Add server sent events (#5279)

* Create server-sent-events.js

* Create sse-change-listener.js

* Implement server sent events

* Convert to ES5 and wrap in function

* Use the host string from tiddlyweb

* Improve comments in sse-server.js

* Can't use object reference as key

* Add retry timeout

* Fix a bug

* bug fix

* Fix formatting

* Fix ES5 compat

* capitalize comments

* more fixes

* Refactor tiddlywek/sse-server.js

* Extract helper functions for handling wikis and connections.
* Replace JSDoc comments.
* Fix formatting according to TW core.
* Simplify the logic for adding and removing connections.

* Fix formatting of tiddlyweb/sse-client.js

Fix formatting according to TW core.

* Fix formatting of server-sent-events.js

Fix formatting and comments following TW core guidelines.

* Extract a debounce function in sse-client.js

* Avoid using startsWith in server-sent-events.js

startsWith is part of ES2015, while TiddlyWiki uses the 5.1 dialect.

* New sse-enabled WebServer parameter

* If not set to "yes", disabled SSE request handling.
* Add documentation for the parameter in core/language/en-GB/Help/listen.tid
* Add new tiddler editions/tw5.com/tiddlers/webserver/WebServer Parameter_ sse-enabled.tid

* Disable polling for changes if SSE is enabled

* Add sse_enabled to /status JSON response
* Store syncer polling status in $:/config/SyncDisablePolling
* Handled disabling polling in core/modules/syncer.js

* Simply boolean logic in syncer.js

* Delete trailing whitespaces in syncer.js

Co-authored-by: Arlen22 <arlenbee@gmail.com>

core/modules/server/routes/get-status.js

@@ -22,6 +22,7 @@ exports.handler = function(request,response,state) {
 		username: state.authenticatedUsername || state.server.get("anon-username") || "",
 		anonymous: !state.authenticatedUsername,
 		read_only: !state.server.isAuthorized("writers",state.authenticatedUsername),
+		sse_enabled: state.server.get("sse-enabled") === "yes",
 		space: {
 			recipe: "default"
 		},

core/modules/server/server-sent-events.js

@@ -0,0 +1,70 @@
+/*\
+title: $:/core/modules/server/server-sent-events.js
+type: application/javascript
+module-type: library
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+parameters:
+		prefix - usually the plugin path, such as `plugins/tiddlywiki/tiddlyweb`. The
+			route will match `/events/${prefix}` exactly.
+
+		handler - a function that will be called each time a request comes in with the
+			request and state from the route and an emit function to call.
+*/
+
+var ServerSentEvents = function ServerSentEvents(prefix, handler) {
+	this.handler = handler;
+	this.prefix = prefix;
+};
+
+ServerSentEvents.prototype.getExports = function() {
+	return {
+		bodyFormat: "stream",
+		method: "GET",
+		path: new RegExp("^/events/" + this.prefix + "$"),
+		handler: this.handleEventRequest.bind(this)
+	};
+};
+
+ServerSentEvents.prototype.handleEventRequest = function(request,response,state) {
+	if(ServerSentEvents.prototype.isEventStreamRequest(request)) {
+		response.writeHead(200, {
+			"Content-Type": "text/event-stream",
+			"Cache-Control": "no-cache",
+			"Connection": "keep-alive"
+		});
+		this.handler(request,state,this.emit.bind(this,response),this.end.bind(this,response));
+	} else {
+		response.writeHead(406,"Not Acceptable",{});
+		response.end();
+	}
+};
+
+ServerSentEvents.prototype.isEventStreamRequest = function(request) {
+	return request.headers.accept &&
+		request.headers.accept.match(/^text\/event-stream/);
+};
+
+ServerSentEvents.prototype.emit = function(response,event,data) {
+	if(typeof event !== "string" || event.indexOf("\n") !== -1) {
+		throw new Error("Type must be a single-line string");
+	}
+	if(typeof data !== "string" || data.indexOf("\n") !== -1) {
+		throw new Error("Data must be a single-line string");
+	}
+	response.write("event: " + event + "\ndata: " + data + "\n\n", "utf8");
+};
+
+ServerSentEvents.prototype.end = function(response) {
+	response.end();
+};
+
+exports.ServerSentEvents = ServerSentEvents;
+
+})();