Rails response header containing git revision sha

application.rb

# config/application.rb

require_relative 'boot'

require 'rails/all'

require_relative '../lib/middleware/revision_middleware'

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module MyApp
  class Application < Rails::Application
    # Initialize configuration defaults for originally generated Rails version.
    config.load_defaults 6.0

    # Set a response header containing the git revision
    config.middleware.use RevisionMiddleware

    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration can go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded after loading
    # the framework and any gems in your application.
  end
end

revision_middleware.rb

# lib/revision_middleware.rb

class RevisionMiddleware
  def initialize(app)
    @app = app
    @revision = ENV["GIT_REV"] || "unknown"
  end

  def call(env)
    status, headers, body = @app.call(env)

    headers["X-Revision"] = @revision

    [status, headers, body]
  end
end