Enforce non-empty constraints for Site Title and Network Title across all contexts

src/wp-admin/css/forms.css

@@ -1222,6 +1222,10 @@ table.form-table td .updated p {
   20.0 - Settings
 ------------------------------------------------------------------------------*/
 
+.form-field #blogname {
+	max-width: 25em;
+}
+
 .timezone-info code {
 	white-space: nowrap;
 }
@@ -1317,7 +1321,8 @@ table.form-table td .updated p {
 .form-field #admin-email,
 .form-field #path,
 .form-field #blog_registered,
-.form-field #blog_last_updated {
+.form-field #blog_last_updated,
+.form-field #site_name {
 	max-width: 25em;
 }
 

src/wp-admin/network/settings.php

@@ -116,6 +116,17 @@
 		}
 	}
 
+	// Ensure the network title (site_name) is not left empty.
+	$network_title_error = '';
+	if ( isset( $_POST['site_name'] ) ) {
+		$site_name = wp_unslash( $_POST['site_name'] );
+
+		if ( ! is_scalar( $site_name ) || '' === trim( (string) $site_name ) ) {
+			unset( $_POST['site_name'] );
+			$network_title_error = __( 'The network title cannot be empty. Please enter a title for your network.' );
+		}
+	}
+
 	foreach ( $options as $option_name ) {
 		if ( ! isset( $_POST[ $option_name ] ) ) {
 			continue;
@@ -131,21 +142,42 @@
 	 */
 	do_action( 'update_wpmu_options' );
 
-	wp_redirect( add_query_arg( 'updated', 'true', network_admin_url( 'settings.php' ) ) );
+	if ( $network_title_error ) {
+		set_transient( 'network_settings_errors', array( $network_title_error ), 30 );
+		wp_redirect( add_query_arg( 'updated', 'error', network_admin_url( 'settings.php' ) ) );
+	} else {
+		wp_redirect( add_query_arg( 'updated', 'true', network_admin_url( 'settings.php' ) ) );
+	}
 	exit;
 }
 
 require_once ABSPATH . 'wp-admin/admin-header.php';
 
 if ( isset( $_GET['updated'] ) ) {
-	wp_admin_notice(
-		__( 'Settings saved.' ),
-		array(
-			'type'        => 'success',
-			'dismissible' => true,
-			'id'          => 'message',
-		)
-	);
+	if ( 'true' === $_GET['updated'] ) {
+		wp_admin_notice(
+			__( 'Settings saved.' ),
+			array(
+				'type'        => 'success',
+				'dismissible' => true,
+				'id'          => 'message',
+			)
+		);
+	} elseif ( 'error' === $_GET['updated'] ) {
+		$network_settings_errors = get_transient( 'network_settings_errors' );
+		delete_transient( 'network_settings_errors' );
+
+		$error_messages = is_array( $network_settings_errors ) ? $network_settings_errors : array( __( 'Some settings could not be saved.' ) );
+		foreach ( $error_messages as $error_message ) {
+			wp_admin_notice(
+				$error_message,
+				array(
+					'type'        => 'error',
+					'dismissible' => true,
+				)
+			);
+		}
+	}
 }
 ?>
 
@@ -155,10 +187,10 @@
 		<?php wp_nonce_field( 'siteoptions' ); ?>
 		<h2 id="wp-settings-section-operational-settings"><?php _e( 'Operational Settings' ); ?></h2>
 		<table class="form-table" role="presentation">
-			<tr>
+			<tr class="form-field form-required">
 				<th scope="row"><label for="site_name"><?php _e( 'Network Title' ); ?></label></th>
 				<td>
-					<input name="site_name" type="text" id="site_name" class="regular-text" value="<?php echo esc_attr( get_network()->site_name ); ?>" />
+					<input name="site_name" type="text" id="site_name" class="regular-text" value="<?php echo esc_attr( get_network()->site_name ); ?>" required aria-required="true" />
 				</td>
 			</tr>
 

src/wp-admin/network/site-settings.php

@@ -60,10 +60,25 @@
 	do_action( 'wpmu_update_blog_options', $id );
 
 	restore_current_blog();
+
+	$update = 'updated';
+
+	/*
+	 * sanitize_option() rejects invalid values (such as an empty, required
+	 * Site Title) by registering a settings error and keeping the stored value.
+	 * Carry those errors across the redirect so an error notice can be shown
+	 * instead of a success message.
+	 */
+	$settings_errors = get_settings_errors();
+	if ( ! empty( $settings_errors ) ) {
+		set_transient( 'settings_errors', $settings_errors, 30 );
+		$update = 'not-updated';
+	}
+
 	wp_redirect(
 		add_query_arg(
 			array(
-				'update' => 'updated',
+				'update' => $update,
 				'id'     => $id,
 			),
 			'site-settings.php'
@@ -72,10 +87,25 @@
 	exit;
 }
 
