[IMP] mail: prevent message loss during connection issue

Before the changes, when a message was posted (on Discuss or in
the chat window), the composer was cleared directly as it didn't
wait for the rpc response. If there was latency or a connection
loss, the message disappeared from the composer but took time to
appear in the content window or didn't appear at all and was lost.
Now, the composer waits for the rpc response and then is cleared.

Task #1957856

addons/mail/static/src/js/composers/basic_composer.js

@@ -510,9 +510,10 @@ var BasicComposer = Widget.extend({
         clearTimeout(this._cannedTimeout);
         var self = this;
         this._preprocessMessage().then(function (message) {
-            self.trigger('post_message', message);
-            self._clearComposerOnSend();
-            self.$input.focus();
+            self.trigger('post_message', message, function() {
+                self._clearComposerOnSend();
+                self.$input.focus();
+            });
         });
     },
     /**

addons/mail/static/src/js/discuss.js

@@ -1461,8 +1461,9 @@ var Discuss = AbstractAction.extend({
     /**
      * @private
      * @param {Object} messageData
+     * @param {Function} callback
      */
-    _onPostMessage: function (messageData) {
+    _onPostMessage: function (messageData, callback) {
         var self = this;
         var options = {};
         if (this._selectedMessage) {
@@ -1483,6 +1484,7 @@ var Discuss = AbstractAction.extend({
                 } else {
                     self._threadWidget.scrollToBottom();
                 }
+                callback();
             });
     },
     /**

addons/mail/static/src/js/thread_windows/abstract_thread_window.js

@@ -394,15 +394,17 @@ var AbstractThreadWindow = Widget.extend({
      *
      * @private
      * @param {Object} messageData
+     * @param {Function} callback
      */
-    _postMessage: function (messageData) {
+    _postMessage: function (messageData, callback) {
         var self = this;
         if (!this.hasThread()) {
             return;
         }
         this._thread.postMessage(messageData)
             .then(function () {
                 self._threadWidget.scrollToBottom();
+                callback();
             });
     },
     /**

addons/mail/static/tests/discuss_tests.js

@@ -3,7 +3,6 @@ odoo.define('mail.discuss_test', function (require) {
 
 var mailTestUtils = require('mail.testUtils');
 
-var concurrency = require('web.concurrency');
 var testUtils = require('web.test_utils');
 
 var createDiscuss = mailTestUtils.createDiscuss;
@@ -1491,5 +1490,54 @@ QUnit.test('custom-named DM conversation', async function (assert) {
     discuss.destroy();
 });
 
+QUnit.test('input not cleared on unresolved message_post rpc', async function (assert) {
+    assert.expect(2);
+
+    // Promise to simulate late server response on message post
+    var messagePostPromise = testUtils.makeTestPromise();
+
+    this.data.initMessaging = {
+        channel_slots: {
+            channel_channel: [{
+                id: 1,
+                channel_type: "channel",
+                name: "general",
+            }],
+        },
+    };
+
+    var discuss = await createDiscuss({
+        id: 1,
+        context: {},
+        params: {},
+        data: this.data,
+        services: this.services,
+        mockRPC: function (route, args) {
+            if (args.method === 'message_post') {
+                return messagePostPromise;
+            }
+            return this._super.apply(this, arguments);
+        },
+    });
+
+    // Click on channel 'general'
+    var $general = discuss.$('.o_mail_discuss_sidebar').find('.o_mail_discuss_item[data-thread-id=1]');
+    await testUtils.dom.click($general);
+
+    // Type message
+    var $input = discuss.$('textarea.o_composer_text_field').first();
+    $input.focus();
+    $input.val('test message');
+
+    // Send message
+    await testUtils.fields.triggerKeydown($input, 'enter');
+    assert.strictEqual($input.val(), 'test message', "composer should not be cleared on send without server response");
+
+    // Simulate server response
+    messagePostPromise.resolve();
+    await testUtils.nextTick();
+    assert.strictEqual($input.val(), '', "composer should be cleared on send after server response");
+    discuss.destroy();
+});
 });
 });