Continuing of HTTP Call & Proxy Integration (4) - Customize Responses

If you are doing AJAX you know that the XMLHttpRequest object is quite brittle - different browsers offer different behaviors and bugs. One such bug is that IE does not properly the more complicated cousins of text/xml, such as application/atom+xml. In such situations it would be nice to have the proxy customizing the response before it is passed back to the browser.

The simplest way of accomplishing conversion of content-type is to take an keyword parameter:

(define (proxy! (url ($pathinfo)) (headers ($headers))
                #:content-type (content-type (lambda (x) x)))
  (define (helper url headers)
    (raise 
     (http-client-response->response
      (case ($method)
        ((post) (http-post url (request-post-data/raw ($request)) headers))
        ((get) (http-get url headers))
        (else (error 'proxy "proxy method ~a not supported" ($method))))
      content-type)))
  (call-with-values 
   (lambda () 
     (normalize-url+headers url headers))
   helper))

This means we should also modify http-client-response->response:

(define (http-client-response->response r content-type)
  (define (get-content-type r)
    (define (helper header)
      (string->bytes/utf-8 
       (content-type (if (not header) 
                         "text/html; charset=utf-8"
                         (cdr header)))))
     (helper (assf (lambda (key)
                     (string-ci=? key "content-type"))
                   (http-client-response-headers r))))
  (define (normalize-headers r)
    (map (lambda (kv)
           (make-header (string->bytes/utf-8 (car kv))
                        (string->bytes/utf-8 (cdr kv))))
         (http-client-response-headers r)))
  (define (make-generator)
    (lambda (output) 
      (let loop ((b (read-bytes 4096 r)))
        (cond ((eof-object? b)
               (void))
              (else
               (output b)
               (loop (read-bytes 4095 r)))))))
  (make-response/incremental (http-client-response-code r)
                             (string->bytes/utf-8 (http-client-response-reason r))
                             (current-seconds)
                             (get-content-type r)
                             (normalize-headers r)
                             (make-generator)))

And now our proxy can be customized if we want to filter out extended XML headers:

;; proxy.shp
(proxy! #:content-type 
 (lambda (ct) 
  (if (regexp-match #px"^application/.+xml.*$" ct)
      "text/xml"
      ct)))