How do I use a Rack middleware only for certain paths? - ruby

Question

I'd like to have MyMiddleware run in my Rack app, but only for certain paths. I was hoping to use Rack::Builder or at least Rack::URLMap, but I can't quite figure out how.
This is what I thought would work, but doesn't:
# in my rackup file or Rails environment.rb:
map '/foo' do
use MyMiddleware, { :some => 'options' }
end
Or, better yet, with a Regexp:
map /^foo/ do
use MyMiddleware, { :some => 'options' }
end
But map seems to demand an app at the end; it won't fall back on just passing control back to its parent. (The actual error is "undefined method 'each' for nil:NilClass" from when Rack tries to turn the end of that do...end block into an app.)
Is there a middleware out there that takes an array of middlewares and a path and only runs them if the path matches?

Answer 1

You could have MyMiddleware check the path and not pass control to the next piece of middle ware if it matches.
class MyMiddleware
def initialize app
#app = app
end
def call env
middlewary_stuff if env['PATH_INFO'] == '/foo'
#app.call env
end
def middlewary_stuff
#...
end
end
Or, you could use URLMap w/o the dslness. It would look something like this:
main_app = MainApp.new
Rack::URLMap.new '/'=>main_app, /^(foo|bar)/ => MyMiddleWare.new(main_app)
URLMap is actually pretty simple to grok.

Answer 2

This doesn't work because #app doesn't exist in the right scope:
# in my_app.ru or any Rack::Builder context:
#app = self
map '/foo' do
use MyMiddleware
run lambda { |env| #app.call(env) }
end
But this will:
# in my_app.ru or any Rack::Builder context:
::MAIN_RACK_APP = self
map '/foo' do
use MyMiddleware
run lambda { |env| ::MAIN_RACK_APP.call(env) }
end
Rack::Builder strips the first argument to map off the front of the path, so it doesn't endlessly recurse. Unfortunately, this means that after that path prefix is stripped off, it's unlikely that the rest of the path will properly match other mappings.
Here's an example:
::MAIN_APP = self
use Rack::ShowExceptions
use Rack::Lint
use Rack::Reloader, 0
use Rack::ContentLength
map '/html' do
use MyContentTypeSettingMiddleware, 'text/html'
run lambda { |env| puts 'HTML!'; ::MAIN_APP.call(env) }
end
map '/xml' do
use MyContentTypeSettingMiddleware, 'application/xml'
run lambda { |env| puts 'XML!'; ::MAIN_APP.call(env) }
end
map '/' do
use ContentType, 'text/plain'
run lambda { |env| [ 200, {}, "<p>Hello!</p>" ] }
end
Going to /html/xml causes the following to go to the log:
HTML!
XML!
127.0.0.1 - - [28/May/2009 17:41:42] "GET /html/xml HTTP/1.1" 200 13 0.3626
That is, the app mapped to '/html' strips of the '/html' prefix and the call now matches the app mapped to '/xml'.