site.rb

  def enter!(readonly: false)
    Current.site = self
    @inside = true
    set_appsignal_tags!
    @shardine = Shardine.new(connection_config_hash: database_config.merge(readonly:))
    @shardine.enter!
  end

switcher.rb

class SwitchingMiddleware
  def initialize(app)
    @app = app
  end

  IDEMPOTENT_HTTP_VERBS = %w( GET HEAD OPTIONS QUERY )
  def call(env)
    site = Site.new(detect_sitename_from_request(env))
    return Mappe::NoSiteApp.new.call(env) unless site.configured?

    req = Rack::Request.new(env)
    readonly = IDEMPOTENT_HTTP_VERBS.include?(req.request_method)
    did_enter = site.enter!(readonly:)
    status, header, upstream_body = @app.call(env)
    body = Rack::BodyProxy.new(upstream_body) { site.leave! }

    [status, header, body]
  rescue
    site.leave! if did_enter
    raise
  end
end