asp.net core 3.1 源码分析之KestrelServer

public class KestrelServer : IServer
    {
        private readonly List<(IConnectionListener, Task)> _transports = new List<(IConnectionListener, Task)>();
        private readonly IServerAddressesFeature _serverAddresses;
        private readonly IConnectionListenerFactory _transportFactory;

        private bool _hasStarted;
        private int _stopping;
        private readonly TaskCompletionSource<object> _stoppedTcs = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);

        public KestrelServer(
            IOptions<KestrelServerOptions> options,
            IConnectionListenerFactory transportFactory,
            ILoggerFactory loggerFactory)
            : this(transportFactory, CreateServiceContext(options, loggerFactory))
        {
        }

        // For testing
        internal KestrelServer(IConnectionListenerFactory transportFactory, ServiceContext serviceContext)
        {
            if (transportFactory == null)
            {
                throw new ArgumentNullException(nameof(transportFactory));
            }

            _transportFactory = transportFactory;
            ServiceContext = serviceContext;

            Features = new FeatureCollection();
            _serverAddresses = new ServerAddressesFeature();
            Features.Set(_serverAddresses);

            HttpCharacters.Initialize();
        }

        private static ServiceContext CreateServiceContext(IOptions<KestrelServerOptions> options, ILoggerFactory loggerFactory)
        {
            if (options == null)
            {
                throw new ArgumentNullException(nameof(options));
            }
            if (loggerFactory == null)
            {
                throw new ArgumentNullException(nameof(loggerFactory));
            }

            var serverOptions = options.Value ?? new KestrelServerOptions();
            var logger = loggerFactory.CreateLogger("Microsoft.AspNetCore.Server.Kestrel");
            var trace = new KestrelTrace(logger);
            var connectionManager = new ConnectionManager(
                trace,
                serverOptions.Limits.MaxConcurrentUpgradedConnections);

            var heartbeatManager = new HeartbeatManager(connectionManager);
            var dateHeaderValueManager = new DateHeaderValueManager();
            var heartbeat = new Heartbeat(
                new IHeartbeatHandler[] { dateHeaderValueManager, heartbeatManager },
                new SystemClock(),
                DebuggerWrapper.Singleton,
                trace);

            return new ServiceContext
            {
                Log = trace,
                HttpParser = new HttpParser<Http1ParsingHandler>(trace.IsEnabled(LogLevel.Information)),
                Scheduler = PipeScheduler.ThreadPool,
                SystemClock = heartbeatManager,
                DateHeaderValueManager = dateHeaderValueManager,
                ConnectionManager = connectionManager,
                Heartbeat = heartbeat,
                ServerOptions = serverOptions,
            };
        }

        public IFeatureCollection Features { get; }

        public KestrelServerOptions Options => ServiceContext.ServerOptions;

        private ServiceContext ServiceContext { get; }

        private IKestrelTrace Trace => ServiceContext.Log;

        private ConnectionManager ConnectionManager => ServiceContext.ConnectionManager;

        public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken)
        {
            try
            {
                if (!BitConverter.IsLittleEndian)
                {
                    throw new PlatformNotSupportedException(CoreStrings.BigEndianNotSupported);
                }

                ValidateOptions();

                if (_hasStarted)
                {
                    // The server has already started and/or has not been cleaned up yet
                    throw new InvalidOperationException(CoreStrings.ServerAlreadyStarted);
                }
                _hasStarted = true;

                ServiceContext.Heartbeat?.Start();

                async Task OnBind(ListenOptions options)
                {
                    // Add the HTTP middleware as the terminal connection middleware
                    options.UseHttpServer(ServiceContext, application, options.Protocols);

                    var connectionDelegate = options.Build();

                    // Add the connection limit middleware
                    if (Options.Limits.MaxConcurrentConnections.HasValue)
                    {
                        connectionDelegate = new ConnectionLimitMiddleware(connectionDelegate, Options.Limits.MaxConcurrentConnections.Value, Trace).OnConnectionAsync;
                    }

                    var connectionDispatcher = new ConnectionDispatcher(ServiceContext, connectionDelegate);
                    var transport = await _transportFactory.BindAsync(options.EndPoint).ConfigureAwait(false);

                    // Update the endpoint
                    options.EndPoint = transport.EndPoint;
                    var acceptLoopTask = connectionDispatcher.StartAcceptingConnections(transport);

                    _transports.Add((transport, acceptLoopTask));
                }

                await AddressBinder.BindAsync(_serverAddresses, Options, Trace, OnBind).ConfigureAwait(false);
            }
            catch (Exception ex)
            {
                Trace.LogCritical(0, ex, "Unable to start Kestrel.");
                Dispose();
                throw;
            }
        }

