http: Bring back handler.js support, mostly. Partly in C, this time.

core/core.js

@@ -860,6 +860,73 @@ function sendStats() {
 	}
 }
 
+let g_handler_index = 0;
+
+exports.callAppHandler = async function callAppHandler(
+	response,
+	app_blob_id,
+	path,
+	query,
+	headers,
+	package_owner,
+	package_name
+) {
+	let answer;
+	try {
+		let do_resolve;
+		let promise = new Promise(async function (resolve, reject) {
+			do_resolve = resolve;
+		});
+		let process;
+		try {
+			process = await getProcessBlob(
+				app_blob_id,
+				'handler_' + g_handler_index++,
+				{
+					script: 'handler.js',
+					imports: {
+						request: {
+							path: path,
+							query: query,
+						},
+						respond: do_resolve,
+					},
+					credentials: await httpd.auth_query(headers),
+					packageOwner: package_owner,
+					packageName: package_name,
+				}
+			);
+			await process.ready;
+			answer = await promise;
+		} finally {
+			if (process?.task) {
+				await process.task.kill();
+			}
+		}
+	} catch (error) {
+		let data = utf8Encode(
+			`Internal Server Error\n\n${error?.message}\n${error?.stack}`
+		);
+		response.writeHead(500, {
+			'Content-Type': 'text/plain; charset=utf-8',
+			'Content-Length': data.length,
+		});
+		response.end(data);
+		return;
+	}
+	if (typeof answer?.data == 'string') {
+		answer.data = utf8Encode(answer.data);
+	}
+	response.writeHead(answer?.status_code, {
+		'Content-Type': answer?.content_type,
+		'Content-Length': answer?.data?.length,
+		'Access-Control-Allow-Origin': '*',
+		'Content-Security-Policy':
+			'sandbox allow-downloads allow-top-navigation-by-user-activation',
+	});
+	response.end(answer?.data);
+};
+
 /**
  * TODOC
  */