Lifecycle hook support

[kibertoad]

It would be helpful to support pre/post-processing of requests/responses. Some of the use-cases that I see are:

• authentication
• logging
• custom container wrapping (e. g. by using Either)

Ideally this feature should come with a certain level of granularity, so that it is either possible to set lifecycle hooks both per Client or globally.

If this is accepted, I can work on implementation.

[mcollina]

I'm positive. This technique can be implemented with very little overhead (see Fastify).

However I would not do global, only per-client and per-pool.

[kibertoad]

@mcollina Can you point out places in fastify code you would recommend to use as reference? Also should I go for full-blown plugin style as fastify does, or implement minimal useful subset of features?

[mcollina]

I would go with minimal features. Here is the hooks implementation: https://github.com/fastify/fastify/blob/master/lib/hooks.js

[kibertoad]

Thanks a lot!

[kibertoad]

@mcollina BTW, there is no engines constraint for Node.js version in package.json, but CI is running on 10+. Do any older Node versions have to be supported?

[ronag]

Do any older Node versions have to be supported?

No only LTS versions.

[arontsang]

I would like to suggest the following signature for extending Dispatchers

function DispatchExtender(dispatcher: Dispatcher, opts: RequestOptions, handler: DispatchHandlers): [RequestOptions, DispatchHandlers]

This way, we can reduce a list of DispatchExtenders when calling dispatch on each dispatcher.

for (const decorator of opts.dispatchExtenders ?? []) {
    [opts, handler] = decorator(this, opts, handler)
}

For example, we can wrap the existing RedirectHandler like this:

function RedirectDecorator (dispatcher, opts, innerHandler) {
  if (dispatcher instanceof Agent) {
    const { maxRedirections = dispatcher[kMaxRedirections] } = opts
    if (maxRedirections != null && maxRedirections !== 0) {
      opts = { ...opts, maxRedirections: 0 } // Stop sub dispatcher from also redirecting.
      return [opts, new RedirectHandler(this, maxRedirections, opts, innerHandler)]
    }
  }
  if (dispatcher instanceof Client) {
    const { maxRedirections = dispatcher[kMaxRedirections] } = opts
    if (maxRedirections) {
      return [opts, new RedirectHandler(this, maxRedirections, opts, innerHandler)]
    }
  }
  return [opts, innerHandler]
}

We can also handle caching as per #1146 using a decorator. The decorator would simply call then innerHandler with the result, then pass a RequestHandler with an already aborted abort signal, so that the Dispatcher will ignore that request.