        // Graceful shutdown if possible
        public async Task StopAsync(CancellationToken cancellationToken)
        {
            if (Interlocked.Exchange(ref _stopping, 1) == 1)
            {
                await _stoppedTcs.Task.ConfigureAwait(false);
                return;
            }

            try
            {
                var tasks = new Task[_transports.Count];
                for (int i = 0; i < _transports.Count; i++)
                {
                    (IConnectionListener listener, Task acceptLoop) = _transports[i];
                    tasks[i] = Task.WhenAll(listener.UnbindAsync(cancellationToken).AsTask(), acceptLoop);
                }

                await Task.WhenAll(tasks).ConfigureAwait(false);

                if (!await ConnectionManager.CloseAllConnectionsAsync(cancellationToken).ConfigureAwait(false))
                {
                    Trace.NotAllConnectionsClosedGracefully();

                    if (!await ConnectionManager.AbortAllConnectionsAsync().ConfigureAwait(false))
                    {
                        Trace.NotAllConnectionsAborted();
                    }
                }

                for (int i = 0; i < _transports.Count; i++)
                {
                    (IConnectionListener listener, Task acceptLoop) = _transports[i];
                    tasks[i] = listener.DisposeAsync().AsTask();
                }

                await Task.WhenAll(tasks).ConfigureAwait(false);

                ServiceContext.Heartbeat?.Dispose();
            }
            catch (Exception ex)
            {
                _stoppedTcs.TrySetException(ex);
                throw;
            }

            _stoppedTcs.TrySetResult(null);
        }

        // Ungraceful shutdown
        public void Dispose()
        {
            var cancelledTokenSource = new CancellationTokenSource();
            cancelledTokenSource.Cancel();
            StopAsync(cancelledTokenSource.Token).GetAwaiter().GetResult();
        }

        private void ValidateOptions()
        {
            Options.ConfigurationLoader?.Load();

            if (Options.Limits.MaxRequestBufferSize.HasValue &&
                Options.Limits.MaxRequestBufferSize < Options.Limits.MaxRequestLineSize)
            {
                throw new InvalidOperationException(
                    CoreStrings.FormatMaxRequestBufferSmallerThanRequestLineBuffer(Options.Limits.MaxRequestBufferSize.Value, Options.Limits.MaxRequestLineSize));
            }

            if (Options.Limits.MaxRequestBufferSize.HasValue &&
                Options.Limits.MaxRequestBufferSize < Options.Limits.MaxRequestHeadersTotalSize)
            {
                throw new InvalidOperationException(
                    CoreStrings.FormatMaxRequestBufferSmallerThanRequestHeaderBuffer(Options.Limits.MaxRequestBufferSize.Value, Options.Limits.MaxRequestHeadersTotalSize));
            }
        }
    }

KestrelServer类本身的代码并不多

主要看下StartAsync核心方法，内部有个OnBind方法

async Task OnBind(ListenOptions options)
                {
                    // Add the HTTP middleware as the terminal connection middleware
                    options.UseHttpServer(ServiceContext, application, options.Protocols);

                    var connectionDelegate = options.Build();

                    // Add the connection limit middleware
                    if (Options.Limits.MaxConcurrentConnections.HasValue)
                    {
                        connectionDelegate = new ConnectionLimitMiddleware(connectionDelegate, Options.Limits.MaxConcurrentConnections.Value, Trace).OnConnectionAsync;
                    }

                    var connectionDispatcher = new ConnectionDispatcher(ServiceContext, connectionDelegate);
                    var transport = await _transportFactory.BindAsync(options.EndPoint).ConfigureAwait(false);

                    // Update the endpoint
                    options.EndPoint = transport.EndPoint;
                    var acceptLoopTask = connectionDispatcher.StartAcceptingConnections(transport);

                    _transports.Add((transport, acceptLoopTask));
                }

看下ListenOptions参数

internal static class HttpConnectionBuilderExtensions
    {
        public static IConnectionBuilder UseHttpServer<TContext>(
            this IConnectionBuilder builder,
            ServiceContext serviceContext,
            IHttpApplication<TContext> application,
            HttpProtocols protocols)
        {
            var middleware = new HttpConnectionMiddleware<TContext>(serviceContext, application, protocols);
            return builder.Use(next =>
            {
                return middleware.OnConnectionAsync;
            });
        }
    }

public interface IConnectionBuilder
    {
        /// <summary>
        /// Gets the <see cref="IServiceProvider"/> that provides access to the application's service container.
        /// </summary>
        IServiceProvider ApplicationServices { get; }

        /// <summary>
        /// Adds a middleware delegate to the application's connection pipeline.
        /// </summary>
        /// <param name="middleware">The middleware delegate.</param>
        /// <returns>The <see cref="IConnectionBuilder"/>.</returns>
        IConnectionBuilder Use(Func<ConnectionDelegate, ConnectionDelegate> middleware);

