Upgrade/Install: Load the .maintenance file non-fatally

src/wp-admin/includes/class-wp-automatic-updater.php

@@ -1766,8 +1766,11 @@ protected function has_fatal_error() {
 			return false;
 		}
 
-		require $maintenance_file;
-		if ( ! is_int( $upgrading ) ) {
+		// The file may be removed between the check above and the load below.
+		// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- Warning emitted in expected failure case.
+		$maintenance_file_loaded = @include $maintenance_file;
+
+		if ( false === $maintenance_file_loaded || ! is_int( $upgrading ) ) {
 			return false;
 		}
 

src/wp-includes/load.php

@@ -441,7 +441,13 @@ function wp_is_maintenance_mode() {
 		return false;
 	}
 
-	require ABSPATH . '.maintenance';
+	// The file may be removed between the check above and the load below.
+	// phpcs:ignore WordPress.PHP.NoSilencedErrors.Discouraged -- Warning emitted in expected failure case.
+	$maintenance_file_loaded = @include ABSPATH . '.maintenance';
+
+	if ( false === $maintenance_file_loaded ) {
+		return false;
+	}
 
 	// If the $upgrading timestamp is older than 10 minutes, consider maintenance over.
 	if ( ( time() - $upgrading ) >= 10 * MINUTE_IN_SECONDS ) {

tests/phpunit/tests/load/wpIsMaintenanceMode.php

@@ -0,0 +1,34 @@
+<?php
+/**
+ * Unit tests for `wp_is_maintenance_mode()`.
+ *
+ * @package WordPress
+ * @subpackage UnitTests
+ *
+ * @group load
+ *
+ * @covers ::wp_is_maintenance_mode
+ */
+class Test_WP_Is_Maintenance_Mode extends WP_UnitTestCase {
+
+	public function tear_down() {
+		if ( is_dir( ABSPATH . '.maintenance' ) ) {
+			rmdir( ABSPATH . '.maintenance' );
+		}
+
+		parent::tear_down();
+	}
+
+	/**
+	 * @ticket 65911
+	 */
+	public function test_should_return_false_when_the_maintenance_file_cannot_be_loaded() {
+		if ( file_exists( ABSPATH . '.maintenance' ) ) {
+			$this->markTestSkipped( 'A .maintenance file already exists in ABSPATH.' );
+		}
+
+		mkdir( ABSPATH . '.maintenance' );
+
+		$this->assertFalse( wp_is_maintenance_mode() );
+	}
+}