HttpClient does not work as expected in heavy load

[erikbra]

The README.md explains more detail, but in summary, I struggle with HttpClient when making multiple simultaneous calls to a REST API (local one made for demonstrating the problem).

Typical test run output:

Expected collection to be empty because there should be no failed calls, but found {175, 224, 339, 409, 499, 522, 540, 761, 775, 788, 826, 831, 841, 893, 895, 896, 897, 910, 914, 918, 923, 978, 979, 987, 993, 994, 995, 996, 999, 1002, 1003, 1004, ...7893 more...}.
   at FluentAssertions.Execution.XUnit2TestFramework.Throw(String message) in C:\projects\fluentassertions-vf06b\Src\FluentAssertions\Execution\XUnit2TestFramework.cs:line 32
   at FluentAssertions.Execution.AssertionScope.FailWith(String message, Object[] args) in C:\projects\fluentassertions-vf06b\Src\FluentAssertions\Execution\AssertionScope.cs:line 197
   at FluentAssertions.Execution.GivenSelector`1.FailWith(String message, Object[] args) in C:\projects\fluentassertions-vf06b\Src\FluentAssertions\Execution\GivenSelector.cs:line 122
   at FluentAssertions.Collections.CollectionAssertions`2.BeEmpty(String because, Object[] becauseArgs) in C:\projects\fluentassertions-vf06b\Src\FluentAssertions\Collections\CollectionAssertions.cs:line 42
   at UnitTests.Call_Api.AssertNoFailedCalls(CallResultSet results) in C:\Users\ErikBrandstadmoen\Source\Repos\HttpClientProblems\UnitTests.NetCore\Call_Api.cs:line 198
   at UnitTests.Call_Api.<Slow_Multiple_Times_In_Parallel>d__16.MoveNext() in C:\Users\ErikBrandstadmoen\Source\Repos\HttpClientProblems\UnitTests.NetCore\Call_Api.cs:line 128
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)

System.Net.Http.HttpRequestException: An error occurred while sending the request. ---> System.Net.Http.WinHttpException: The connection with the server was terminated abnormally
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Threading.Tasks.RendezvousAwaitable`1.GetResult()
   at System.Net.Http.WinHttpHandler.<StartRequest>d__105.MoveNext()
   --- End of inner exception stack trace ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.ConfiguredTaskAwaitable`1.ConfiguredTaskAwaiter.GetResult()
   at System.Net.Http.HttpClient.<FinishSendAsyncBuffered>d__58.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
   at UnitTests.Call_Api.<CallApi>d__22.MoveNext() in C:\Users\ErikBrandstadmoen\Source\Repos\HttpClientProblems\UnitTests.NetCore\Call_Api.cs:line 204
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
   at UnitTests.Call_Api.<Measure>d__23.MoveNext() in C:\Users\ErikBrandstadmoen\Source\Repos\HttpClientProblems\UnitTests.NetCore\Call_Api.cs:line 219
Time elapsed: 1416.7396ms. Called 10000, failed 7925. Avg call time: 1146.4329ms