        /// <summary>
        /// Builds the delegate used by this application to process connections.
        /// </summary>
        /// <returns>The connection handling delegate.</returns>
        ConnectionDelegate Build();
    }

public class ConnectionBuilder : IConnectionBuilder
    {
        private readonly IList<Func<ConnectionDelegate, ConnectionDelegate>> _components = new List<Func<ConnectionDelegate, ConnectionDelegate>>();

        public IServiceProvider ApplicationServices { get; }

        public ConnectionBuilder(IServiceProvider applicationServices)
        {
            ApplicationServices = applicationServices;
        }

        public IConnectionBuilder Use(Func<ConnectionDelegate, ConnectionDelegate> middleware)
        {
            _components.Add(middleware);
            return this;
        }

        public ConnectionDelegate Build()
        {
            ConnectionDelegate app = features =>
            {
                return Task.CompletedTask;
            };

            foreach (var component in _components.Reverse())
            {
                app = component(app);
            }

            return app;
        }
    }

ConnectionBuilder构建一个处理http连接的委托链

public delegate Task ConnectionDelegate(ConnectionContext connection);

HttpConnectionMiddleware建立一个HttpConnection对象，调用ProcessRequestsAsync方法处理请求

先看下服务器是如何监听http请求的

public interface IConnectionListenerFactory
    {
        /// <summary>
        /// Creates an <see cref="IConnectionListener"/> bound to the specified <see cref="EndPoint"/>.
        /// </summary>
        /// <param name="endpoint">The <see cref="EndPoint" /> to bind to.</param>
        /// <param name="cancellationToken">The token to monitor for cancellation requests.</param>
        /// <returns>A <see cref="ValueTask{IConnectionListener}"/> that completes when the listener has been bound, yielding a <see cref="IConnectionListener" /> representing the new listener.</returns>
        ValueTask<IConnectionListener> BindAsync(EndPoint endpoint, CancellationToken cancellationToken = default);
    }

public sealed class SocketTransportFactory : IConnectionListenerFactory
    {
        private readonly SocketTransportOptions _options;
        private readonly SocketsTrace _trace;

        public SocketTransportFactory(
            IOptions<SocketTransportOptions> options,
            ILoggerFactory loggerFactory)
        {
            if (options == null)
            {
                throw new ArgumentNullException(nameof(options));
            }

            if (loggerFactory == null)
            {
                throw new ArgumentNullException(nameof(loggerFactory));
            }

            _options = options.Value;
            var logger = loggerFactory.CreateLogger("Microsoft.AspNetCore.Server.Kestrel.Transport.Sockets");
            _trace = new SocketsTrace(logger);
        }

        public ValueTask<IConnectionListener> BindAsync(EndPoint endpoint, CancellationToken cancellationToken = default)
        {
            var transport = new SocketConnectionListener(endpoint, _options, _trace);
            transport.Bind();
            return new ValueTask<IConnectionListener>(transport);
        }
    }

internal sealed class SocketConnectionListener : IConnectionListener
    {
        private readonly MemoryPool<byte> _memoryPool;
        private readonly int _numSchedulers;
        private readonly PipeScheduler[] _schedulers;
        private readonly ISocketsTrace _trace;
        private Socket _listenSocket;
        private int _schedulerIndex;
        private readonly SocketTransportOptions _options;

        public EndPoint EndPoint { get; private set; }

        internal SocketConnectionListener(
            EndPoint endpoint,
            SocketTransportOptions options,
            ISocketsTrace trace)
        {
            EndPoint = endpoint;
            _trace = trace;
            _options = options;
            _memoryPool = _options.MemoryPoolFactory();
            var ioQueueCount = options.IOQueueCount;

            if (ioQueueCount > 0)
            {
                _numSchedulers = ioQueueCount;
                _schedulers = new IOQueue[_numSchedulers];

                for (var i = 0; i < _numSchedulers; i++)
                {
                    _schedulers[i] = new IOQueue();
                }
            }
            else
            {
                var directScheduler = new PipeScheduler[] { PipeScheduler.ThreadPool };
                _numSchedulers = directScheduler.Length;
                _schedulers = directScheduler;
            }
        }

        internal void Bind()
        {
            if (_listenSocket != null)
            {
                throw new InvalidOperationException(SocketsStrings.TransportAlreadyBound);
            }

            Socket listenSocket;

            // Unix domain sockets are unspecified
            var protocolType = EndPoint is UnixDomainSocketEndPoint ? ProtocolType.Unspecified : ProtocolType.Tcp;

            listenSocket = new Socket(EndPoint.AddressFamily, SocketType.Stream, protocolType);

            // Kestrel expects IPv6Any to bind to both IPv6 and IPv4
            if (EndPoint is IPEndPoint ip && ip.Address == IPAddress.IPv6Any)
            {
                listenSocket.DualMode = true;
            }

            try
            {
                listenSocket.Bind(EndPoint);
            }
            catch (SocketException e) when (e.SocketErrorCode == SocketError.AddressAlreadyInUse)
            {
                throw new AddressInUseException(e.Message, e);
            }

            EndPoint = listenSocket.LocalEndPoint;

            listenSocket.Listen(512);

            _listenSocket = listenSocket;
        }

        public async ValueTask<ConnectionContext> AcceptAsync(CancellationToken cancellationToken = default)
        {
            while (true)
            {
                try
                {
                    var acceptSocket = await _listenSocket.AcceptAsync();

