Add optional method ProxyTLSConnection (closes #779)

[sleeyax]

Previously, it was impossible to specify both Proxy and NetDialTLSContext on the websocket Dialer without experiencing connection issues. This commit brings a change to the proxy CONNECT flow so that the initial connection is always a normal proxy CONNECT over TCP, while allowing TLS customizations on the existing connection at a later point in time via the new ProxyTLSConnection method.

Fixes #779

Summary of Changes

Please see my comment on the relevant issue for a detailed explanation and my thought process: #779 (comment)

Tests are not included, I'd appreciate some help or guidance on that front because I'm unsure what to test, thanks!

[jaitaiwan]

Thanks for the commit. I know it's been a long time but it would be good to see a test to go with this PR. I'm not fully confident in merging without associated tests.

[jaitaiwan]

It also looks like this PR does something similar: https://github.com/gorilla/websocket/pull/740/files maybe consider using that PR to bolster your own.

[sleeyax]

It also looks like this PR does something similar: https://github.com/gorilla/websocket/pull/740/files maybe consider using that PR to bolster your own.

Not sure if this fixes the issue completely. The PR you mention adds support for HTTPS proxies whereas this PR fixes a bug where NetDialTLSContext and Proxy fields on the websocket.Dialer don't work together. See the referenced issues for more details.

[sleeyax]

Thanks for the commit. I know it's been a long time but it would be good to see a test to go with this PR. I'm not fully confident in merging without associated tests.

It's been so long. Can't say when/if I'll be able to find the time to look into this again and add tests.