Widgets: Preserve classic sidebars when switching to a block theme.

When switching to a block theme, classic sidebars were orphaned and their widgets remapping to the `'wp_inactive_widgets'` sidebar . This changeset preserves the sidebars and their widgets, providing a migration path to a block theme without losing the widgets.

Classic sidebars are now:

* Stored in a new theme mod called `'wp_classic_sidebars'`;
* Restored to the `$wp_registered_sidebars` global variable when the `'widgets_init'` action fires (via a new internal function called `_wp_block_theme_register_classic_sidebars()`);
* And marked as `'inactive'` when interacting with sidebars REST API endpoint.

References:
* [https://github.com/WordPress/gutenberg/pull/45509 Gutenberg PR 45509] which adds an option for importing widgets from sidebars into template parts.

Follow-up to [50995], [6334].

Props mamaduka, audrasjb, hellofromTonya, ironprogrammer, jameskoster, joen, matveb, mukesh27, noisysocks, poena, youknowriad.
Fixes #57531.

git-svn-id: https://develop.svn.wordpress.org/trunk@55200 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/rest-api/rest-sidebars-controller.php

@@ -860,6 +860,73 @@ class WP_Test_REST_Sidebars_Controller extends WP_Test_REST_Controller_Testcase
 		);
 	}
 
+	/**
+	 * @ticket 57531
+	 * @covers WP_Test_REST_Sidebars_Controller::prepare_item_for_response
+	 */
+	public function test_prepare_item_for_response_to_set_inactive_on_theme_switch() {
+		$request = new WP_REST_Request( 'GET', '/wp/v2/sidebars/sidebar-1' );
+
+		// Set up the test.
+		wp_widgets_init();
+		$this->setup_widget(
+			'widget_rss',
+			1,
+			array(
+				'title' => 'RSS test',
+			)
+		);
+		$this->setup_widget(
+			'widget_text',
+			1,
+			array(
+				'text' => 'Custom text test',
+			)
+		);
+		$this->setup_sidebar(
+			'sidebar-1',
+			array(
+				'name' => 'Sidebar 1',
+			),
+			array( 'text-1', 'rss-1' )
+		);
+
+		// Validate the state before a theme switch.
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$data     = $this->remove_links( $data );
+
+		$this->assertSame( 'active', $data['status'] );
+		$this->assertFalse(
+			get_theme_mod( 'wp_classic_sidebars' ),
+			'wp_classic_sidebars theme mod should not exist before switching to block theme'
+		);
+
+		switch_theme( 'block-theme' );
+		wp_widgets_init();
+
+		// Validate the state after a theme switch.
+		$response = rest_get_server()->dispatch( $request );
+		$data     = $response->get_data();
+		$data     = $this->remove_links( $data );
+
+		$this->assertSame(
+			'inactive',
+			$data['status'],
+			'Sidebar status should have changed to inactive'
+		);
+		$this->assertSame(
+			array( 'text-1', 'rss-1' ),
+			$data['widgets'],
+			'The text and rss widgets should still in sidebar-1'
+		);
+		$this->assertArrayHasKey(
+			'sidebar-1',
+			get_theme_mod( 'wp_classic_sidebars' ),
+			'sidebar-1 should be in "wp_classic_sidebars" theme mod'
+		);
+	}
+
 	/**
 	 * @ticket 41683
 	 */

tests/phpunit/tests/widgets/wpBlockThemeRegisterClassicSidebar.php

@@ -0,0 +1,89 @@
+<?php
+/**
+ * Tests for _wp_block_theme_register_classic_sidebars().
+ *
+ * @group widgets
+ * @covers ::_wp_block_theme_register_classic_sidebars
+ */
+class Tests_Widgets_WpBlockThemeRegisterClassicSidebars extends WP_UnitTestCase {
+	/**
+	 * Original global $wp_registered_sidebars.
+	 *
+	 * @var array
+	 */
+	private static $wp_registered_sidebars;
+
+	public static function set_up_before_class() {
+		global $wp_registered_sidebars;
+		parent::set_up_before_class();
+
+		// Store the original global before running tests.
+		static::$wp_registered_sidebars = $wp_registered_sidebars;
+	}
+
+	public function tear_down() {
+		// Restore the global after each test.
+		global $wp_registered_sidebars;
+		$wp_registered_sidebars = static::$wp_registered_sidebars;
+
+		parent::tear_down();
+	}
+
+	public function test_a_sidebar_should_be_registered() {
+		global $wp_registered_sidebars;
+
+		$sidebar_id = array_key_first( $wp_registered_sidebars );
+		$this->assertNotEmpty( $sidebar_id );
+	}
+
+	/**
+	 * @ticket 57531
+	 */
+	public function test_should_reregister_previous_theme_sidebar() {
+		global $wp_registered_sidebars;
+
+		$sidebar_id = array_key_first( $wp_registered_sidebars );
+
+		switch_theme( 'block-theme' );
+		unregister_sidebar( $sidebar_id );
+
+		// Test before.
+		$this->assertArrayNotHasKey(
+			$sidebar_id,
+			$wp_registered_sidebars,
+			'Sidebar should not be in registered sidebars after unregister'
+		);
+
+		_wp_block_theme_register_classic_sidebars();
+
+		// Test after.
+		$this->assertArrayHasKey(
+			$sidebar_id,
+			$wp_registered_sidebars,
+			'Sidebar should be in registered sidebars after invoking _wp_block_theme_register_classic_sidebars()'
+		);
+	}
+
+	/**
+	 * @ticket 57531
+	 */
+	public function test_should_bail_out_when_theme_mod_is_empty() {
+		global $wp_registered_sidebars;
+
+		// Test state before invoking.
+		$this->assertFalse(
+			get_theme_mod( 'wp_classic_sidebars' ),
+			'Theme mod should not be set before invoking _wp_block_theme_register_classic_sidebars()'
+		);
+
+		$before = $wp_registered_sidebars;
+		_wp_block_theme_register_classic_sidebars();
+
+		// Test state after invoking.
+		$this->assertSameSetsWithIndex(
+			$before,
+			$wp_registered_sidebars,
+			'No change should happen after invoking _wp_block_theme_register_classic_sidebars()'
+		);
+	}
+}