                    // Only apply no delay to Tcp based endpoints
                    if (acceptSocket.LocalEndPoint is IPEndPoint)
                    {
                        acceptSocket.NoDelay = _options.NoDelay;
                    }

                    var connection = new SocketConnection(acceptSocket, _memoryPool, _schedulers[_schedulerIndex], _trace, _options.MaxReadBufferSize, _options.MaxWriteBufferSize);

                    connection.Start();

                    _schedulerIndex = (_schedulerIndex + 1) % _numSchedulers;

                    return connection;
                }
                catch (ObjectDisposedException)
                {
                    // A call was made to UnbindAsync/DisposeAsync just return null which signals we're done
                    return null;
                }
                catch (SocketException e) when (e.SocketErrorCode == SocketError.OperationAborted)
                {
                    // A call was made to UnbindAsync/DisposeAsync just return null which signals we're done
                    return null;
                }
                catch (SocketException)
                {
                    // The connection got reset while it was in the backlog, so we try again.
                    _trace.ConnectionReset(connectionId: "(null)");
                }
            }
        }

        public ValueTask UnbindAsync(CancellationToken cancellationToken = default)
        {
            _listenSocket?.Dispose();
            return default;
        }

        public ValueTask DisposeAsync()
        {
            _listenSocket?.Dispose();
            // Dispose the memory pool
            _memoryPool.Dispose();
            return default;
        }
    }

SocketTransportFactory创建SocketConnectionListener，监听对应的端口

并返回SocketConnection

 /// <summary>
    /// Encapsulates all information about an individual connection.
    /// </summary>
    public abstract class ConnectionContext : IAsyncDisposable
    {
        /// <summary>
        /// Gets or sets a unique identifier to represent this connection in trace logs.
        /// </summary>
        public abstract string ConnectionId { get; set; }

        /// <summary>
        /// Gets the collection of features provided by the server and middleware available on this connection.
        /// </summary>
        public abstract IFeatureCollection Features { get; }

        /// <summary>
        /// Gets or sets a key/value collection that can be used to share data within the scope of this connection.
        /// </summary>
        public abstract IDictionary<object, object> Items { get; set; }

        /// <summary>
        /// Gets or sets the <see cref="IDuplexPipe"/> that can be used to read or write data on this connection.
        /// </summary>
        public abstract IDuplexPipe Transport { get; set; }

        /// <summary>
        /// Triggered when the client connection is closed.
        /// </summary>
        public virtual CancellationToken ConnectionClosed { get; set; }

        /// <summary>
        /// Gets or sets the local endpoint for this connection.
        /// </summary>
        public virtual EndPoint LocalEndPoint { get; set; }

        /// <summary>
        /// Gets or sets the remote endpoint for this connection.
        /// </summary>
        public virtual EndPoint RemoteEndPoint { get; set; }

        /// <summary>
        /// Aborts the underlying connection.
        /// </summary>
        /// <param name="abortReason">An optional <see cref="ConnectionAbortedException"/> describing the reason the connection is being terminated.</param>
        public virtual void Abort(ConnectionAbortedException abortReason)
        {
            // We expect this to be overridden, but this helps maintain back compat
            // with implementations of ConnectionContext that predate the addition of
            // ConnectionContext.Abort()
            Features.Get<IConnectionLifetimeFeature>()?.Abort();
        }

        /// <summary>
        /// Aborts the underlying connection.
        /// </summary>
        public virtual void Abort() => Abort(new ConnectionAbortedException("The connection was aborted by the application via ConnectionContext.Abort()."));

        /// <summary>
        /// Releases resources for the underlying connection.
        /// </summary>
        /// <returns>A <see cref="ValueTask"/> that completes when resources have been released.</returns>
        public virtual ValueTask DisposeAsync()
        {
            return default;
        }
    }

internal abstract partial class TransportConnection : ConnectionContext
    {
        private IDictionary<object, object> _items;
        private string _connectionId;

        public TransportConnection()
        {
            FastReset();
        }

        public override EndPoint LocalEndPoint { get; set; }
        public override EndPoint RemoteEndPoint { get; set; }

        public override string ConnectionId
        {
            get
            {
                if (_connectionId == null)
                {
                    _connectionId = CorrelationIdGenerator.GetNextId();
                }

                return _connectionId;
            }
            set
            {
                _connectionId = value;
            }
        }

        public override IFeatureCollection Features => this;

        public virtual MemoryPool<byte> MemoryPool { get; }

        public override IDuplexPipe Transport { get; set; }

        public IDuplexPipe Application { get; set; }

        public override IDictionary<object, object> Items
        {
            get
            {
                // Lazily allocate connection metadata
                return _items ?? (_items = new ConnectionItems());
            }
            set
            {
                _items = value;
            }
        }

