Return status code 303 after a branch DELETE operation to avoid project deletion

Closes #14994

CHANGELOG

@@ -2,6 +2,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.7.0 (unreleased)
   - All images in discussions and wikis now link to their source files !3464 (Connor Shea).
+  - Return status code 303 after a branch DELETE operation to avoid project deletion (Stan Hu)
   - Improved Markdown rendering performance !3389 (Yorick Peterse)
   - Don't attempt to look up an avatar in repo if repo directory does not exist (Stan Hu)
   - Preserve time notes/comments have been updated at when moving issue

app/controllers/projects/application_controller.rb

@@ -68,7 +68,9 @@ class Projects::ApplicationController < ApplicationController
   end
 
   def require_non_empty_project
-    redirect_to namespace_project_path(@project.namespace, @project) if @project.empty_repo?
+    # Be sure to return status code 303 to avoid a double DELETE:
+    # http://api.rubyonrails.org/classes/ActionController/Redirecting.html
+    redirect_to namespace_project_path(@project.namespace, @project), status: 303 if @project.empty_repo?
   end
 
   def require_branch_head

app/controllers/projects/branches_controller.rb

@@ -48,7 +48,7 @@ class Projects::BranchesController < Projects::ApplicationController
     respond_to do |format|
       format.html do
         redirect_to namespace_project_branches_path(@project.namespace,
-                                                    @project)
+                                                    @project), status: 303
       end
       format.js { render status: status[:return_code] }
     end

spec/controllers/projects/branches_controller_spec.rb

@@ -93,6 +93,20 @@ describe Projects::BranchesController do
     end
   end
 
+  describe "POST destroy with HTML format" do
+    render_views
+
+    it 'returns 303' do
+      post :destroy,
+           format: :html,
+           id: 'foo/bar/baz',
+           namespace_id: project.namespace.to_param,
+           project_id: project.to_param
+
+      expect(response.status).to eq(303)
+    end
+  end
+
   describe "POST destroy" do
     render_views