Support HTTP headers with CORS-method (#2957)

Adds support for sending and receiving HTTP-headers when using the
CORS-method.

This change is required for the Yr weather-provider introduced in
https://github.com/MichMich/MagicMirror/pull/2948.

To make it easier to add unit tests I moved the server-functions into a
separate file.

js/server_functions.js

@@ -0,0 +1,127 @@
+const fetch = require("./fetch");
+const fs = require("fs");
+const path = require("path");
+const Log = require("logger");
+
+/**
+ * Gets the config.
+ *
+ * @param {Request} req - the request
+ * @param {Response} res - the result
+ */
+function getConfig(req, res) {
+	res.send(config);
+}
+
+/**
+ * A method that forewards HTTP Get-methods to the internet to avoid CORS-errors.
+ *
+ * Example input request url: /cors?sendheaders=header1:value1,header2:value2&expectedheaders=header1,header2&url=http://www.test.com/path?param1=value1
+ *
+ * Only the url-param of the input request url is required. It must be the last parameter.
+ *
+ * @param {Request} req - the request
+ * @param {Response} res - the result
+ */
+async function cors(req, res) {
+	try {
+		const urlRegEx = "url=(.+?)$";
+		let url = "";
+
+		const match = new RegExp(urlRegEx, "g").exec(req.url);
+		if (!match) {
+			url = "invalid url: " + req.url;
+			Log.error(url);
+			res.send(url);
+		} else {
+			url = match[1];
+
+			const headersToSend = getHeadersToSend(req.url);
+			const expectedRecievedHeaders = geExpectedRecievedHeaders(req.url);
+
+			Log.log("cors url: " + url);
+			const response = await fetch(url, { headers: headersToSend });
+
+			for (const header of expectedRecievedHeaders) {
+				const headerValue = response.headers.get(header);
+				if (header) res.set(header, headerValue);
+			}
+			const data = await response.text();
+			res.send(data);
+		}
+	} catch (error) {
+		Log.error(error);
+		res.send(error);
+	}
+}
+
+/**
+ * Gets headers and values to attatch to the web request.
+ *
+ * @param {string} url - The url containing the headers and values to send.
+ * @returns {object} An object specifying name and value of the headers.
+ */
+function getHeadersToSend(url) {
+	const headersToSend = { "User-Agent": "Mozilla/5.0 MagicMirror/" + global.version };
+	const headersToSendMatch = new RegExp("sendheaders=(.+?)(&|$)", "g").exec(url);
+	if (headersToSendMatch) {
+		const headers = headersToSendMatch[1].split(",");
+		for (const header of headers) {
+			const keyValue = header.split(":");
+			if (keyValue.length !== 2) {
+				throw new Error(`Invalid format for header ${header}`);
+			}
+			headersToSend[keyValue[0]] = decodeURIComponent(keyValue[1]);
+		}
+	}
+	return headersToSend;
+}
+
+/**
+ * Gets the headers expected from the response.
+ *
+ * @param {string} url - The url containing the expected headers from the response.
+ * @returns {string[]} headers - The name of the expected headers.
+ */
+function geExpectedRecievedHeaders(url) {
+	const expectedRecievedHeaders = ["Content-Type"];
+	const expectedRecievedHeadersMatch = new RegExp("expectedheaders=(.+?)(&|$)", "g").exec(url);
+	if (expectedRecievedHeadersMatch) {
+		const headers = expectedRecievedHeadersMatch[1].split(",");
+		for (const header of headers) {
+			expectedRecievedHeaders.push(header);
+		}
+	}
+	return expectedRecievedHeaders;
+}
+
+/**
+ * Gets the HTML to display the magic mirror.
+ *
+ * @param {Request} req - the request
+ * @param {Response} res - the result
+ */
+function getHtml(req, res) {
+	let html = fs.readFileSync(path.resolve(`${global.root_path}/index.html`), { encoding: "utf8" });
+	html = html.replace("#VERSION#", global.version);
+
+	let configFile = "config/config.js";
+	if (typeof global.configuration_file !== "undefined") {
+		configFile = global.configuration_file;
+	}
+	html = html.replace("#CONFIG_FILE#", configFile);
+
+	res.send(html);
+}
+
+/**
+ * Gets the MagicMirror version.
+ *
+ * @param {Request} req - the request
+ * @param {Response} res - the result
+ */
+function getVersion(req, res) {
+	res.send(global.version);
+}
+
+module.exports = { cors, getConfig, getHtml, getVersion };