Make the global dispatcher truly global

[delvedor]

The current implementation of the global dispatcher is that it stores within the library the global agent. This works well when the user is directly using undici. Still, suppose the user is using a library that internally uses undici and the user didn't install the exact same version of the library. In that case, the global dispatcher API won't work as the user would expect.

To clarify what I think a typical user expects: it should work in the same way as nock.
nock does not suffer from this problem because it will always use the same version of Node.js, while for undici this is not true (yet).

Even if it's kinda ugly, I would suggest attaching the global dispatcher to the Node.js's global object, so different versions (withing the major possibly) of undici can use the same global dispatcher.

If this solution is fine for you (or you have a better idea), I can work on it.

[ronag]

@mcollina I think this is more your area. Wdyt?

[delvedor]

A problem might be compatibility between major versions, but that can be detected by adding some metadata to the global object.

For example:

global.___undiciGlobalDispatcher = {
  version: 5,
  dispatcher: Dispatcher
}

And have undici checking if the global dispatcher uses the same major version as the one configured in the global.

[mcollina]

I thought quite a lot about this in the last few weeks.
The fundamental problem is the incompatibility of multiple versions of Undici. However, the interface for Dispatcher looks quite simple to me: https://undici.nodejs.org/#/docs/api/Dispatcher.

I think we could do this. However, we should be extra careful when doing so and validate compatility.

@ronag do you think it would be possible to use the Agent from one version of undici in the one inside node core?

[mcollina]

This could solve it nodejs/node#42814.

[ronag]

@ronag do you think it would be possible to use the Agent from one version of undici in the one inside node core?

Yes. But then we can't break the interface. Ever

[mcollina]

Pretty much. Are we reading for it?

[delvedor]

Yes. But then we can't break the interface. Ever

Yes and no IMHO. This problem could be solved similarly to what we are doing with Fastify plugins.