Add pinghub handler to wpcom-xhr-wrapper

[jsnajdr]

Add pinghub support for WebSocket connections to wpcom-xhr-wrapper, the wpcom handler used by Desktop app. The Desktop app uses OAuth and doesn't use the REST proxy iframe.

The pinghub handler intercepts requests to /pinghub/* paths and instead of making HTTP requests (XHR or fetch), handles them specially with WebSockets. In the web version of Calypso, this is done by the REST proxy provider loaded in the iframe.

This code doesn't fully work yet, it's just a proof of concept that shows what needs to be done to make Pinghub WebSockets work in Desktop app.

[matticbot]

Test live: https://calypso.live/?branch=add/wpcom-pinghub-handler

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

App Entrypoints (~1265 bytes added 📈 [gzipped])

name                   parsed_size           gzip_size
entry-main                  +946 B  (+0.1%)     +318 B  (+0.1%)
entry-login                 +946 B  (+0.1%)     +311 B  (+0.1%)
entry-gutenboarding         +946 B  (+0.1%)     +319 B  (+0.1%)
entry-domains-landing       +946 B  (+0.2%)     +317 B  (+0.2%)

Common code that is always downloaded and parsed every time the app is loaded, no matter which route is used.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[nsakaimbo]

Thanks for putting this PR together! Overall makes sense and looks reasonable. Couple quick clarifications for my own education:

1. Is the intention for this logic to "just work" and intercept the existing call to pinghub in the notifications rest client? (I think the answer is "yes" but didn't want to take any details for granted!)

2. Per the proposal in your deep-dive, this would need to be augmented with the OAuth token, yeah? (Assuming the backend will support it?)

[jsnajdr]

Is the intention for this logic to "just work" and intercept the existing call to pinghub in the notifications rest client?

Yes, it will intercept all requests to /pinghub/* and instead of issuing a HTTP request, it will interpret the request as WebSocket processing.

Per the proposal in your deep-dive, this would need to be augmented with the OAuth token, yeah?

Yes, the patch is not finished yet. I didn't add the token to URL, because I have no way to test it. Also, I think the format of responses sent to the callback will need to be different. Instead of { body: { type }, code } it will be { response: { type }, code }? With no way to test it against a real server, I don't know.

[jsnajdr]

An alternative solution to the issue was merged in #46070. This one can be closed.