Implement basic dev server HTTPS certificate error detection

inc/manifest/namespace.php

@@ -31,8 +31,128 @@ function load_asset_file( $path ) {
 function get_assets_list( string $manifest_path ) {
 	$dev_assets = load_asset_file( $manifest_path );
 	if ( ! empty( $dev_assets ) ) {
+		maybe_setup_ssl_cert_error_handling( $dev_assets );
 		return array_values( $dev_assets );
 	}
 
 	return null;
 }
+
+/**
+ * Check to see if the manifest contains HTTPS localhost URLs, and set up error
+ * detection to display a notice reminding the developer to accept the dev
+ * server's SSL certificate if any of those HTTPS scripts fail to load.
+ *
+ * @param array $dev_assets Array of script URLs to load.
+ * @return void
+ */
+function maybe_setup_ssl_cert_error_handling( $dev_assets ) {
+	preg_match_all( '#https://localhost:\d+#', implode( "\n", $dev_assets ), $matches );
+	if ( empty( $matches ) || empty( $matches[0] ) ) {
+		// No HTTPS URLs? Carry on.
+		return;
+	}
+	if ( is_admin() ) {
+		add_action( 'admin_head', __NAMESPACE__ . '\\detect_localhost_script_errors' );
+		add_filter( 'script_loader_tag', __NAMESPACE__ . '\\add_onerror_to_scripts', 10, 3 );
+	}
+}
+
+/**
+ * Render inline JS into the page header to register a function which will be
+ * called should any of our registered HTTPS localhost scripts fail to load.
+ *
+ * @return void
+ */
+function detect_localhost_script_errors() {
+	?>
+<script>
+( function() {
+	var scriptsWithErrors = [];
+
+	/**
+	 * @param HTMLScriptElement The script which experienced an error.
+	 */
+	window.maybeSSLError = function( script ) {
+		scriptsWithErrors.push( script );
+	};
+
+	/**
+	 * Check whether an error has occurred, then attempt to display a Block Editor
+	 * notice to alert the developer if so.
+	 *
+	 * @return void
+	 */
+	function processErrors() {
+		if ( ! scriptsWithErrors.length ) {
+			// There are no problems to highlight.
+			return;
+		}
+
+		var notices = null;
+		if ( window.wp && window.wp.data && window.wp.data.dispatch ) {
+			notices = window.wp.data.dispatch( 'core/notices' );
+		}
+		if ( ! notices ) {
+			// We're not in a context where it is easy to display a notice from JS.
+			return;
+		}
+
+		// Build a list of problem hosts.
+		var hosts = scriptsWithErrors.reduce(
+			function( hosts, script ) {
+				var src = script.getAttribute( 'src' );
+				if ( ! src || ! /https:\/\/localhost/i.test( src ) ) {
+					return hosts;
+				}
+				src = src.replace( /^(https:\/\/localhost:\d+).*$/i, '$1' );
+				hosts[ src ] = true;
+				return hosts;
+			},
+			{}
+		);
+		hosts = Object.keys( hosts );
+
+		// Build the error markup.
+		const messageHTML = [
+			'<strong>Error loading scripts from localhost!</strong>',
+			'<br>',
+			'Ensure that ',
+			( hosts.length > 1 ? 'these hosts are ' : 'this host is ' ),
+			'accessible, and that you have accepted any development server SSL certificates:',
+			'<ul>',
+			hosts.map( host => '<li><a target="_blank" href="' + host + '">' + host + '</a></li>' ).join( '' ),
+			'</ul>'
+		].join( '' );
+
+		notices.createErrorNotice( messageHTML, { __unstableHTML: true } );
+	}
+
+	// Set up processErrors to run 1 second after page load.
+	document.addEventListener( 'DOMContentLoaded', function() {
+		setTimeout( processErrors, 1000 );
+	} );
+} )();
+</script>
+	<?php
+}
+
+/**
+ * Inject an onerror attribute into the rendered script tag for any script
+ * loaded from localhost with an HTTPS protocol.
+ *
+ * @param string $tag    The HTML of the script tag to render.
+ * @param string $handle The registered script handle for this tag.
+ * @param string $src    The src URI of the JavaScript file this script loads.
+ * @return string The script tag HTML, conditionally transformed.
+ */
+function add_onerror_to_scripts( string $tag, string $handle, string $src ) : string {
+	if ( ! preg_match( '#https://localhost#', $src ) ) {
+		return $tag;
+	}
+	return preg_replace(
+		'/<script/',
+		'<script onerror="maybeSSLError && maybeSSLError( this );"',
+		$tag
+	);
+}