Add ioxide.Kestrel: ASP.NET Core Kestrel transport on the io_uring reactor

[MDA2AV]

Summary

Adds ioxide.Kestrel — an ASP.NET Core Kestrel transport backed by ioxide. Each connection runs on an ioxide reactor, and Kestrel's whole request loop (recv → parse → handler → send) is pinned to the reactor thread — no ThreadPool hop on the hot path. Drop-in via builder.WebHost.UseIoxide().

Engine impact: minimal and opt-in

Only 3 additive touch points in the engine, with no behavioral change to the native inline path:

• Reactor/Reactor.Post.cs (new): ScheduleOnReactor(Action<object?>, object?) + OnReactorThread, built on the existing _remoteOps/WakeFdWrite machinery — a queue drained once per loop iteration.
• Reactor.cs and Reactor.Incremental.cs: one DrainPostQ(); per loop.

When nothing uses the scheduler (the native runtime, GenHTTP engine, pg/redis/file clients), DrainPostQ is a single TryDequeue on an always-empty queue — effectively free, and inline execution is byte-for-byte unchanged. If you don't reference ioxide.Kestrel, the engine behaves exactly as before.

How the transport works

HopDuplexPipe hands Kestrel two System.IO.Pipelines.Pipes whose reader schedulers are a ReactorPipeScheduler (over Reactor.ScheduleOnReactor). A recv pump copies received bytes into the inbound pipe and a send pump drains the response into the connection's send slab — both on the reactor. Routing the pipe reader continuations to the reactor is what keeps Kestrel's loop there (it's the one Kestrel-sanctioned hook for loop placement).

Usage

var builder = WebApplication.CreateBuilder(args);
builder.WebHost.UseIoxide();                 // replaces the default sockets transport

var app = builder.Build();
app.MapGet("/", () => "Hello, World!");
app.Run();

Options: UseIoxide(o => { o.ReactorCount = …; o.ConfigureServer = cfg => cfg with { … }; }).

Benchmarks (single box, plaintext)

i9-14900K, net11, server pinned to 4 P-cores / 8 reactors, wrk on 16 E-cores, median of 3:

conns	sockets transport	ioxide.Kestrel
64	914K rps (p99 2.65 ms)	1.36M rps (p99 0.52 ms)
256	915K rps	1.45M rps
1024	916K rps	1.37M rps

~1.5× the stock sockets transport, with ~100% of request processing on the reactor thread (verified by thread sampling), 0 errors. Caveats: single-box (the load generator shares the machine), small messages, Linux/io_uring only.

Notes

• New package targets net11.0, references Microsoft.AspNetCore.App, added to ioxide.slnx.
• Public surface is just UseIoxide() + IoxideTransportOptions; everything else is internal.
• Thread-per-core caveat: middleware runs on the reactor thread, so blocking work in a handler stalls every connection on that reactor — keep handlers async.