Can't implement proxy headers handling with a middleware

[wrzasa]

Hey! I run into this issue when trying to implement handling standard proxy headers into my application and make starlette generate HTTP 307 to the correct URL in case of trailing slashes on the URL. I have a case with a stripped-path proxy that also changes HTTP host (communication with workers via network sockets) and protocol (proxy is a TLS terminator).

Some of the proxy-related headers seem to be handled correctly (e.g. scheme is changed to X-Forwarded-Proto) but some are not.

I tried to implement a middleware to handle the missing pieces. I was able to set X-Forwarded-Prefix to set root_path on the fly and this works. But I failed to handle X-Forwarded-Host. That's because updating server in the Starlette scope to does not change the host in the URL it redirects to. It seems to be caused by this line of code of the URL class:

starlette/starlette/datastructures.py

Line 44 in b588ebe

if host_header is not None:

that instead of looking for a parsed (and maybe updated) host information in the scope's server attribute, goes directly to raw request headers to get it.

I don't understand how this is an expected behavior, I'd expect server attribute to be used here. But before I provide a PR I prefer to ask if using server is an expected fix here. If not, how can I implement handling the X-Forwarded-Host correctly n my app?

[wrzasa]

There seem to be more issues coming from the same piece of code: #1855 Is there a chance for an answer here?