Fix acceptance of username for Mattermost service update via API

changelogs/unreleased/39895-cant-set-mattermost-username-channel-from-api.yml

+---
+title: Fix acceptance of username for Mattermost service update
+merge_request: 15275
+author:
+type: fixed

doc/api/services.md

@@ -572,7 +572,7 @@ Parameters:
 | Attribute | Type | Required | Description |
 | --------- | ---- | -------- | ----------- |
 | `token` | string | yes | The Mattermost token |
-
+| `username` | string | no | The username to use to post the message |
 
 ### Delete Mattermost slash command service
 

lib/api/services.rb

@@ -522,6 +522,12 @@ module API
           name: :webhook,
           type: String,
           desc: 'The Mattermost webhook. e.g. http://mattermost_host/hooks/...'
+        },
+        {
+          required: false,
+          name: :username,
+          type: String,
+          desc: 'The username to use to post the message'
         }
       ],
       'teamcity' => [

spec/requests/api/services_spec.rb

@@ -175,4 +175,25 @@ describe API::Services do
       end
     end
   end
+
+  describe 'Mattermost service' do
+    let(:service_name) { 'mattermost' }
+    let(:params) do
+      { webhook: 'https://hook.example.com', username: 'username' }
+    end
+
+    before do
+      project.create_mattermost_service(
+        active: true,
+        properties: params
+      )
+    end
+
+    it 'accepts a username for update' do
+      put api("/projects/#{project.id}/services/mattermost", user), params.merge(username: 'new_username')
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response['properties']['username']).to eq('new_username')
+    end
+  end
 end