Setting headers on `ctx.request` in middleware (request proxy bug?)

[ptim]

Hi! Thanks for your work on Sunder - loving it, learning plenty 🙏

I'm not sure if this issue is a bug, or user error…

I'm trying to add headers on the request object so that I can tweak the behaviour of sub-fetches (for caching and image resizing, etc).

I see that HeadersShorthands makes provision for this, but what I'm seeing is:

// in my middleware...
ctx.request['x-foo-header'] = 'bar
console.log(request['x-foo-header']) // 'bar'
console.log(request.headers.get('x-foo-header')) // null

request.headers.set('x-foo-header', 'bar')
// TypeError: immutable
//      at Headers.set ([/project/node_modules/undici/lib/fetch/headers.js:293:13]())

I'm expecting that:

• ctx.request['x-foo-header'] will be handled by the get trap on Sunder's proxied request, and return ctx.request.headers.get('x-foo-header')
• ctx.request['x-foo-header'] = 'bar' would similarly trigger the set trap and mutate request.headers

So I guess there are two issues here:

• am I using the proxy correctly?
• do the HeadersShorthands work as expected? I figure if the set trap was getting hit as expected, then I'd see a type error (I didn't find any relevant tests)

Regards, Tim

[gzuidhof]

Hi Tim,

The shorthands are only for the get, set and delete functions on the Headers object, which is proxied on the request or response object.

So these should be equivalent:

ctx.response.set("my-header", "my-value");
ctx.response.headers.set("my-header", "my-value");

ctx.request.get("some-header");
ctx.request.headers.get("some-header");

I decided against also proxying any ["some-value"] values, as that would stop you from being able to add fields to Request and Response (not sure why you would) and it's a bit too much magic.

[gzuidhof]

As for the request headers being immutable: I haven't run into this before, perhaps incoming request headers are mutable and you have to init a new request using new Request(ctx.request)?
That's not necessarily Sunder specific, here's a thread I found about it: https://community.cloudflare.com/t/how-to-modify-immutable-headers-and-add-nonces-to-response-header/146165.

[ptim]

Thanks very much @gzuidhof 👍

I'm reconsidering my approach, as I think I'm building a footgun!

incoming request headers are mutable immutable and you have to init a new request using new Request(ctx.request)

Indeed! (IIUC, headers on a Response received from fetch are also immutable, but everything works fine in Sunder because the response is created, not received? Related: workers/examples/alter-headers)

It's possible to work around this like so:

import { proxyRequest } from 'sunder/util/request'

// in middleware
ctx.request = proxyRequest(new Request(ctx.request.url, ctx.request))
ctx.request.headers.set('x-foo', 'bar')

...but this is probably a bad idea (better to set the specific request headers when constructing my sub-requests!)

I wonder if it would be a good idea to remove Sunder's Request.set shorthand, or add an errorHint? As it is, Sunder's request.set helper throws "TypeError: immutable"...

[MikelHearon]