Same content passed through Response.Filter twice

First, please let me emphasise- this is not a question about .Write() being called multiple times on a Response.Filter. I am aware chunks of the response body are sent through the filter and as such, Write() gets called multiple times.

The problem

The same content is getting passed through a custom HttpResonse.Filter object twice- and I do mean the exact same content. If the hypothetical page content "My Test" was being rendered, then it would be passed through the HttpResponse.Filter's Write() method twice. It only happens under some very specific conditions, which I will explain below.

The setup

Firstly, there is a custom HttpModule that intercepts requests and inserts a custom Stream into the HttpResponse.Filter property. It subscribes to the HttpApplication.OnPreSendRequestHeaders event (and a few others).

There are two pages- start.aspx and target.aspx. Start.aspx makes a call to Server.Transfer, transferring to target.aspx. Target.aspx will at multiple times call Response.Flush(), ensuring that the content-type header gets set to "chunked" and http chunking is used.

The weird part The Write() method of the HttpResonse.Filter will get invoked twice for the buffered content of Target.aspx. However if target.aspx does not invoke Response.Flush(), Filter is invoked as expected. If the HttpModule does not subscribe to the OnPreSendRequestHeaders event, Filter is invoked as expected. If you visit target.aspx directly, Filter is invoked as expected.

Additionally On the first set of calls to Response.Filter, the stack trace looks like this:

     at MyResponseFilter.Write(Byte[] buffer, Int32 offset, Int32 count) 
   at System.Web.HttpWriter.FilterIntegrated(Boolean finalFiltering, IIS7WorkerRequest wr)
   at System.Web.HttpResponse.UpdateNativeResponse(Boolean sendHeaders)
   at System.Web.HttpResponse.Flush(Boolean finalFlush)
   at System.Web.HttpResponse.Flush()
   at System.Web.HttpResponse.End()
   at System.Web.HttpServerUtility.Transfer(String path, Boolean preserveForm)
   at System.Web.HttpServerUtility.Transfer(String path)
   at Target.ButtonHandler(Object sneder, EventArgs e) in C:\Testspace\ModTest\ModTest\TransferMe.aspx.vb:line 9
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.Target.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
   at System.Web.HttpApplication.PipelineStepManager.ResumeSteps(Exception error)
   at System.Web.HttpApplication.BeginProcessRequestNotification(HttpContext context, AsyncCallback cb)
   at System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context)
   at Syst开发者_如何学Pythonem.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)

The second set looks like this:

   at MyResponseFilter.Write(Byte[] buffer, Int32 offset, Int32 count) 
   at System.Web.HttpWriter.FilterIntegrated(Boolean finalFiltering, IIS7WorkerRequest wr)
   at System.Web.HttpResponse.UpdateNativeResponse(Boolean sendHeaders)
   at System.Web.HttpRuntime.FinishRequestNotification(IIS7WorkerRequest wr, HttpContext context, RequestNotificationStatus& status)
   at System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.UnsafeIISMethods.MgdExplicitFlush(IntPtr context)
   at System.Web.Hosting.IIS7WorkerRequest.ExplicitFlush()
   at System.Web.HttpResponse.Flush(Boolean finalFlush)
   at System.Web.HttpResponse.Flush()
   at System.Web.HttpResponse.End()
   at System.Web.HttpServerUtility.Transfer(String path, Boolean preserveForm)
   at System.Web.HttpServerUtility.Transfer(String path)
   at Target.ButtonHandler(Object sneder, EventArgs e) 
   at System.Web.UI.WebControls.Button.OnClick(EventArgs e)
   at System.Web.UI.WebControls.Button.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.WebControls.Button.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument)
   at System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
   at System.Web.UI.Page.ProcessRequest()
   at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
   at System.Web.UI.Page.ProcessRequest(HttpContext context)
   at ASP.Target.ProcessRequest(HttpContext context)
   at System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
   at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously)
   at System.Web.HttpApplication.PipelineStepManager.ResumeSteps(Exception error)
   at System.Web.HttpApplication.BeginProcessRequestNotification(HttpContext context, AsyncCallback cb)
   at System.Web.HttpRuntime.ProcessRequestNotificationPrivate(IIS7WorkerRequest wr, HttpContext context)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.UnsafeIISMethods.MgdIndicateCompletion(IntPtr pHandler, RequestNotificationStatus& notificationStatus)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotificationHelper(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)
   at System.Web.Hosting.PipelineRuntime.ProcessRequestNotification(IntPtr managedHttpContext, IntPtr nativeRequestContext, IntPtr moduleData, Int32 flags)

If anyone can shed some light on this issue, I will shower you with gifts fit for a king.