NSURLConnection HttpClient implementation seems to cancel running requests when app moves to background

[leonluc-dev]

Steps to Reproduce

1. Set up a HttpClient (with NSUrlConnection (iOS 7+) set as the HttpClient implementation in the project settings)
2. Send a post or get request using the http client (preferrably a slow loading url to have time to do step 3)
3. While the request is running, move the app to the background (home button or swipe)

Expected Behavior

The http request finishes properly in the background

As is the case with the Managed (default) and CFNetwork (iOS 6+) HttpClient handlers

Actual Behavior

The http request throws a TaskCanceledException just after the app moves to the background.

System.Threading.Tasks.TaskCanceledException: A task was canceled.
  at System.Net.Http.NSUrlSessionHandler.SendAsync (System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) [0x001d4] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.20.2.2/src/Xamarin.iOS/Foundation/NSUrlSessionHandler.cs:527 
  at System.Net.Http.HttpClient.FinishSendAsyncBuffered (System.Threading.Tasks.Task`1[TResult] sendTask, System.Net.Http.HttpRequestMessage request, System.Threading.CancellationTokenSource cts, System.Boolean disposeCts) [0x0017e] in /Library/Frameworks/Xamarin.iOS.framework/Versions/Current/src/Xamarin.iOS/external/corefx/src/System.Net.Http/src/System/Net/Http/HttpClient.cs:506 
  at SessionTestApp.ViewController+<TestHttpBtn_TouchUpInside>d__7.MoveNext () [0x0003b] in C:\Users\appdev\source\repos\SessionTestApp\SessionTestApp\ViewController.cs:29 

Environment

Microsoft Visual Studio Professional 2019
Version 16.7.1
VisualStudio.16.Release/16.7.1+30406.217
Microsoft .NET Framework
Version 4.8.03752

Installed Version: Professional

ASP.NET and Web Tools 2019   16.7.532.28833
ASP.NET and Web Tools 2019

ASP.NET Core Razor Language Services   16.1.0.2035807+72d099b977d3a85e65fa3b0614ca8cfc803fef02
Provides languages services for ASP.NET Core Razor.

ASP.NET Web Frameworks and Tools 2019   16.7.532.28833
For additional information, visit https://www.asp.net/

Azure App Service Tools v3.0.0   16.7.532.28833
Azure App Service Tools v3.0.0

Azure Functions and Web Jobs Tools   16.7.532.28833
Azure Functions and Web Jobs Tools

C# Tools   3.7.0-6.20375.2+34202cc2f3e869fd70a26d8237f4552cf9e192cf
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Common Azure Tools   1.10
Provides common services for use by Azure Mobile Services and Microsoft Azure Tools.

Extensibility Message Bus   1.2.6 (master@34d6af2)
Provides common messaging-based MEF services for loosely coupled Visual Studio extension components communication and integration.

IntelliCode Extension   1.0
IntelliCode Visual Studio Extension Detailed Info

Microsoft Azure Tools   2.9
Microsoft Azure Tools for Microsoft Visual Studio 2019 - v2.9.30701.1

Microsoft Continuous Delivery Tools for Visual Studio   0.4
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Microsoft JVM Debugger   1.0
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Microsoft Library Manager   2.1.79+ge3567815aa.RR
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Microsoft MI-Based Debugger   1.0
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Microsoft Visual Studio Tools for Containers   1.1
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Mono Debugging for Visual Studio   16.7.5 (112c7bc)
Support for debugging Mono processes with Visual Studio.

NuGet Package Manager   5.7.0
NuGet Package Manager in Visual Studio. For more information about NuGet, visit https://docs.nuget.org/

ProjectServicesPackage Extension   1.0
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SQL Server Data Tools   16.0.62008.03130
Microsoft SQL Server Data Tools

TypeScript Tools   16.0.20702.2001
TypeScript Tools for Microsoft Visual Studio

Visual Basic Tools   3.7.0-6.20375.2+34202cc2f3e869fd70a26d8237f4552cf9e192cf
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Visual F# Tools 10.10.0.0 for F# 4.7   16.7.0-beta.20361.3+3ef6f0b514198c0bfa6c2c09fefe41a740b024d5
Microsoft Visual F# Tools 10.10.0.0 for F# 4.7

Visual Studio Code Debug Adapter Host Package   1.0
Interop layer for hosting Visual Studio Code debug adapters in Visual Studio

Visual Studio Container Tools Extensions (Preview)   1.0
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Visual Studio Tools for Containers   1.0
Visual Studio Tools for Containers

VisualStudio.DeviceLog   1.0
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VisualStudio.Foo   1.0
Information about my package

VisualStudio.Mac   1.0
Mac Extension for Visual Studio

Xamarin   16.7.000.440 (d16-7@358f3c6)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin Designer   16.7.0.495 (remotes/origin/d16-7@79c0c522c)
Visual Studio extension to enable Xamarin Designer tools in Visual Studio.

Xamarin Templates   16.7.85 (1bcbbdf)
Templates for building iOS, Android, and Windows apps with Xamarin and Xamarin.Forms.

Xamarin.Android SDK   11.0.0.3 (d16-7/aca845b)
Xamarin.Android Reference Assemblies and MSBuild support.
    Mono: 83105ba
    Java.Interop: xamarin/java.interop/d16-7@1f3388a
    ProGuard: Guardsquare/proguard/proguard6.2.2@ebe9000
    SQLite: xamarin/sqlite/3.32.1@1a3276b
    Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-7@017078f


Xamarin.iOS and Xamarin.Mac SDK   13.20.2.2 (817b6f72a)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Example Project (If Possible)

The example project contains a simple call to http://slowwly.robertomurray.co.uk/delay/7000/url/{url} , which delays the response for testing purposes. But the issue occurs with all urls, both HTTP and HTTPS.

NSUrlSessionTestApp.zip

[mandel-macaque]

Hello,

This is not an issue but a 'feature' that was added to workaround an issue that was present in the mono runtime. This workaround will be remove as soon as we have a good way to reproduce the fact that it was fixed as per issue #7080

What you are describing is a duplicate of issue #6443 In that issue, please read the following comment. If you do not want to have this in your requests and you do know that the time of the request will be sort and won't be affected by the OS reclaiming resources I added the following fix.

To workaround it, simply create a NSUrlSessionHandler that set the property BypassBackgroundSessionCheck to true. For example:

// create your own handler instance rather than using the one provided by default.
var handler = new NSUrlSessionHandler() {
  BypassBackgroundSessionCheck = false, // readd the hack
};
var httpClient = new HttpClient (handler); // use the handler with the hack

This won't be needed in the next preview of the next stable version of Xamarin.iOS but it is needed in the current stable one as per #8296

[leonluc-dev]

I see. Ill look into the workaround. Thanks for the information!

The reason this came up is because we had a massive failure of Testflight tests recently. The initialization code of our app does some http requests after startup to check status etc.
But after starting a Testflight app build for the first time it shows a test info screen to the user (this happens for all Testflight apps distributed through iTunes connect groups). This info screen moves the actual app to the background at the same time its running startup code causing all requests during initialization to fail for every single user.

[mandel-macaque]

@leonluc-dev use the property and everything should be fine. AFAIK the threadpool was fixed already. New release will have it disabled by default, and we will leave the 'feature' just in case we have issues.