Remove duplication in serving with and without graceful shutdown

[jplatte]

Simpler implementation of the same idea from #2478, against latest main.

[mladedav]

This is great.

I just wonder if this couldn't go one step further and merge the Serve and WithGracefulShutdown into a single generic type. As it stands now, Serve<M, S> is just WithGracefulShutdown<M, S, Pending>` so why not embrace it?

The only difference I know of is that with the current separation users cannot call with_graceful_shutdown multiple times but I don't think that would be a problem.

[jplatte]

Sure, could do that. But it feels like leaking implementation details into the public API, and I don't see the little bit of extra code as much of a problem.

[jplatte]

I'm so confused about that error. Need to look into it, it's definitely no flake.

[mladedav]

I've taken a look. It's because the graceful shutdown serve code has additional tracing with trace!("connection {remote_addr} accepted"); and another one for closing the connection, but this test tries to parse each tracing line as a json with excpected fields.

So the easiest solutions are to pass the filter in tracing_helpers.rs as a function argument and pass axum::rejection=trace instead of the current axum=trace filter.

Right now the helper is used from just this one test so it could also just be changed in place, but passing the filter in is a bit cleaner.

[jplatte]

Thanks for investigating!
I've pushed a potentially overengineered fix, let me know what you think 😄

[mladedav]

Chaining the with_filter call certainly looks better on the callsite than just passing it in, so this is good with me. Even if a custom future impl maybe is a bit complex solution for a test helper.

[mladedav]

Just curious, why did you do this? I assume that since it's DeserializeOwned, the T will not be a reference so I don't see the point of going out of your way to make it contravariant. Am I missing something?

[jplatte]

fn(T) would be contravariant, fn() -> T is covariant. It's just good form for generic "output" parameters, doesn't actually matter here. See https://doc.rust-lang.org/nomicon/phantom-data.html#table-of-phantomdata-patterns for details.