Editor: Make sure resetEditorBlocks is synchronous. (#21839)

* Data Controls: Add a synchronous `select` control.

* Editor: Use `syncSelect` in `resetEditorBlocks`.

* Data Controls: Make `syncSelect` "__unstable".

packages/data-controls/src/index.js

@@ -63,6 +63,38 @@ export function select( storeKey, selectorName, ...args ) {
 	};
 }
 
+/**
+ * Dispatches a control action for triggering a registry select.
+ *
+ * Note: This functions like the `select` control, but does not wait
+ * for resolvers.
+ *
+ * @param {string} storeKey     The key for the store the selector belongs to.
+ * @param {string} selectorName The name of the selector.
+ * @param {Array}  args         Arguments for the select.
+ *
+ * @example
+ * ```js
+ * import { __unstableSyncSelect } from '@wordpress/data-controls';
+ *
+ * // Action generator using `__unstableSyncSelect`.
+ * export function* myAction() {
+ * 	const isEditorSideBarOpened = yield __unstableSyncSelect( 'core/edit-post', 'isEditorSideBarOpened' );
+ * 	// Do stuff with the result from the `__unstableSyncSelect`.
+ * }
+ * ```
+ *
+ * @return {Object} The control descriptor.
+ */
+export function __unstableSyncSelect( storeKey, selectorName, ...args ) {
+	return {
+		type: 'SYNC_SELECT',
+		storeKey,
+		selectorName,
+		args,
+	};
+}
+
 /**
  * Dispatches a control action for triggering a registry dispatch.
  *
@@ -133,6 +165,11 @@ export const controls = {
 			]( storeKey )[ selectorName ]( ...args );
 		}
 	),
+	SYNC_SELECT: createRegistryControl(
+		( registry ) => ( { storeKey, selectorName, args } ) => {
+			return registry.select( storeKey )[ selectorName ]( ...args );
+		}
+	),
 	DISPATCH: createRegistryControl(
 		( registry ) => ( { storeKey, actionName, args } ) => {
 			return registry.dispatch( storeKey )[ actionName ]( ...args );

packages/editor/src/store/actions.js

@@ -7,7 +7,12 @@ import { has, castArray } from 'lodash';
  * WordPress dependencies
  */
 import deprecated from '@wordpress/deprecated';
-import { dispatch, select, apiFetch } from '@wordpress/data-controls';
+import {
+	dispatch,
+	select,
+	__unstableSyncSelect,
+	apiFetch,
+} from '@wordpress/data-controls';
 import { parse, synchronizeBlocksWithTemplate } from '@wordpress/blocks';
 
 /**
@@ -672,7 +677,7 @@ export function* resetEditorBlocks( blocks, options = {} ) {
 	if ( __unstableShouldCreateUndoLevel !== false ) {
 		const { id, type } = yield select( STORE_KEY, 'getCurrentPost' );
 		const noChange =
-			( yield select(
+			( yield __unstableSyncSelect(
 				'core',
 				'getEditedEntityRecord',
 				'postType',