ObjectDisposedException on shutdown

从相关的释放注册处的 WebApiAppBuilderExtensions.cs 和 HttpMessageHandlerAdapter.cs 开始，然后跟踪 host.OnAppDisposing 和 HostingEnvironment.StopListening 流程。确定如何在释放时避免中断正在处理的请求，以及应由哪个生命周期事件或机制执行最终释放；完成的标准是关闭不再引发 ObjectDisposedException，并且资源最终得到清理。

An app running in IIS with OWIN integration gets ObjectDisposedExceptions on shutdown rather than gracefully draining requests.

Exception Details: System.ObjectDisposedException, Cannot access a disposed object.
Object name: '[Redacted].AuthenticationHandler'.,
    at System.Net.Http.DelegatingHandler.CheckDisposed()
   at System.Net.Http.DelegatingHandler.SetOperationStarted()
   at System.Net.Http.DelegatingHandler.SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
   at [Redacted].AuthenticationHandler.<SendAsync>d__12.MoveNext()

Here's where dispose is called:

   at [Redacted].AuthenticationHandler.Dispose(Boolean disposing)
   at System.Net.Http.DelegatingHandler.Dispose(Boolean disposing)
   at System.Web.Http.HttpServer.Dispose(Boolean disposing)
   at System.Net.Http.HttpMessageInvoker.Dispose(Boolean disposing)
   at System.Web.Http.Owin.HttpMessageHandlerAdapter.OnAppDisposing()
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
   at System.Threading.CancellationCallbackInfo.ExecuteCallback()
   at System.Threading.CancellationTokenSource.ExecuteCallbackHandlers(Boolean throwOnFirstException)
   at System.Threading.CancellationTokenSource.NotifyCancellation(Boolean throwOnFirstException)
   at Microsoft.Owin.Host.SystemWeb.ShutdownDetector.Cancel()
   at Microsoft.Owin.Host.SystemWeb.ShutdownDetector.Stop(Boolean immediate)
   at System.Web.Hosting.HostingEnvironment.StopRegisteredObjects(Boolean immediate)
   at System.Web.Hosting.HostingEnvironment.InitiateShutdownWorkItemCallback(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.QueueUserWorkItemCallback.System.Threading.IThreadPoolWorkItem.ExecuteWorkItem()
   at System.Threading.ThreadPoolWorkQueue.Dispatch()'

The root event here is HostingEnvironment.StopListening. "Occurs when the worker process or application pool associated with this host has stopped listening for new requests and will eventually shut down." The app is shutting down, but there may still be requests in flight that should be completed if possible.

Microsoft.Owin wired into the StopListening event so that SignalR could gracefully shut down long polling and WebSocket requests. WebAPI however wired up it's pipeline disposal to this event. This causes the pipeline to be disposed while request are in flight.
https://github.com/aspnet/AspNetWebStack/blob/ba26cfbfbf958d548e4c0a96e853250f13450dc6/src/System.Web.Http.Owin/WebApiAppBuilderExtensions.cs#L111
https://github.com/aspnet/AspNetWebStack/blob/ba26cfbfbf958d548e4c0a96e853250f13450dc6/src/System.Web.Http.Owin/WebApiAppBuilderExtensions.cs#L119-L128
https://github.com/aspnet/AspNetWebStack/blame/ba26cfbfbf958d548e4c0a96e853250f13450dc6/src/System.Web.Http.Owin/HttpMessageHandlerAdapter.cs#L87

There is no alternate event in IIS that fires when requests have drained.

Workaround: Remove the CancellationToken from the IAppBuilder so WebAPI cannot register with it.

appBuilder.Properties.Remove("host.OnAppDisposing");

Proposed fix: WebApi should not register for disposal with this event.
https://github.com/aspnet/AspNetWebStack/blame/ba26cfbfbf958d548e4c0a96e853250f13450dc6/src/System.Web.Http.Owin/HttpMessageHandlerAdapter.cs#L87
(When should it be disposed then?)