        public override CancellationToken ConnectionClosed { get; set; }

        // DO NOT remove this override to ConnectionContext.Abort. Doing so would cause
        // any TransportConnection that does not override Abort or calls base.Abort
        // to stack overflow when IConnectionLifetimeFeature.Abort() is called.
        // That said, all derived types should override this method should override
        // this implementation of Abort because canceling pending output reads is not
        // sufficient to abort the connection if there is backpressure.
        public override void Abort(ConnectionAbortedException abortReason)
        {
            Application.Input.CancelPendingRead();
        }
    }

internal partial class TransportConnection : IConnectionIdFeature,
                                                 IConnectionTransportFeature,
                                                 IConnectionItemsFeature,
                                                 IMemoryPoolFeature,
                                                 IConnectionLifetimeFeature
    {
        // NOTE: When feature interfaces are added to or removed from this TransportConnection class implementation,
        // then the list of `features` in the generated code project MUST also be updated.
        // See also: tools/CodeGenerator/TransportConnectionFeatureCollection.cs

        MemoryPool<byte> IMemoryPoolFeature.MemoryPool => MemoryPool;

        IDuplexPipe IConnectionTransportFeature.Transport
        {
            get => Transport;
            set => Transport = value;
        }

        IDictionary<object, object> IConnectionItemsFeature.Items
        {
            get => Items;
            set => Items = value;
        }

        CancellationToken IConnectionLifetimeFeature.ConnectionClosed
        {
            get => ConnectionClosed;
            set => ConnectionClosed = value;
        }

        void IConnectionLifetimeFeature.Abort() => Abort(new ConnectionAbortedException("The connection was aborted by the application via IConnectionLifetimeFeature.Abort()."));
    }

 internal partial class TransportConnection : IFeatureCollection
    {
        private static readonly Type IConnectionIdFeatureType = typeof(IConnectionIdFeature);
        private static readonly Type IConnectionTransportFeatureType = typeof(IConnectionTransportFeature);
        private static readonly Type IConnectionItemsFeatureType = typeof(IConnectionItemsFeature);
        private static readonly Type IMemoryPoolFeatureType = typeof(IMemoryPoolFeature);
        private static readonly Type IConnectionLifetimeFeatureType = typeof(IConnectionLifetimeFeature);

        private object _currentIConnectionIdFeature;
        private object _currentIConnectionTransportFeature;
        private object _currentIConnectionItemsFeature;
        private object _currentIMemoryPoolFeature;
        private object _currentIConnectionLifetimeFeature;

        private int _featureRevision;

        private List<KeyValuePair<Type, object>> MaybeExtra;

        private void FastReset()
        {
            _currentIConnectionIdFeature = this;
            _currentIConnectionTransportFeature = this;
            _currentIConnectionItemsFeature = this;
            _currentIMemoryPoolFeature = this;
            _currentIConnectionLifetimeFeature = this;

        }

        // Internal for testing
        internal void ResetFeatureCollection()
        {
            FastReset();
            MaybeExtra?.Clear();
            _featureRevision++;
        }

        private object ExtraFeatureGet(Type key)
        {
            if (MaybeExtra == null)
            {
                return null;
            }
            for (var i = 0; i < MaybeExtra.Count; i++)
            {
                var kv = MaybeExtra[i];
                if (kv.Key == key)
                {
                    return kv.Value;
                }
            }
            return null;
        }

        private void ExtraFeatureSet(Type key, object value)
        {
            if (MaybeExtra == null)
            {
                MaybeExtra = new List<KeyValuePair<Type, object>>(2);
            }

            for (var i = 0; i < MaybeExtra.Count; i++)
            {
                if (MaybeExtra[i].Key == key)
                {
                    MaybeExtra[i] = new KeyValuePair<Type, object>(key, value);
                    return;
                }
            }
            MaybeExtra.Add(new KeyValuePair<Type, object>(key, value));
        }

        bool IFeatureCollection.IsReadOnly => false;

        int IFeatureCollection.Revision => _featureRevision;

        object IFeatureCollection.this[Type key]
        {
            get
            {
                object feature = null;
                if (key == IConnectionIdFeatureType)
                {
                    feature = _currentIConnectionIdFeature;
                }
                else if (key == IConnectionTransportFeatureType)
                {
                    feature = _currentIConnectionTransportFeature;
                }
                else if (key == IConnectionItemsFeatureType)
                {
                    feature = _currentIConnectionItemsFeature;
                }
                else if (key == IMemoryPoolFeatureType)
                {
                    feature = _currentIMemoryPoolFeature;
                }
                else if (key == IConnectionLifetimeFeatureType)
                {
                    feature = _currentIConnectionLifetimeFeature;
                }
                else if (MaybeExtra != null)
                {
                    feature = ExtraFeatureGet(key);
                }

                return feature;
            }

            set
            {
                _featureRevision++;

                if (key == IConnectionIdFeatureType)
                {
                    _currentIConnectionIdFeature = value;
                }
                else if (key == IConnectionTransportFeatureType)
                {
                    _currentIConnectionTransportFeature = value;
                }
                else if (key == IConnectionItemsFeatureType)
                {
                    _currentIConnectionItemsFeature = value;
                }
                else if (key == IMemoryPoolFeatureType)
                {
                    _currentIMemoryPoolFeature = value;
                }
                else if (key == IConnectionLifetimeFeatureType)
                {
                    _currentIConnectionLifetimeFeature = value;
                }
                else
                {
                    ExtraFeatureSet(key, value);
                }
            }
        }

