feat: add proxy-agent

[RafaelGSS]

Address: #569.

There is a remaining point to be discussed. I've implemented it and tested it against other HTTP libraries (request, axios) and here are a quick HTTP response summary:

Undici

Axios

Request

You will notice that other HTTP libraries answer the HTTP request with Connection: close through the proxy. Nonetheless, undici keeps the connection open with keep-alive. I'm supposing that it's was designed by default, I'm sharing it just to align expectations.

---

TODO:

• Add ProxyAgent test
• Https test
• Add ProxyAgent types test
• Docs

[codecov-commenter]

Codecov Report

Merging #1070 (f4e177a) into main (a821ba7) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1070      +/-   ##
==========================================
+ Coverage   94.93%   94.96%   +0.03%     
==========================================
  Files          38       39       +1     
  Lines        3709     3734      +25     
==========================================
+ Hits         3521     3546      +25     
  Misses        188      188              

Impacted Files	Coverage Δ
lib/core/symbols.js	100.00% <ø> (ø)
index.js	100.00% <100.00%> (ø)
lib/proxy-agent.js	100.00% <100.00%> (ø)

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[mcollina]

Could you add another test with a path as well?

Looks good!

[ronag]

docs?

[RafaelGSS]

I've improved the test coverage and included API documentation in this new class. There is a remaining check to do before making it ready for review which is the usage with HTTPS. I'll do it and share feedback ASAP.

[simoneb]

I would consider rewording this:

• after the first release where this will be available, the version in which it is available will not be the "latest" anymore. When this code is released this comment is basically redundant because the ProxyAgent is available and that's all users need to know. This mention should simply go in the release notes. I would also move it to the top as it's probably going to be the preferred way to set it up
• "avoid some settings" could be reworded as well

[RafaelGSS]

Do you think that this entire document should be refactored to use ProxyAgent or a recommendation on top of the file:

Connecting through a proxy is possible by:

- Using [AgentProxy](docs/api/ProxyAgent.md).
- Configuring `Client` or `Pool` constructor.

should be enough?

[simoneb]

in the agent implementation I see it's dealing with possibly multiple clients. is it something worth testing as well?

[RafaelGSS]

If I'm not mistaken, when dealing with Proxy just one client is set.

Example:

setGlobalDispatcher(new ProxyAgent('http:https://localhost:8000/'))

async function main() {
  const res1 = await request('http:https://localhost:3000/undici')
  console.log('response received', res1.statusCode)

  const res2 = await request('http:https://localhost:4000/undici')
  console.log('response received', res2.statusCode)

}
main()

It will only set http:https://localhost:8000 as a client.

setting client http:https://localhost:8000/
response received 200
response received 201

[RafaelGSS]

Communication with origin HTTPS is now working. It wasn't working because I've missed the Host header and the servers were answering the as bad requests.

[mcollina]

Let's remove this user-agent if it's not strictly needed. If it's strictly required by proxies, let's make sure it's user-configurable (right now is not if I read the code correctly).

[RafaelGSS]

It's not required by proxies and it's totally user-configurable:

  const {
    statusCode,
    headers,
    trailers,
    body
    // send the request via the http:https://localhost:8000/ HTTP proxy
  } = await request('http:https://localhost:3000/undici', { headers: { 'User-Agent': 'Example/0.2.1' } })

Removed support to pre-defined UserAgent.

[mcollina]

lgtm

[RA80533]

This line does not correspond with the actual import of ProxyAgent (index.js#L17).