fix(quinn-udp): add Apple fast path API stubs for windows and fallback

[mxinden]

Mirror the Apple fast datapath APIs to windows.rs and fallback.rs.

See discussion in #2463 (comment).

[thomaseizinger]

LGTM

Given that they are "experimental" as you said, maybe they should be gated on the feature being enabled?

[mxinden]

Turns out, this is a bit more complex than I thought:

In unix.rs they are currently gated on apple_fast:

quinn/quinn-udp/src/unix.rs

Lines 345 to 364 in c67e9d8

	/// Enables Apple's fast UDP datapath using private `sendmsg_x`/`recvmsg_x` APIs.
	/// Once enabled, this also updates [`max_gso_segments`] to allow batched sends.
	///
	/// # Safety
	///
	/// These APIs may crash on unsupported OS versions, so callers must verify
	/// availability before enabling.
	///
	/// [`max_gso_segments`]: Self::max_gso_segments
	#[cfg(apple_fast)]
	pub unsafe fn set_apple_fast_path(&self) {
	self.apple_fast_path.store(true, Ordering::Relaxed);
	self.max_gso_segments.store(BATCH_SIZE, Ordering::Relaxed);
	}
	/// Returns whether Apple's fast UDP datapath is enabled for this socket.
	#[cfg(apple_fast)]
	pub fn is_apple_fast_path_enabled(&self) -> bool {
	self.apple_fast_path.load(Ordering::Relaxed)
	}

apple_fast is only true on Apple platforms:

quinn/quinn-udp/build.rs

Line 30 in c67e9d8

apple_fast: { all(apple, feature = "fast-apple-datapath") },

Thus e.g. on Linux, the APIs are currently not available.

@thomaseizinger how should we proceed?

1. Make them available on all platforms, including Linux?
2. Make them available on MacOS only (i.e. status quo).

[thomaseizinger]

Make them available on MacOS only (i.e. status quo).

I guess this is the better option then!

[mxinden]

👍 will keep as is then, i.e. closing here.