Don't allow someone to deploy a feature branch when there's already one active

deploy.rb

if ENV['BRANCH']
  before "deploy", "deploy:check_branch"
end

after "deploy:finalize_update", "deploy:put_branch"

namespace :deploy do
  task :check_branch do
    current_branch = capture("cat #{current_path}/BRANCH; true")

    if branch != 'master' && current_branch != 'master' && current_branch != `git rev-parse --abbrev-rev HEAD`
      puts 'Sorry, someone else is already deploying a feature branch.'
      exit
    end
  end

  task :put_branch do
    put "#{branch}", "#{release_path}/BRANCH"
  end
end