templates: Access to proxy response (closes #4420)

[mholt]

This WIP branch allows accessing the reverse proxy response in templates:

Content-Type: {{.ProxyResponse.Header.Get "Content-Type"}}
Body: {{call .ProxyResponse.Body 1024}}

This, of course, reads and drains the response body. I'm not really sure what I'm doing here, so I'm deferring to @francislavoie for a review on this. Maybe there's a better way to handle this. The argument 1024 here is the maximum size to read (since the body has to be buffered).

Accessing the body is admittedly a little awkward currently, since you have to use call. I tried without call and it said Body has arguments but cannot be invoked as function, so 🤷‍♂️ I guess you can't invoke functions as functions when they're struct fields: https://go.dev/src/text/template/exec.go?s=5814:5878

@byoungdale could you please give this a try?

[mholt]

@byoungdale Is there still interest in this? Would be a shame to have to close it if we can't confirm this meets the requirements of the original feature request.

[byoungdale]

I would still be interested, but unfortunately, I have no time at the moment to try it out.

[mholt]

@byoungdale When will you try this?

[mholt]

Closing due to inactivity :(

We can merge this if someone who needs it tests it to confirm it works for them (rather than me just assuming it works for a real use case).

[frink]

I'm facing similar needs and can try. I know it's been awhile but If you want to reopen I'll test it.

[mholt]

@frink Go for it 👍

[mholt]

Closing due to merge conflicts and inactivity. If anyone wants to take this up and fix the conflict, and test that it works for their use case, we can reopen.

[MurrayGroves]

Would it be easy to modify this to allow passing the response body into another reverse_proxy? My usecase is to intercept all HTML responses from a service and add a script to their head section. From what I can tell the easiest way to do that would be to use some modified version of this feature to pass the response body to a service that returns a new HTML body with the script added.