ObjectDisposedException in FileStreamCompletionSource.CompleteCallback crashes the process

[Tratcher]

See dotnet/aspnetcore#4422

Application: dotnet.exe
CoreCLR Version: 4.6.27110.4
Description: The process was terminated due to an unhandled exception.
Exception Info: System.ObjectDisposedException: The CancellationTokenSource has been disposed.
   at System.Threading.CancellationTokenSource.ThrowObjectDisposedException()
   at System.IO.FileStream.FileStreamCompletionSource.CompleteCallback(UInt64 packedResult)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state)
--- End of stack trace from previous location where exception was thrown ---
   at System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)

In this situation the application is reading files, cancels the reads, and apparently disposing the CancellationTokenSource.

[Tratcher]

@stephentoub this looks like a regression introduced here:
dotnet/coreclr@5fd50d8#diff-742d93872313d38656fb88ad801f4d7bR174
_cancellationRegistration.Token can throw if the CTS was disposed.

[stephentoub]

_cancellationRegistration.Token can throw if the CTS was disposed.

Ugh. That's a bug in CTR.Token; it should never throw.

[stephentoub]

Re-opening to track potential ports.

[karelz]

Part of 2.2.1 release (via PR dotnet/corefx#21417)