        TFeature IFeatureCollection.Get<TFeature>()
        {
            TFeature feature = default;
            if (typeof(TFeature) == typeof(IConnectionIdFeature))
            {
                feature = (TFeature)_currentIConnectionIdFeature;
            }
            else if (typeof(TFeature) == typeof(IConnectionTransportFeature))
            {
                feature = (TFeature)_currentIConnectionTransportFeature;
            }
            else if (typeof(TFeature) == typeof(IConnectionItemsFeature))
            {
                feature = (TFeature)_currentIConnectionItemsFeature;
            }
            else if (typeof(TFeature) == typeof(IMemoryPoolFeature))
            {
                feature = (TFeature)_currentIMemoryPoolFeature;
            }
            else if (typeof(TFeature) == typeof(IConnectionLifetimeFeature))
            {
                feature = (TFeature)_currentIConnectionLifetimeFeature;
            }
            else if (MaybeExtra != null)
            {
                feature = (TFeature)(ExtraFeatureGet(typeof(TFeature)));
            }

            return feature;
        }

        void IFeatureCollection.Set<TFeature>(TFeature feature)
        {
            _featureRevision++;
            if (typeof(TFeature) == typeof(IConnectionIdFeature))
            {
                _currentIConnectionIdFeature = feature;
            }
            else if (typeof(TFeature) == typeof(IConnectionTransportFeature))
            {
                _currentIConnectionTransportFeature = feature;
            }
            else if (typeof(TFeature) == typeof(IConnectionItemsFeature))
            {
                _currentIConnectionItemsFeature = feature;
            }
            else if (typeof(TFeature) == typeof(IMemoryPoolFeature))
            {
                _currentIMemoryPoolFeature = feature;
            }
            else if (typeof(TFeature) == typeof(IConnectionLifetimeFeature))
            {
                _currentIConnectionLifetimeFeature = feature;
            }
            else
            {
                ExtraFeatureSet(typeof(TFeature), feature);
            }
        }

        private IEnumerable<KeyValuePair<Type, object>> FastEnumerable()
        {
            if (_currentIConnectionIdFeature != null)
            {
                yield return new KeyValuePair<Type, object>(IConnectionIdFeatureType, _currentIConnectionIdFeature);
            }
            if (_currentIConnectionTransportFeature != null)
            {
                yield return new KeyValuePair<Type, object>(IConnectionTransportFeatureType, _currentIConnectionTransportFeature);
            }
            if (_currentIConnectionItemsFeature != null)
            {
                yield return new KeyValuePair<Type, object>(IConnectionItemsFeatureType, _currentIConnectionItemsFeature);
            }
            if (_currentIMemoryPoolFeature != null)
            {
                yield return new KeyValuePair<Type, object>(IMemoryPoolFeatureType, _currentIMemoryPoolFeature);
            }
            if (_currentIConnectionLifetimeFeature != null)
            {
                yield return new KeyValuePair<Type, object>(IConnectionLifetimeFeatureType, _currentIConnectionLifetimeFeature);
            }

            if (MaybeExtra != null)
            {
                foreach (var item in MaybeExtra)
                {
                    yield return item;
                }
            }
        }

        IEnumerator<KeyValuePair<Type, object>> IEnumerable<KeyValuePair<Type, object>>.GetEnumerator() => FastEnumerable().GetEnumerator();

        IEnumerator IEnumerable.GetEnumerator() => FastEnumerable().GetEnumerator();
    }

internal sealed class SocketConnection : TransportConnection
    {
        private static readonly int MinAllocBufferSize = SlabMemoryPool.BlockSize / 2;
        private static readonly bool IsWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
        private static readonly bool IsMacOS = RuntimeInformation.IsOSPlatform(OSPlatform.OSX);

        private readonly Socket _socket;
        private readonly ISocketsTrace _trace;
        private readonly SocketReceiver _receiver;
        private readonly SocketSender _sender;
        private readonly CancellationTokenSource _connectionClosedTokenSource = new CancellationTokenSource();

        private readonly object _shutdownLock = new object();
        private volatile bool _socketDisposed;
        private volatile Exception _shutdownReason;
        private Task _processingTask;
        private readonly TaskCompletionSource<object> _waitForConnectionClosedTcs = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);
        private bool _connectionClosed;

        internal SocketConnection(Socket socket,
                                  MemoryPool<byte> memoryPool,
                                  PipeScheduler scheduler,
                                  ISocketsTrace trace,
                                  long? maxReadBufferSize = null,
                                  long? maxWriteBufferSize = null)
        {
            Debug.Assert(socket != null);
            Debug.Assert(memoryPool != null);
            Debug.Assert(trace != null);

            _socket = socket;
            MemoryPool = memoryPool;
            _trace = trace;

