4 files changed, 41 insertions, 14 deletions
diff --git a/app/assets/javascripts/task_list.js b/app/assets/javascripts/task_list.js
index dfe24d1fb33..b1402c0a880 100644
--- a/app/assets/javascripts/task_list.js
+++ b/app/assets/javascripts/task_list.js
@@ -1,3 +1,4 @@
+/* global Flash */
 require('vendor/task_list');
 
 class TaskList {
@@ -6,6 +7,16 @@ class TaskList {
     this.dataType = options.dataType;
     this.fieldName = options.fieldName;
     this.onSuccess = options.onSuccess || (() => {});
+    this.onError = function showFlash(response) {
+      let errorMessages = '';
+
+      if (response.responseJSON) {
+        errorMessages = response.responseJSON.errors.join(' ');
+      }
+
+      return new Flash(errorMessages || 'Update failed', 'alert');
+    };
+
     this.init();
   }
 
@@ -32,6 +43,7 @@ class TaskList {
       url: $target.data('update-url') || $('form.js-issuable-update').attr('action'),
       data: patchData,
       success: this.onSuccess,
+      error: this.onError,
     });
   }
 }
diff --git a/app/controllers/concerns/issuable_actions.rb b/app/controllers/concerns/issuable_actions.rb
index 0821974aa93..3ccf2a9ce33 100644
--- a/app/controllers/concerns/issuable_actions.rb
+++ b/app/controllers/concerns/issuable_actions.rb
@@ -26,6 +26,23 @@ module IssuableActions
 
   private
 
+  def render_conflict_response
+    respond_to do |format|
+      format.html do
+        @conflict = true
+        render :edit
+      end
+
+      format.json do
+        render json: {
+          errors: [
+            "Someone edited this #{issuable.human_class_name} at the same time you did. Please refresh your browser and make sure your changes will not unintentionally remove theirs."
+          ]
+        }, status: 409
+      end
+    end
+  end
+
   def labels
     @labels ||= LabelsFinder.new(current_user, project_id: @project.id).execute
   end
diff --git a/app/controllers/projects/issues_controller.rb b/app/controllers/projects/issues_controller.rb
index ca5e81100da..1151555b8fa 100644
--- a/app/controllers/projects/issues_controller.rb
+++ b/app/controllers/projects/issues_controller.rb
@@ -134,8 +134,7 @@ class Projects::IssuesController < Projects::ApplicationController
     end
 
   rescue ActiveRecord::StaleObjectError
-    @conflict = true
-    render :edit
+    render_conflict_response
   end
 
   def referenced_merge_requests
diff --git a/app/controllers/projects/merge_requests_controller.rb b/app/controllers/projects/merge_requests_controller.rb
index 365c49a20d4..dda7a92550a 100644
--- a/app/controllers/projects/merge_requests_controller.rb
+++ b/app/controllers/projects/merge_requests_controller.rb
@@ -296,22 +296,21 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   def update
     @merge_request = MergeRequests::UpdateService.new(project, current_user, merge_request_params).execute(@merge_request)
 
-    if @merge_request.valid?
-      respond_to do |format|
-        format.html do
-          redirect_to([@merge_request.target_project.namespace.becomes(Namespace),
-                       @merge_request.target_project, @merge_request])
-        end
-        format.json do
-          render json: @merge_request.to_json(include: { milestone: {}, assignee: { methods: :avatar_url }, labels: { methods: :text_color } }, methods: [:task_status, :task_status_short])
+    respond_to do |format|
+      format.html do
+        if @merge_request.valid?
+          redirect_to([@merge_request.target_project.namespace.becomes(Namespace), @merge_request.target_project, @merge_request])
+        else
+          render :edit
         end
       end
-    else
-      render "edit"
+
+      format.json do
+        render json: @merge_request.to_json(include: { milestone: {}, assignee: { methods: :avatar_url }, labels: { methods: :text_color } }, methods: [:task_status, :task_status_short])
+      end
     end
   rescue ActiveRecord::StaleObjectError
-    @conflict = true
-    render :edit
+    render_conflict_response
   end
 
   def remove_wip