Fix infinite save loop when editing a lesson with a quiz block

assets/blocks/quiz/data.js

@@ -15,6 +15,49 @@ import categoryQuestionBlock from './category-question-block';
  */
 import { mapKeys, mapValues, isObject, omit } from 'lodash';
 
+/**
+ * Question attributes that are derived from the server and must not be sent
+ * back as editable structure.
+ *
+ * @type {string[]}
+ */
+export const READ_ONLY_ATTRIBUTES = [
+	'categories',
+	'shared',
+	'options.studentHelp',
+	'media',
+	'categoryName',
+	'lock',
+];
+
+/**
+ * Normalize a quiz REST response into the structure the editor tracks.
+ *
+ * Only the fields that make up the editor's structure are kept. Other plugins
+ * (e.g. Polylang) may inject extra top-level fields such as `lang` and
+ * `translations` into the REST response; including them here would make the
+ * structure comparison detect a permanent diff and trigger an infinite save
+ * loop.
+ *
+ * @param {Object} structure The quiz response.
+ *
+ * @return {Object} The normalized structure.
+ */
+export function normalizeServerStructure( structure ) {
+	if ( ! structure ) {
+		return {};
+	}
+
+	return {
+		lesson_status: structure.lesson_status,
+		lesson_title: structure.lesson_title,
+		options: structure.options,
+		questions: ( structure.questions || [] ).map( ( question ) =>
+			omit( question, READ_ONLY_ATTRIBUTES )
+		),
+	};
+}
+
 /**
  * Quiz settings and questions data.
  *

assets/blocks/quiz/data.test.js

@@ -0,0 +1,69 @@
+/**
+ * Internal dependencies
+ */
+import { normalizeServerStructure } from './data';
+
+describe( 'normalizeServerStructure', () => {
+	it( 'returns an empty object for a falsy response', () => {
+		expect( normalizeServerStructure( undefined ) ).toEqual( {} );
+		expect( normalizeServerStructure( null ) ).toEqual( {} );
+	} );
+
+	it( 'strips injected top-level fields (e.g. Polylang lang/translations)', () => {
+		const structure = {
+			lesson_status: 'publish',
+			lesson_title: 'Lesson',
+			options: { pass_required: true },
+			questions: [ { id: 1, title: 'Q1' } ],
+			lang: 'en',
+			translations: { en: 42 },
+		};
+
+		const normalized = normalizeServerStructure( structure );
+
+		expect( normalized ).not.toHaveProperty( 'lang' );
+		expect( normalized ).not.toHaveProperty( 'translations' );
+		expect( normalized ).toEqual( {
+			lesson_status: 'publish',
+			lesson_title: 'Lesson',
+			options: { pass_required: true },
+			questions: [ { id: 1, title: 'Q1' } ],
+		} );
+	} );
+
+	it( 'removes read-only attributes from questions', () => {
+		const structure = {
+			lesson_status: 'publish',
+			lesson_title: 'Lesson',
+			options: {},
+			questions: [
+				{
+					id: 1,
+					title: 'Q1',
+					shared: true,
+					categories: [ 5 ],
+					lock: true,
+					options: { grade: 1, studentHelp: 'help text' },
+				},
+			],
+		};
+
+		expect( normalizeServerStructure( structure ).questions[ 0 ] ).toEqual(
+			{
+				id: 1,
+				title: 'Q1',
+				options: { grade: 1 },
+			}
+		);
+	} );
+
+	it( 'defaults questions to an empty array when missing', () => {
+		const structure = {
+			lesson_status: 'publish',
+			lesson_title: 'Lesson',
+			options: {},
+		};
+
+		expect( normalizeServerStructure( structure ).questions ).toEqual( [] );
+	} );
+} );

assets/blocks/quiz/quiz-store.js

@@ -18,19 +18,12 @@ import {
 	parseQuestionBlocks,
 	syncQuestionBlocks,
 	normalizeAttributes,
+	normalizeServerStructure,
+	READ_ONLY_ATTRIBUTES,
 } from './data';
 
 export const QUIZ_STORE = 'sensei/quiz-structure';
 
-const READ_ONLY_ATTRIBUTES = [
-	'categories',
-	'shared',
-	'options.studentHelp',
-	'media',
-	'categoryName',
-	'lock',
-];
-
 /**
  * Syncronize this block with quiz data.
  *
@@ -204,16 +197,5 @@ registerStructureStore( {
 	 *
 	 * @return {Object} The modified response.
 	 */
-	setServerStructure( structure ) {
-		if ( ! structure ) {
-			return {};
-		}
-
-		return {
-			...structure,
-			questions: structure.questions.map( ( question ) =>
-				omit( question, READ_ONLY_ATTRIBUTES )
-			),
-		};
-	},
+	setServerStructure: normalizeServerStructure,
 } );

changelog/fix-quiz-save-loop-injected-rest-fields

@@ -0,0 +1,4 @@
+Significance: patch
+Type: fixed
+
+Fix an infinite save loop when editing a lesson with a quiz block while a plugin (e.g. Polylang) injects extra fields into the REST response.