get field key.
 			if err != nil {
 				return err
 			}
 
 			key := t.(string)
 			if keys[key] { // Check for duplicates.
-				return ErrJSONDuplicate
+				return ErrJSONDuplicate(fmt.Errorf("Coze: JSON duplicate field %q", key))
 			}
 			keys[key] = true
 
@@ -433,7 +432,5 @@ func checkDuplicate(d *json.Decoder) error {
 	return nil
 }
 
-// ErrJSONDuplicate is for applications that need to check for the JSON
-// duplicate error.  Alternatively, applications need to check for the error
-// string, which may change.
-var ErrJSONDuplicate = errors.New("Coze: JSON duplicate field name")
+// ErrJSONDuplicate allows applications to check for JSON duplicate error.
+type ErrJSONDuplicate error

coze_test.go

@@ -159,12 +159,11 @@ func ExamplePay_String_custom() {
 func ExamplePay_UnmarshalJSON_duplicate() {
 	h := &Pay{}
 	msg := []byte(`{"alg":"ES256","alg":"ES384"}`)
-
 	err := json.Unmarshal(msg, h)
 	fmt.Println(err)
 
 	// Output:
-	// Coze: JSON duplicate field name
+	// Coze: JSON duplicate field "alg"
 }
 
 // Example demonstrating that unmarshalling a `coze` that has duplicate field
@@ -176,7 +175,7 @@ func ExampleCoze_UnmarshalJSON_duplicate() {
 	fmt.Println(err)
 
 	// Output:
-	// Coze: JSON duplicate field name
+	// Coze: JSON duplicate field "pay"
 }
 
 // ExampleCoze_embed demonstrates how to embed a JSON `coze` into a third party
@@ -429,25 +428,25 @@ func ExampleMarshal_jsonRawMessage() {
 }
 
 func Test_checkDuplicate(t *testing.T) {
-	// Duplicate, should error.
-	data := `{"a": "b", "a":true,"c":["field_3 string 1","field3 string2"], "d": {"e": 1, "e": 2}}`
+	// Happy path; no duplicate.  Should not error.
+	data := `{"a": "b", "c":"d", "d": {"e": 1, "f": 2}}`
 	err := checkDuplicate(json.NewDecoder(strings.NewReader(data)))
-	if err != ErrJSONDuplicate {
-		t.Fatal("Should have found duplicate.")
+	if err != nil {
+		t.Fatal(err)
 	}
 
-	// Recursive check with duplicate in inner struct.  Should error.
-	data = `{"a": "b", "c":"d", "d": {"e": 1, "e": 2}}`
+	// Duplicate, should error.
+	data = `{"a": "aValue", "a":true,"c":["field_3 string 1","field3 string2"], "d": {"e": 1, "e": 2}}`
 	err = checkDuplicate(json.NewDecoder(strings.NewReader(data)))
-	if err != ErrJSONDuplicate {
+	if err == nil {
 		t.Fatal("Should have found duplicate.")
 	}
 
-	// No duplicate.  Should not error.
-	data = `{"a": "b", "c":"d", "d": {"e": 1, "f": 2}}`
+	// Recursive check with duplicate in inner struct.  Should error.
+	data = `{"a": "aValue", "c":"cValue", "d": {"e": 1, "e": 2}}`
 	err = checkDuplicate(json.NewDecoder(strings.NewReader(data)))
-	if err != nil {
-		t.Fatal(err)
+	if err == nil {
+		t.Fatal("Recursive check should have found duplicate.")
 	}
 }
 

key.go

@@ -104,9 +104,14 @@ func Thumbprint(c *Key) (tmb B64, err error) {
 
 // UnmarshalJSON always populates `tmb` even if it isn't given.
 func (c *Key) UnmarshalJSON(b []byte) error {
+	err := checkDuplicate(json.NewDecoder(bytes.NewReader(b)))
+	if err != nil {
+		return err
+	}
+
 	type key2 Key // Break infinite unmarshal loop
 	czk2 := new(key2)
-	err := json.Unmarshal(b, czk2)
+	err = json.Unmarshal(b, czk2)
 	if err != nil {
 		return err
 	}

key_test.go

@@ -258,6 +258,18 @@ func ExampleKey_unmarshal() {
 	//  {"alg":"ES256","iat":1623132000,"kid":"Zami's Majuscule Key.","tmb":"cLj8vsYtMBwYkzoFVZHBZo6SNL8wSdCIjCKAwXNuhOk","x":"2nTOaFVm2QLxmUO_SjgyscVHBtvHEfo2rq65MvgNRjORojq39Haq9rXNxvXxwba_Xj0F5vZibJR3isBdOWbo5g"}
 }
 
+// Example demonstrating that unmarshalling a `pay` that has duplicate field
+// names results in an error.
+func ExampleKey_UnmarshalJSON_duplicate() {
+	k := &Key{}
+	msg := []byte(`{"alg":"ES256","alg":"ES256"}`)
+	err := json.Unmarshal(msg, k)
+	fmt.Println(err)
+
+	// Output:
+	// Coze: JSON duplicate field "alg"
+}
+
 func ExampleKey_SignCoze() {
 	cz := new(Coze)
 	cz.Pay = json.RawMessage(GoldenPay)