Validate presence of essential params for diff rendering

This will avoid application errors generated by the assumption of the
presence of these params.

app/controllers/projects/blob_controller.rb

@@ -16,6 +16,7 @@ class Projects::BlobController < Projects::ApplicationController
   before_action :from_merge_request, only: [:edit, :update]
   before_action :require_branch_head, only: [:edit, :update]
   before_action :editor_variables, except: [:show, :preview, :diff]
+  before_action :validate_diff_params, only: :diff
   def new
     commit unless @repository.empty?
@@ -146,4 +147,10 @@ class Projects::BlobController < Projects::ApplicationController
       file_content_encoding: params[:encoding]
     }
   end
+
+  def validate_diff_params
+    if [:since, :to, :offset].any? { |key| params[key].blank? }
+      render nothing: true
+    end
+  end
 end

spec/controllers/projects/blob_controller_spec.rb

+require 'rails_helper'
+
+describe Projects::BlobController do
+  let(:project) { create(:project) }
+  let(:user)    { create(:user) }
+
+  before do
+    user = create(:user)
+    project.team << [user, :master]
+
+    sign_in(user)
+  end
+
+  describe 'GET diff' do
+    render_views
+
+    def do_get(opts = {})
+      params = { namespace_id: project.namespace.to_param,
+                 project_id: project.to_param,
+                 id: 'master/CHANGELOG' }
+      get :diff, params.merge(opts)
+    end
+
+    context 'when essential params are missing' do
+      it 'renders nothing' do
+        do_get
+
+        expect(response.body).to be_blank
+      end
+    end
+
+    context 'when essential params are present' do
+      it 'renders the diff content' do
+        do_get(since: 1, to: 5, offset: 10)
+
+        expect(response.body).to be_present
+      end
+    end
+  end
+end