Make calculateDelay promisable

[Giotino]

Checklist

• I have read the documentation.
• I have included some tests.
• If it's a new feature, I have included documentation updates.

Fixes #1262

[szmarczak]

Excellent! This just needs tests :)

Also Node.js 14.2.0 is failing somehow:

  Uncaught exception in test/retry.ts
  RequestError: Cannot call end after a stream was destroyed
  › PromisableRequest._destroy (dist/source/core/index.js:96:618)
  › PromisableRequest.destroy (internal/streams/destroy.js:36:8)
  › errorOrDestroy (internal/streams/destroy.js:146:12)
  › dist/source/core/index.js:94:579
  › endRequest (dist/source/core/index.js:94:221)
  › PromisableRequest._final (dist/source/core/index.js:94:706)

[Giotino]

That issue is less trivial than it seems.
It's a problem with the request destruction.

https://github.com/sindresorhus/got/blob/master/source/as-promise/index.ts#L151,L224

request.once('error', async (error: RequestError) => {
  // Some slow logic
  request.destroy();
});

After the error event, the request fire the response event, but this is prevented by request.destroy().
The problem in my PR is that being the error listener async, the emitter sees it as immediately completed and goes on with the response event because request.destroy() has not been called yet.

While the previous commit make some sense (a86d7b0) that thing should never happen, hence it's not a fix.

[Giotino]

The only thing that came to my mind was restoring the order of the two events (error and response) in the async environment. I used a sync barrier (ae24c2f).

Note: my code needs a lot of cleaup and rewriting.

[Giotino]

BTW, this commit is passing on my Travic CI (and also on my PC).
I think the last commit of sindresorhus/got (2e349d1) broke everything.
https://travis-ci.org/github/Giotino/got/builds/687143936

[szmarczak]

@sindresorhus Is this looking good? I'd like to release a patch version before any API depracations.

[szmarczak]

Nice one! 🙌