reverseproxy: Support multiple 'to' upstreams in reverse-proxy cmd

[parrotmac]

From #4692

This is a proposal to add the ability to specify multiple upstream/to targets to the reverse-proxy command.

Example usage:

caddy reverse-proxy --from :8080 --to host1.example.com,host2.example.com

What do we think of the implementation? Any suggestions for a different interface?

[CLAassistant]

All committers have signed the CLA.

[mholt]

Thanks for the contribution!

I think it is more typical to repeat a flag than to have custom parsing within a flag's value, i.e. --to host1 --to host2 instead of --to host1,host2. This would probably be my preference, anyway.

That said, I'm not entirely sure how nicely our flags API handles that at the moment. What do you think?

[francislavoie]

@mholt is there even a way to repeat flags with Go's built-in stuff? We probably can with cobra #4565 but I don't think we can as-is? I don't see a fl.Array() or fl.Slice() etc, cause it wouldn't be type safe 🤔

[mholt]

@francislavoie Yeah, I had to look this up but I knew I did it once before, years ago. We would need to define our own type: https://stackoverflow.com/questions/45487377/golang-flag-ignore-missing-flag-and-parse-multiple-duplicate-flags

Which, I think maybe that's suitable for this. It's just a few lines and very simple. I do think the CLI looks cleaner this way, personally; but maybe that's just my preference. 🤷‍♂️

[francislavoie]

Ah ok, that's not so bad. Yeah I suppose we should do it that way then 👍

[parrotmac]

I think it is more typical to repeat a flag than to have custom parsing within a flag's value, i.e. --to host1 --to host2 instead of --to host1,host2. This would probably be my preference, anyway.

That said, I'm not entirely sure how nicely our flags API handles that at the moment. What do you think?

Yeah, I actually quite prefer the multiple flags form but wasn't sure if it was worth going that route. Here's an example of how that could be implemented. If that's preferred I'm happy to switch to something like that. Edit ha! Looks like y'all beat me to exactly this same thing! 😂

[mholt]

@parrotmac So, did we decide to use multiple --to or to stick with comma separated values? Honestly I'm probably OK with either, I just think the multiple flag form will be clearer.

[mholt]

I refactored it to use multiple --to flags rather than comma-separating them. I think it is slightly more natural. Thank you for the contribution! Sorry it took me a while.