content_type_provided called twice, or where to set headers

[zoggy]

Hello,

I want to set some headers (cache-control) but there is no method to do so in the resource class. So I set it in the content_type_provided method:

class wm_file content_type blob =
  let md5 = Digest.to_hex (Digest.string blob) in
  fun () ->
  object(self)
    inherit [Cohttp_lwt_body.t] Wm.resource
    method allowed_methods rd = Wm.continue [`GET ] rd
    method content_types_provided rd =
      let resp_headers =
          Cohttp.Header.add_multi rd.Wm.Rd.resp_headers
          "cache-control" ["public"; "must-revalidate"]
      in
      let rd = { rd with Wm.Rd.resp_headers } in
      Wm.continue [ content_type, Wm.continue (`String blob) ] rd
    method content_types_accepted rd = Wm.continue [] rd
    method generate_etag rd = Wm.continue (Some md5) rd
  end

But it seems that this method is called twice (I can see it by adding print instructions). Is this a bug ?
And is there another way to specify headers ? Currently, I end up with duplicated values in the header I specify.

[seliopou]

content_types_provided is indeed called in a few different places. In general none of the resource methods are guaranteed to be called once in the course of processing a request. For example, allowed_methods is called at the b10 state of the diagram, but is also used in the default implementation of the options method. There may be a few more examples like this in the code, but those are the ones that I can point out off the top of my head.

Having said that there is a way to accomplish what you want. The handlers associated with content types are indeed only called once. (If they were called multiple times per request, that would definitely be a bug.) If you set the cache control headers in the content type handler, you will get your desired behavior.

To make it concrete, take a look at this resource from issue #41. If you change the to_content method from that resource to instead read:

method to_content rd =
  let add_cache_control =
    Rd.with_resp_headers (fun headers ->
      Cohtt.Header.add_multi headers "cache-control" ["public"; "must-revalidate"])
  in
  serve_file filename (add_cache_control rd)

You will get your desired behavior (modulo the file reading, etc.).

[zoggy]

Thanks !