            LocalEndPoint = _socket.LocalEndPoint;
            RemoteEndPoint = _socket.RemoteEndPoint;

            ConnectionClosed = _connectionClosedTokenSource.Token;

            // On *nix platforms, Sockets already dispatches to the ThreadPool.
            // Yes, the IOQueues are still used for the PipeSchedulers. This is intentional.
            // https://github.com/aspnet/KestrelHttpServer/issues/2573
            var awaiterScheduler = IsWindows ? scheduler : PipeScheduler.Inline;

            _receiver = new SocketReceiver(_socket, awaiterScheduler);
            _sender = new SocketSender(_socket, awaiterScheduler);

            maxReadBufferSize ??= 0;
            maxWriteBufferSize ??= 0;

            var inputOptions = new PipeOptions(MemoryPool, PipeScheduler.ThreadPool, scheduler, maxReadBufferSize.Value, maxReadBufferSize.Value / 2, useSynchronizationContext: false);
            var outputOptions = new PipeOptions(MemoryPool, scheduler, PipeScheduler.ThreadPool, maxWriteBufferSize.Value, maxWriteBufferSize.Value / 2, useSynchronizationContext: false);

            var pair = DuplexPipe.CreateConnectionPair(inputOptions, outputOptions);

            // Set the transport and connection id
            Transport = pair.Transport;
            Application = pair.Application;
        }

        public PipeWriter Input => Application.Output;

        public PipeReader Output => Application.Input;

        public override MemoryPool<byte> MemoryPool { get; }

        public void Start()
        {
            _processingTask = StartAsync();
        }

        private async Task StartAsync()
        {
            try
            {
                // Spawn send and receive logic
                var receiveTask = DoReceive();
                var sendTask = DoSend();

                // Now wait for both to complete
                await receiveTask;
                await sendTask;

                _receiver.Dispose();
                _sender.Dispose();
            }
            catch (Exception ex)
            {
                _trace.LogError(0, ex, $"Unexpected exception in {nameof(SocketConnection)}.{nameof(StartAsync)}.");
            }
        }

        public override void Abort(ConnectionAbortedException abortReason)
        {
            // Try to gracefully close the socket to match libuv behavior.
            Shutdown(abortReason);

            // Cancel ProcessSends loop after calling shutdown to ensure the correct _shutdownReason gets set.
            Output.CancelPendingRead();
        }

        // Only called after connection middleware is complete which means the ConnectionClosed token has fired.
        public override async ValueTask DisposeAsync()
        {
            Transport.Input.Complete();
            Transport.Output.Complete();

            if (_processingTask != null)
            {
                await _processingTask;
            }

            _connectionClosedTokenSource.Dispose();
        }

        private async Task DoReceive()
        {
            Exception error = null;

            try
            {
                await ProcessReceives();
            }
            catch (SocketException ex) when (IsConnectionResetError(ex.SocketErrorCode))
            {
                // This could be ignored if _shutdownReason is already set.
                error = new ConnectionResetException(ex.Message, ex);

                // There's still a small chance that both DoReceive() and DoSend() can log the same connection reset.
                // Both logs will have the same ConnectionId. I don't think it's worthwhile to lock just to avoid this.
                if (!_socketDisposed)
                {
                    _trace.ConnectionReset(ConnectionId);
                }
            }
            catch (Exception ex)
                when ((ex is SocketException socketEx && IsConnectionAbortError(socketEx.SocketErrorCode)) ||
                       ex is ObjectDisposedException)
            {
                // This exception should always be ignored because _shutdownReason should be set.
                error = ex;

                if (!_socketDisposed)
                {
                    // This is unexpected if the socket hasn't been disposed yet.
                    _trace.ConnectionError(ConnectionId, error);
                }
            }
            catch (Exception ex)
            {
                // This is unexpected.
                error = ex;
                _trace.ConnectionError(ConnectionId, error);
            }
            finally
            {
                // If Shutdown() has already bee called, assume that was the reason ProcessReceives() exited.
                Input.Complete(_shutdownReason ?? error);

                FireConnectionClosed();

                await _waitForConnectionClosedTcs.Task;
            }
        }

        private async Task ProcessReceives()
        {
            // Resolve `input` PipeWriter via the IDuplexPipe interface prior to loop start for performance.
            var input = Input;
            while (true)
            {
                // Wait for data before allocating a buffer.
                await _receiver.WaitForDataAsync();

