add X-Git-Commit header to Rack application

git_commit_header.rb

# Rack middleware to insert git commit id into http response header
# For Rails, put this script under RAILS_ROOT/config/initializers

module Rack
  class GitCommitHeader
    def initialize(app)
      @app = app
      c = `git rev-parse HEAD`.chomp
      @commit = (c =~ /^[0-9a-f]{40}$/) ? c : nil
    end
    def call(env)
      status, headers, body = @app.call(env)
      headers['X-Git-Commit'] = @commit if @commit
      [status, headers, body]
    end
  end
end
ActionController::Dispatcher.middleware.use Rack::GitCommitHeader rescue nil