[Blueprints] Prevent WSOD when autologin is enabled and a plugin logs a notice

packages/playground/website/playwright/e2e/blueprints.spec.ts

@@ -449,3 +449,45 @@ test('should correctly redirect to a multisite wp-admin url', async ({
 		await expect(wordpress.locator('body')).toContainText('Escritorio');
 	});
 });
+
+test('WordPress homepage loads when mu-plugin prints a notice', async ({
+	wordpress,
+	website,
+	page,
+}) => {
+	// Load a blueprint that enables debug mode and adds a mu-plugin that prints a notice
+	const blueprint = {
+		landingPage: '/',
+		preferredVersions: {
+			wp: '6.7',
+			php: '8.0',
+		},
+		steps: [
+			{
+				step: 'defineWpConfigConsts',
+				consts: {
+					WP_DEBUG: true,
+					WP_DEBUG_DISPLAY: true,
+				},
+			},
+			{
+				step: 'writeFile',
+				path: '/wordpress/wp-content/mu-plugins/000-print-notice.php',
+				data: `<?php
+					echo 'This is a notice printed by an mu-plugin.';
+				`,
+			},
+		],
+	};
+
+	const encodedBlueprint = JSON.stringify(blueprint);
+	await website.goto(`./#${encodedBlueprint}`);
+
+	// Wait for the page to load and verify it contains both WordPress content and the notice
+	await expect(wordpress.locator('body')).toContainText(
+		'Welcome to WordPress. This is your first post.'
+	);
+
+	// Verify there's no admin bar
+	await expect(wordpress.locator('body')).not.toContainText('Dashboard');
+});

packages/playground/wordpress/src/index.ts

@@ -147,15 +147,45 @@ export async function setupPlatformLevelMuPlugins(php: UniversalPHP) {
 			if (!$user) {
 				return;
 			}
+
+			/**
+			 * We're about to set cookies and redirect. It will log the user in
+			 * if the headers haven't been sent yet.
+			 *
+			 * However, if they have been sent already – e.g. there a PHP
+			 * notice was printed, we'll exit the script with a bunch of errors
+			 * on the screen and without the user being logged in. This
+			 * will happen on every page load and will effectively make Playground
+			 * unusable.
+			 *
+			 * Therefore, we just won't auto-login if headers have been sent. Maybe
+			 * we'll be able to finish the operation in one of the future requests
+			 * or maybe not, but at least we won't end up with a permanent white screen.
+			 */
+			if (headers_sent()) {
+				_doing_it_wrong('playground_auto_login', 'Headers already sent, the Playground runtime will not auto-login the user', '1.0.0');
+				return;
+			}
+
 			/**
 			 * This approach is described in a comment on
 			 * https://developer.wordpress.org/reference/functions/wp_set_current_user/
 			 */
 			wp_set_current_user( $user->ID, $user->user_login );
 			wp_set_auth_cookie( $user->ID );
 			do_action( 'wp_login', $user->user_login, $user );
+
 			setcookie('playground_auto_login_already_happened', '1');
 
+			/**
+			 * Confirm that nothing in WordPress, plugins, or filters have finalized
+			 * the headers sending phase. See the comment above for more context.
+			 */
+			if (headers_sent()) {
+				_doing_it_wrong('playground_auto_login', 'Headers already sent, the Playground runtime will not auto-login the user', '1.0.0');
+				return;
+			}
+
 			/**
 			 * Reload page to ensure the user is logged in correctly.
 			 * WordPress uses cookies to determine if the user is logged in,