                // Ensure we have some reasonable amount of buffer space
                var buffer = input.GetMemory(MinAllocBufferSize);

                var bytesReceived = await _receiver.ReceiveAsync(buffer);

                if (bytesReceived == 0)
                {
                    // FIN
                    _trace.ConnectionReadFin(ConnectionId);
                    break;
                }

                input.Advance(bytesReceived);

                var flushTask = input.FlushAsync();

                var paused = !flushTask.IsCompleted;

                if (paused)
                {
                    _trace.ConnectionPause(ConnectionId);
                }

                var result = await flushTask;

                if (paused)
                {
                    _trace.ConnectionResume(ConnectionId);
                }

                if (result.IsCompleted || result.IsCanceled)
                {
                    // Pipe consumer is shut down, do we stop writing
                    break;
                }
            }
        }

        private async Task DoSend()
        {
            Exception shutdownReason = null;
            Exception unexpectedError = null;

            try
            {
                await ProcessSends();
            }
            catch (SocketException ex) when (IsConnectionResetError(ex.SocketErrorCode))
            {
                shutdownReason = new ConnectionResetException(ex.Message, ex);
                _trace.ConnectionReset(ConnectionId);
            }
            catch (Exception ex)
                when ((ex is SocketException socketEx && IsConnectionAbortError(socketEx.SocketErrorCode)) ||
                       ex is ObjectDisposedException)
            {
                // This should always be ignored since Shutdown() must have already been called by Abort().
                shutdownReason = ex;
            }
            catch (Exception ex)
            {
                shutdownReason = ex;
                unexpectedError = ex;
                _trace.ConnectionError(ConnectionId, unexpectedError);
            }
            finally
            {
                Shutdown(shutdownReason);

                // Complete the output after disposing the socket
                Output.Complete(unexpectedError);

                // Cancel any pending flushes so that the input loop is un-paused
                Input.CancelPendingFlush();
            }
        }

        private async Task ProcessSends()
        {
            // Resolve `output` PipeReader via the IDuplexPipe interface prior to loop start for performance.
            var output = Output;
            while (true)
            {
                var result = await output.ReadAsync();

                if (result.IsCanceled)
                {
                    break;
                }

                var buffer = result.Buffer;

                var end = buffer.End;
                var isCompleted = result.IsCompleted;
                if (!buffer.IsEmpty)
                {
                    await _sender.SendAsync(buffer);
                }

                output.AdvanceTo(end);

                if (isCompleted)
                {
                    break;
                }
            }
        }

        private void FireConnectionClosed()
        {
            // Guard against scheduling this multiple times
            if (_connectionClosed)
            {
                return;
            }

            _connectionClosed = true;

            ThreadPool.UnsafeQueueUserWorkItem(state =>
            {
                state.CancelConnectionClosedToken();

                state._waitForConnectionClosedTcs.TrySetResult(null);
            },
            this,
            preferLocal: false);
        }

        private void Shutdown(Exception shutdownReason)
        {
            lock (_shutdownLock)
            {
                if (_socketDisposed)
                {
                    return;
                }

                // Make sure to close the connection only after the _aborted flag is set.
                // Without this, the RequestsCanBeAbortedMidRead test will sometimes fail when
                // a BadHttpRequestException is thrown instead of a TaskCanceledException.
                _socketDisposed = true;

                // shutdownReason should only be null if the output was completed gracefully, so no one should ever
                // ever observe the nondescript ConnectionAbortedException except for connection middleware attempting
                // to half close the connection which is currently unsupported.
                _shutdownReason = shutdownReason ?? new ConnectionAbortedException("The Socket transport's send loop completed gracefully.");

                _trace.ConnectionWriteFin(ConnectionId, _shutdownReason.Message);

                try
                {
                    // Try to gracefully close the socket even for aborts to match libuv behavior.
                    _socket.Shutdown(SocketShutdown.Both);
                }
                catch
                {
                    // Ignore any errors from Socket.Shutdown() since we're tearing down the connection anyway.
                }

                _socket.Dispose();
            }
        }

        private void CancelConnectionClosedToken()
        {
            try
            {
                _connectionClosedTokenSource.Cancel();
            }
            catch (Exception ex)
            {
                _trace.LogError(0, ex, $"Unexpected exception in {nameof(SocketConnection)}.{nameof(CancelConnectionClosedToken)}.");
            }
        }

        private static bool IsConnectionResetError(SocketError errorCode)
        {
            // A connection reset can be reported as SocketError.ConnectionAborted on Windows.
            // ProtocolType can be removed once https://github.com/dotnet/corefx/issues/31927 is fixed.
            return errorCode == SocketError.ConnectionReset ||
                   errorCode == SocketError.Shutdown ||
                   (errorCode == SocketError.ConnectionAborted && IsWindows) ||
                   (errorCode == SocketError.ProtocolType && IsMacOS);
        }

        private static bool IsConnectionAbortError(SocketError errorCode)
        {
            // Calling Dispose after ReceiveAsync can cause an "InvalidArgument" error on *nix.
            return errorCode == SocketError.OperationAborted ||
                   errorCode == SocketError.Interrupted ||
                   (errorCode == SocketError.InvalidArgument && !IsWindows);
        }
    }

再来看下HttpConnection如何处理请求

如果是Http1.0协议，则调用Http1Connection的ProcessRequestsAsync方法处理请求

 Http1Connection继承HttpProtocol并实现了以上的所有接口

当调用IHttpApplication的CreateContext方法时，需要的IFeatureCollection参数其实就是Http1Connection对象

通过Http1Connection对象来创建HttpContext

然后就进入到了asp.net的处理管道中