ReadAsync accepts a CancellationToken but ignores it

Which service(blob, file, queue, table) does this issue concern?

Blob, probably others

Which version of the SDK was used?

9.3.1

Which platform are you using? (ex: .NET Core 2.1)

NET Core 2.1

What problem was encountered?

CancellationToken is ignored when reading from a blob read stream. The code below will happily read to the end. For my test file which is 42,397,696 bytes, I get the following output:

True
True
True
True
True
True
True
True
True
True
True
True
Done

I’ve had a nose through the code and can see that the problem comes down to the ReadAsync, which takes a CancellationToken, but doesn’t use it.

How can we reproduce the problem in the simplest way?

var cts = new CancellationTokenSource();
var buffer = new byte[4 * 1024 * 1024];
var readStream = await blobRef.OpenReadAsync(null, null, null, cts.Token);
cts.Cancel();
int bytesRead = 0;
do
{
    bytesRead = await readStream.ReadAsync(buffer, 0, buffer.Length, cts.Token);
    Console.WriteLine(cts.Token.IsCancellationRequested);
}
while (bytesRead > 0);
Console.WriteLine("Done");
Console.ReadLine();

Have you found a mitigation/solution?

I’m using the following extension method found here

public static Task<T> WithCancellation<T>(this Task<T> task, CancellationToken cancellationToken)
{
    return task.IsCompleted
        ? task
        : task.ContinueWith(
            completedTask => completedTask.GetAwaiter().GetResult(),
            cancellationToken,
            TaskContinuationOptions.ExecuteSynchronously,
            TaskScheduler.Default);
}

like so:

bytesRead = await readStream.ReadAsync(buffer, 0, buffer.Length, cts.Token).WithCancellation(cts.Token);

[kfarmer-msft]

Tests pass. I expect this will be in our next release.

[lostllama]

I’m working on accepting the PR

Thank you 😃 and thanks to @SimonGustavsson for creating the PR.