PadMessageHandler: Deprecate `client` context property

CHANGELOG.md

@@ -37,6 +37,9 @@
 
 #### For plugin authors
 
+* The `client` context property for the `handleMessageSecurity` and
+  `handleMessage` server-side hooks is deprecated; use the `socket` context
+  property instead.
 * Changes to the `src/static/js/Changeset.js` library:
   * The following attribute processing functions are deprecated (use the new
     attribute APIs instead):

src/node/handler/PadMessageHandler.js

@@ -26,6 +26,7 @@ const ChatMessage = require('../../static/js/ChatMessage');
 const AttributePool = require('../../static/js/AttributePool');
 const AttributeManager = require('../../static/js/AttributeManager');
 const authorManager = require('../db/AuthorManager');
+const {padutils} = require('../../static/js/pad_utils');
 const readOnlyManager = require('../db/ReadOnlyManager');
 const settings = require('../utils/Settings');
 const securityManager = require('../db/SecurityManager');
@@ -270,7 +271,16 @@ exports.handleMessage = async (socket, message) => {
   thisSession.author = authorID;
 
   // Allow plugins to bypass the readonly message blocker
-  const context = {message, socket, client: socket}; // `client` for backwards compatibility.
+  const context = {
+    message,
+    socket,
+    get client() {
+      padutils.warnDeprecated(
+          'the `client` context property for the handleMessageSecurity and handleMessage hooks ' +
+          'is deprecated; use the `socket` property instead');
+      return this.socket;
+    },
+  };
   if ((await hooks.aCallAll('handleMessageSecurity', context)).some((w) => w === true)) {
     thisSession.readonly = false;
   }