I don't really see what this brings other than maybe better ergonomics? The dispatcher interface is already quite extensible? Could this live outside as an npm package?

How would the DispatcherDecorator work?

@mcollina Here is another commit showing what I mean arontsang@d54794f?diff=unified

In effect we add to the constructor of Dispatcher { interceptors: Record<string, DispatchInterceptor[]> }

Where type DispatchInterceptor = Dispatcher[kDispatch] => Dispatcher[kDispatch]
Where type Dispatcher[kDispatch] = (options: Dispatcher.DispatchOptions, handler: Dispatcher.DispatchHandlers) => boolean

We then reduce the array of DispatchInterceptors over this[kDispatch] to produce an intercepted dispatch method.

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I've just noticed that ProxyAgent could be replaced with a DispatchInterceptor quite trivially.

@ronag PTAL

@ronag PTAL

@ronag Any chance you can take a look?

I'll have. A look next week.

I think this can be achieved without modifying existing internals and I think I would maybe prefer that, i.e. implement a compose/decorate/intercept (or other name) function that implements a compose dispatcher that applies this logic. So it would essentially be possible to do the following without any modification of existing code:

const client = undici.intercept(new Client('https://localhost:3000', [ 
  dispatch => function insertHeader(opts, handler){
    opts.headers.push('Authorization', 'Bearer [Some token]')
    return dispatch(opts, handler)
  }
])

Or maybe:

const client = undici.compose(
  () => new Client('https://localhost:3000', 
  dispatch => function insertHeader(opts, handler){
    opts.headers.push('Authorization', 'Bearer [Some token]')
    return dispatch(opts, handler)
  }
)

This might need a few iterations and since it doesn't require any internal modifications we should consider whether or not we should first have it as an external package. Not saying we should. Just to consider.

undici.request(url, {
   decorate: [insertHeaders({ foo: 'bar' }, redirect({ maxRetries: 3 )]
})

This would not work for my use case. I NEED to apply the decorator AT the client level of an Agent.

The decorator needs to be scoped to a single TCP connection to be able to implement NTLM authentication.

Or maybe:

const client = undici.compose(
  () => new Client('https://localhost:3000', 
  dispatch => function insertHeader(opts, handler){
    opts.headers.push('Authorization', 'Bearer [Some token]')
    return dispatch(opts, handler)
  }
)

This works, but it would require an inordinate amount of boiler plate to bring up a full Agent with decorators at the right levels, Agent, Pool, Client etc...

@arontsang Can you provide any details on

Well, if you are already thinking about it. I would like to change the
interface for the DispatchHandler to return headers as string[] rather than
ByteArray[].

This would help reduce the number of Utf8 decodes.

? I can't find where in the code this is happening

@mcollina @ronag I've resolved conflicts and addressed all the outstanding comments, could you please rereview?

upd: type tests are failing, I'll fix them shortly, shouldn't affect any of the runtime code

@mcollina @ronag All green and ready for rereview.

@mcollina Comments addressed!

I would avoid introducing any more global settings, could you avoid them? Thanxk

@mcollina Do you mean new options for Client/Agent/Pool? Where would you place them instead?

@mcollina @ronag What's left before this can be merged?
Also it would be great to release a new version sometime after the merge, would allow our team to start working on actual Undici interceptors :D

Does this support async logic in interceptors? For example, read credentials from the disk in a request interceptor and then add them to request headers.

Also I find it annoying when extending request headers in a request interceptor, the opts.headers type is vague, according to typescript declaration they could be IncomingHttpHeaders | string[] | null, which makes it hard when extending...