TaskCanceledException raised when attempting an HttpClient.GetAsync

[wadebaird]

I realize the root of this issue is most likely related to #6443 and #5463, however the application in this case is not in the background. In addition, I also think this is probably not the most ideal breaking change to make, especially since it immediately cancels all http calls when an app is placed in the background (such as simply swiping up from the bottom of the phone to load the iPhones bottom menu). This behavior will cause everyone to implement the background session handler for ALL http calls in order to prevent their simple calls from being cancelled, or simply go back to the version of Xamarin.Forms before this change was made.

Steps to Reproduce

1. Load the attached project, it needs to be run on an actual device with TouchId enabled.
2. Fingerprint auth when the TouchId dialog pops up

Expected Behavior

No CanceledTaskException occurs (there is no real behavior in this repro as I minimized the code and project down to the bare minimum.

Actual Behavior

TaskCanceledException occurs on the Http.GetAsync call at callstack:

 	0xC in System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:157,13	C#
 	0x1B in System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/external/corert/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/TaskAwaiter.cs:175,25	C#
 	0x29 in System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/external/corert/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/TaskAwaiter.cs:154,48	C#
 	0x9 in System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/external/corert/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/TaskAwaiter.cs:125,17	C#
 	0xFFFFFFFFFFFFFFFF in System.Runtime.CompilerServices.TaskAwaiter<System.Net.Http.HttpResponseMessage>.GetResult at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/external/corert/src/System.Private.CoreLib/shared/System/Runtime/CompilerServices/TaskAwaiter.cs	C#
>	0xBD in CanceledTaskRepro.Services.ProboscisService.EstablishUserSession at C:\source\CanceledTaskExceptionRepro\CanceledTaskExceptionRepro\Services\ProboscisService.cs:103	C#
 	0x6 in System.Runtime.CompilerServices.AsyncMethodBuilderCore.MoveNextRunner.InvokeMoveNext at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1092,17	C#
 	0x73 in System.Threading.ExecutionContext.RunInternal at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:968,17	C#
 	0x4 in System.Threading.ExecutionContext.Run at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/threading/executioncontext.cs:910,13	C#
 	0x32 in System.Runtime.CompilerServices.AsyncMethodBuilderCore.MoveNextRunner.Run at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1073,25	C#
 	0x6 in System.Threading.Tasks.SynchronizationContextAwaitTaskContinuation.<>c.<.cctor>b__7_0 at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/external/corert/src/System.Private.CoreLib/src/System/Threading/Tasks/TaskContinuation.cs:379,78	C#
 	0xC in Foundation.NSAsyncSynchronizationContextDispatcher.Apply at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/Foundation/NSAction.cs:178,5	C#
 	0xFFFFFFFFFFFFFFFF in UIKit.UIApplication.UIApplicationMain	C#
 	0xB in UIKit.UIApplication.Main at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/UIKit/UIApplication.cs:86,4	C#
 	0x11 in UIKit.UIApplication.Main at /Library/Frameworks/Xamarin.iOS.framework/Versions/12.14.0.114/src/Xamarin.iOS/UIKit/UIApplication.cs:65,5	C#
 	0x8 in CanceledTaskExceptionRepro.iOS.Application.Main at C:\source\CanceledTaskExceptionRepro\CanceledTaskExceptionRepro.iOS\Main.cs:17,13	C#

Environment

Microsoft Visual Studio Professional 2019 (2)
Version 16.2.3
VisualStudio.16.Release/16.2.3+29215.179
Microsoft .NET Framework
Version 4.8.03761

Installed Version: Professional

Visual C++ 2019   00435-60000-00000-AA183
Microsoft Visual C++ 2019

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C# Tools   3.2.1-beta4-19408-03+2fc6a04980f800c59e8ede97e6ae294ff47d666e
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Node.js Tools   1.5.10610.1 Commit Hash:529a87de2769e143655e31ea81795c42e5775ef8
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Xamarin.iOS and Xamarin.Mac SDK   12.14.0.114 (c669116)
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Build Logs

N/A

Example Project (If Possible)

CanceledTaskExceptionRepro.zip

[deyupopp]

I confirm we have the exact same issue. We are executing the authentication request immediately after decryption using TouchId, and this worked until the last update. We made a workaround for this particular case, by delaying the request a few milliseconds, so it executes after the app regains "foreground" state, but this is definitely not ideal.

[czuck]

I am having this issue when iOS prompts the user to accept the required permissions. I've put a work around in to catch the exception and re-initiate the HTTP call if the caught exception is a TaskCanceledException, but this is not ideal.

[mandel-macaque]

I am closing the issues since we provided a workaround to stop this behavior from happening in #6918

The issue is that we added this workaround because the mono runtime has issues with the ThreadPool in certain situations in which a request is performed and the application is moved to the background. In that case, we cancel the task so that we are in a known state and we can recover.

The workaround is a temporary fix until the mono team fixes the issue. But if you do not want the new behavior, please take a look at the new property added in the PR mentioned above and voluntarily make your HttpClient not to cancel requests (which is something that, for small requests, will work correctly).