Add AcceptSocketAsync to SocketTransportOptions

src/Servers/Kestrel/Transport.Sockets/src/PublicAPI.Unshipped.txt

@@ -9,4 +9,7 @@ static Microsoft.AspNetCore.Hosting.WebHostBuilderSocketExtensions.UseSockets(th
 static Microsoft.AspNetCore.Hosting.WebHostBuilderSocketExtensions.UseSockets(this Microsoft.AspNetCore.Hosting.IWebHostBuilder! hostBuilder, System.Action<Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions!>! configureOptions) -> Microsoft.AspNetCore.Hosting.IWebHostBuilder!
 static Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateDefaultBoundListenSocket(System.Net.EndPoint! endpoint) -> System.Net.Sockets.Socket!
 Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateBoundListenSocket.get -> System.Func<System.Net.EndPoint!, System.Net.Sockets.Socket!>!
-Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateBoundListenSocket.set -> void
+Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.CreateBoundListenSocket.set -> void
+static Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.DefaultAcceptSocketAsync(System.Net.Sockets.Socket! listenSocket, System.Threading.CancellationToken cancellationToken) -> System.Threading.Tasks.ValueTask<System.Net.Sockets.Socket!>
+Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.AcceptSocketAsync.get -> System.Func<System.Net.Sockets.Socket!, System.Threading.CancellationToken, System.Threading.Tasks.ValueTask<System.Net.Sockets.Socket!>>!
+Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets.SocketTransportOptions.AcceptSocketAsync.set -> void

src/Servers/Kestrel/Transport.Sockets/src/SocketConnectionListener.cs

@@ -117,7 +117,7 @@ internal void Bind()
                 {
                     Debug.Assert(_listenSocket != null, "Bind must be called first.");
 
-                    var acceptSocket = await _listenSocket.AcceptAsync(cancellationToken);
+                    var acceptSocket = await _options.AcceptSocketAsync(_listenSocket, cancellationToken);
 
                     // Only apply no delay to Tcp based endpoints
                     if (acceptSocket.LocalEndPoint is IPEndPoint)

src/Servers/Kestrel/Transport.Sockets/src/SocketTransportOptions.cs

@@ -80,6 +80,25 @@ public class SocketTransportOptions
         /// </remarks>
         public Func<EndPoint, Socket> CreateBoundListenSocket { get; set; } = CreateDefaultBoundListenSocket;
 
+        /// <summary>
+        /// A function used to accept a new <see cref="Socket"/> given a listening <see cref="Socket"/>.
+        /// </summary>
+        /// <remarks>
+        /// The listening <see cref="Socket"/> passed is the one created by a previous call to <see cref="CreateBoundListenSocket"/>.
+        ///
+        /// This property defaults to <see cref="DefaultAcceptSocketAsync"/>.
+        /// </remarks>
+        public Func<Socket, CancellationToken, ValueTask<Socket>> AcceptSocketAsync { get; set; } = DefaultAcceptSocketAsync;
+
+        /// <summary>
+        /// Accepts a new <see cref="Socket"/> from a listen <see cref="Socket"/> previously obtained from <see cref="CreateBoundListenSocket"/>.
+        /// </summary>
+        /// <param name="listenSocket">A listening <see cref="Socket"/>.</param>
+        /// <param name="cancellationToken">Indicates if the accept operation should be aborted.</param>
+        /// <returns>A newly accepted <see cref="Socket"/>.</returns>
+        public static ValueTask<Socket> DefaultAcceptSocketAsync(Socket listenSocket, CancellationToken cancellationToken)
+        => listenSocket.AcceptAsync(cancellationToken);
+
         /// <summary>
         /// Creates a default instance of <see cref="Socket"/> for the given <see cref="EndPoint"/>
         /// that can be used by a connection listener to listen for inbound requests. <see cref="Socket.Bind"/>

src/Servers/Kestrel/test/Sockets.BindTests/SocketTransportOptionsTests.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Net;
+using System.Net.Http;
 using System.Net.Sockets;
 using System.Runtime.InteropServices;
 using System.Threading.Tasks;
@@ -80,6 +81,37 @@ public void CreateDefaultBoundListenSocket_PreservesLocalEndpointFromFileHandleE
             Assert.Equal(fileHandleSocket.LocalEndPoint, listenSocket.LocalEndPoint);
         }
 
+        [Fact]
+        public async Task VerifySocketTransportCallsAcceptSocketAsync()
+        {
+            var wasCalled = false;
+
+            ValueTask<Socket> AcceptSocketAsync(Socket socket, CancellationToken cancellationToken)
+            {
+                wasCalled = true;
+                return socket.AcceptAsync(cancellationToken);
+            }
+
+            using var host = CreateWebHost(
+                new IPEndPoint(IPAddress.Loopback, 0),
+                options =>
+                {
+                    options.AcceptSocketAsync = AcceptSocketAsync;
+                }
+            );
+
+            await host.StartAsync();
+            using var client = new HttpClient();
+
+            var response = await client.GetAsync($"http://127.0.0.1:{host.GetPort()}/");
+            response.EnsureSuccessStatusCode();
+
+            await host.StopAsync();
+
+            Assert.True(wasCalled, $"Expected {nameof(SocketTransportOptions.AcceptSocketAsync)} to be called.");
+            await host.StopAsync();
+        }
+
         public static IEnumerable<object[]> GetEndpoints()
         {
             // IPv4