Prohibit accidental renames of product config json fields

Can be caused by `json:"omitempty"` instead of `json:",omitempty"`

Bug: 69076024
Test: m -j # which runs unit tests
Change-Id: I92e3193d00a740c72d36a56748e0b0a8ad1d772e

Android.bp

@@ -64,6 +64,7 @@ bootstrap_go_package {
         "android/env.go",
     ],
     testSrcs: [
+        "android/config_test.go",
         "android/expand_test.go",
         "android/paths_test.go",
         "android/prebuilt_test.go",

android/config_test.go

@@ -0,0 +1,86 @@
+// Copyright 2017 Google Inc. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package android
+
+import (
+	"fmt"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func validateConfigAnnotations(configurable jsonConfigurable) (err error) {
+	reflectType := reflect.TypeOf(configurable)
+	reflectType = reflectType.Elem()
+	for i := 0; i < reflectType.NumField(); i++ {
+		field := reflectType.Field(i)
+		jsonTag := field.Tag.Get("json")
+		// Check for mistakes in the json tag
+		if jsonTag != "" && !strings.HasPrefix(jsonTag, ",") {
+			if !strings.Contains(jsonTag, ",") {
+				// Probably an accidental rename, most likely "omitempty" instead of ",omitempty"
+				return fmt.Errorf("Field %s.%s has tag %s which specifies to change its json field name to %q.\n"+
+					"Did you mean to use an annotation of %q?\n"+
+					"(Alternatively, to change the json name of the field, rename the field in source instead.)",
+					reflectType.Name(), field.Name, field.Tag, jsonTag, ","+jsonTag)
+			} else {
+				// Although this rename was probably intentional,
+				// a json annotation is still more confusing than renaming the source variable
+				requestedName := strings.Split(jsonTag, ",")[0]
+				return fmt.Errorf("Field %s.%s has tag %s which specifies to change its json field name to %q.\n"+
+					"To change the json name of the field, rename the field in source instead.",
+					reflectType.Name(), field.Name, field.Tag, requestedName)
+
+			}
+		}
+	}
+	return nil
+}
+
+type configType struct {
+	populateMe *bool `json:"omitempty"`
+}
+
+func (c *configType) SetDefaultConfig() {
+}
+
+// tests that ValidateConfigAnnotation works
+func TestValidateConfigAnnotations(t *testing.T) {
+	config := configType{}
+	err := validateConfigAnnotations(&config)
+	expectedError := `Field configType.populateMe has tag json:"omitempty" which specifies to change its json field name to "omitempty".
+Did you mean to use an annotation of ",omitempty"?
+(Alternatively, to change the json name of the field, rename the field in source instead.)`
+	if err.Error() != expectedError {
+		t.Errorf("Incorrect error; expected:\n"+
+			"%s\n"+
+			"got:\n"+
+			"%s",
+			expectedError, err.Error())
+	}
+}
+
+// run validateConfigAnnotations against each type that might have json annotations
+func TestProductConfigAnnotations(t *testing.T) {
+	err := validateConfigAnnotations(&productVariables{})
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+
+	validateConfigAnnotations(&FileConfigurableOptions{})
+	if err != nil {
+		t.Errorf(err.Error())
+	}
+}