From 85919ff914067bb49a14ed08473125f9973446e0 Mon Sep 17 00:00:00 2001
From: Richard Hansen <rhansen@rhansen.org>
Date: Tue, 16 Nov 2021 02:51:36 -0500
Subject: [PATCH] PadMessageHandler: Replace `channels` package with
 async-friendly class

---
 src/node/handler/PadMessageHandler.js | 42 +++++++++++++++++++++++----
 src/package-lock.json                 |  5 ----
 src/package.json                      |  1 -
 3 files changed, 36 insertions(+), 12 deletions(-)

diff --git a/src/node/handler/PadMessageHandler.js b/src/node/handler/PadMessageHandler.js
index ca706e325..1b327a92d 100644
--- a/src/node/handler/PadMessageHandler.js
+++ b/src/node/handler/PadMessageHandler.js
@@ -34,11 +34,9 @@ const messageLogger = log4js.getLogger('message');
 const accessLogger = log4js.getLogger('access');
 const _ = require('underscore');
 const hooks = require('../../static/js/pluginfw/hooks.js');
-const channels = require('channels');
 const stats = require('../stats');
 const assert = require('assert').strict;
 const {RateLimiterMemory} = require('rate-limiter-flexible');
-const util = require('util');
 const webaccess = require('../hooks/express/webaccess');
 
 let rateLimiter;
@@ -84,11 +82,44 @@ stats.gauge('activePads', () => {
   return padIds.size;
 });
 
+/**
+ * Processes one task at a time per channel.
+ */
+class Channels {
+  /**
+   * @param {(ch, task) => any} [exec] - Task executor. If omitted, tasks are assumed to be
+   *     functions that will be executed with the channel as the only argument.
+   */
+  constructor(exec = (ch, task) => task(ch)) {
+    this._exec = exec;
+    this._promiseChains = new Map();
+  }
+
+  /**
+   * Schedules a task for execution. The task will be executed once all previously enqueued tasks
+   * for the named channel have completed.
+   *
+   * @param {any} ch - Identifies the channel.
+   * @param {any} task - The task to give to the executor.
+   * @returns {Promise<any>} The value returned by the executor.
+   */
+  async enqueue(ch, task) {
+    const p = (this._promiseChains.get(ch) || Promise.resolve()).then(() => this._exec(ch, task));
+    const pc = p
+        .catch(() => {}) // Prevent rejections from halting the queue.
+        .then(() => {
+          // Clean up this._promiseChains if there are no more tasks for the channel.
+          if (this._promiseChains.get(ch) === pc) this._promiseChains.delete(ch);
+        });
+    this._promiseChains.set(ch, pc);
+    return await p;
+  }
+}
+
 /**
  * A changeset queue per pad that is processed by handleUserChanges()
  */
-const padChannels =
-    new channels.channels(({socket, message}, cb) => handleUserChangesCb(socket, message, cb));
+const padChannels = new Channels((ch, {socket, message}) => handleUserChanges(socket, message));
 
 /**
  * This Method is called by server.js to tell the message handler on which socket it should send
@@ -265,7 +296,7 @@ exports.handleMessage = async (socket, message) => {
       messageLogger.warn('Dropped message, COLLABROOM for readonly pad');
     } else if (message.data.type === 'USER_CHANGES') {
       stats.counter('pendingEdits').inc();
-      padChannels.emit(message.padId, {socket, message}); // add to pad queue
+      await padChannels.enqueue(message.padId, {socket, message});
     } else if (message.data.type === 'USERINFO_UPDATE') {
       await handleUserInfoUpdate(socket, message);
     } else if (message.data.type === 'CHAT_MESSAGE') {
@@ -690,7 +721,6 @@ const handleUserChanges = async (socket, message) => {
 
   stopWatch.end();
 };
-const handleUserChangesCb = util.callbackify(handleUserChanges);
 
 exports.updatePadClients = async (pad) => {
   // skip this if no-one is on this pad
diff --git a/src/package-lock.json b/src/package-lock.json
index 3565f4b40..037648d44 100644
--- a/src/package-lock.json
+++ b/src/package-lock.json
@@ -1233,11 +1233,6 @@
         "supports-color": "^5.3.0"
       }
     },
-    "channels": {
-      "version": "0.0.4",
-      "resolved": "https://registry.npmjs.org/channels/-/channels-0.0.4.tgz",
-      "integrity": "sha1-G+4yPt6hUrue8E9BvG5rD1lIqUE="
-    },
     "character-entities-html4": {
       "version": "1.1.4",
       "resolved": "https://registry.npmjs.org/character-entities-html4/-/character-entities-html4-1.1.4.tgz",
diff --git a/src/package.json b/src/package.json
index 3dba9c9db..2fd5e985e 100644
--- a/src/package.json
+++ b/src/package.json
@@ -31,7 +31,6 @@
   ],
   "dependencies": {
     "async": "^3.2.1",
-    "channels": "0.0.4",
     "clean-css": "^5.2.1",
     "cookie-parser": "1.4.5",
     "cross-spawn": "^7.0.3",