+$messages       = array();
+$error_messages = array();
+
 if ( isset( $_GET['update'] ) ) {
-	$messages = array();
 	if ( 'updated' === $_GET['update'] ) {
 		$messages[] = __( 'Site options updated.' );
+	} elseif ( 'not-updated' === $_GET['update'] ) {
+		$settings_errors = get_transient( 'settings_errors' );
+		delete_transient( 'settings_errors' );
+
+		if ( is_array( $settings_errors ) ) {
+			foreach ( $settings_errors as $settings_error ) {
+				$error_messages[] = $settings_error['message'];
+			}
+		}
+
+		if ( empty( $error_messages ) ) {
+			$error_messages[] = __( 'Some settings could not be saved.' );
+		}
 	}
 }
 
@@ -103,16 +133,27 @@
 	)
 );
 
+if ( ! empty( $error_messages ) ) {
+	foreach ( $error_messages as $msg ) {
+		wp_admin_notice(
+			$msg,
+			array(
+				'type'        => 'error',
+				'dismissible' => true,
+			)
+		);
+	}
+}
+
 if ( ! empty( $messages ) ) {
-	$notice_args = array(
-		'type'        => 'success',
-		'dismissible' => true,
-		'id'          => 'message',
+	wp_admin_notice(
+		$messages[0],
+		array(
+			'type'        => 'success',
+			'dismissible' => true,
+			'id'          => 'message',
+		)
 	);
-
-	foreach ( $messages as $msg ) {
-		wp_admin_notice( $msg, $notice_args );
-	}
 }
 ?>
 <form method="post" action="site-settings.php?action=update-site">

src/wp-admin/options-general.php

@@ -72,9 +72,9 @@
 
 <table class="form-table" role="presentation">
 
-<tr>
+<tr class="form-field form-required">
 <th scope="row"><label for="blogname"><?php _e( 'Site Title' ); ?></label></th>
-<td><input name="blogname" type="text" id="blogname" value="<?php form_option( 'blogname' ); ?>" class="regular-text" /></td>
+<td><input name="blogname" type="text" id="blogname" value="<?php form_option( 'blogname' ); ?>" class="regular-text" required aria-required="true" /></td>
 </tr>
 
 <?php

src/wp-includes/formatting.php

@@ -5006,6 +5006,11 @@ function sanitize_option( $option, $value ) {
 				$error = $value->get_error_message();
 			} else {
 				$value = esc_html( $value );
+
+				// The site title (blogname) cannot be empty.
+				if ( 'blogname' === $option && '' === trim( $value ) ) {
+					$error = __( 'The site title cannot be empty. Please enter a title for your site.' );
+				}
 			}
 			break;
 

src/wp-includes/option.php

@@ -2746,7 +2746,11 @@ function register_initial_settings() {
 		'blogname',
 		array(
 			'show_in_rest' => array(
-				'name' => 'title',
+				'name'   => 'title',
+				'schema' => array(
+					'minLength' => 1,
+					'pattern'   => '\S',
+				),
 			),
 			'type'         => 'string',
 			'label'        => __( 'Title' ),

src/wp-includes/rest-api/endpoints/class-wp-rest-settings-controller.php

@@ -198,6 +198,14 @@ public function update_item( $request ) {
 					);
 				}
 
+				if ( 'blogname' === $args['option_name'] ) {
+					return new WP_Error(
+						'rest_invalid_param',
+						__( 'The site title cannot be empty. Please enter a title for your site.' ),
+						array( 'status' => 400 )
+					);
+				}
+
 				delete_option( $args['option_name'] );
 			} else {
 				update_option( $args['option_name'], $request[ $name ] );

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

@@ -598,6 +598,36 @@ public function test_update_item_with_invalid_type() {
 		$this->assertErrorResponse( 'rest_invalid_param', $response, 400 );
 	}
 
+	/**
+	 * @ticket 65718
+	 *
+	 * @dataProvider data_update_item_with_invalid_title
+	 */
+	public function test_update_item_with_invalid_title( $invalid_title ) {
+		wp_set_current_user( self::$administrator );
+		$original_title = get_option( 'blogname' );
+
+		$request = new WP_REST_Request( 'PUT', '/wp/v2/settings' );
+		$request->set_param( 'title', $invalid_title );
+		$response = rest_get_server()->dispatch( $request );
+
+		$this->assertErrorResponse( 'rest_invalid_param', $response, 400 );
+		$this->assertSame( $original_title, get_option( 'blogname' ) );
+	}
+
+	/**
+	 * Data provider.
+	 *
+	 * @return array[]
+	 */
+	public function data_update_item_with_invalid_title() {
+		return array(
+			'empty string'    => array( '' ),
+			'whitespace only' => array( '   ' ),
+			'null'            => array( null ),
+		);
+	}
+
 	public function test_update_item_with_integer() {
 		wp_set_current_user( self::$administrator );
 		$request = new WP_REST_Request( 'PUT', '/wp/v2/settings' );

tests/qunit/fixtures/wp-api-generated.js

@@ -11143,6 +11143,8 @@ mockedApiResponse.Schema = {
                             "title": "Title",
                             "description": "Site title.",
                             "type": "string",
+                            "minLength": 1,
+                            "pattern": "\\S",
                             "required": false
                         },
                         "description": {