gRPC C#: how to cancel upload correctly

Question

For uploading images from a client to the server I use chunking.

Here is the client code:

private async Task UploadPersonImage(int personId, string fileName, CancellationToken cancellationToken)
{
    var stream = Client.UploadPersonImage();
    PersonImageMessage personImageMessage = new PersonImageMessage();
    personImageMessage.PersonId = personId;
    personImageMessage.ImageType = ImageType.Jpg;
    byte[] image = File.ReadAllBytes(fileName);
    int imageOffset = 0;
    byte[] imageChunk = new byte[imageChunkSize];
    while (imageOffset < image.Length && !cancellationToken.IsCancellationRequested)
    {
        int length = Math.Min(imageChunkSize, image.Length - imageOffset);
        Buffer.BlockCopy(image, imageOffset, imageChunk, 0, length);
        imageOffset += length;
        ByteString byteString = ByteString.CopyFrom(imageChunk);
        personImageMessage.ImageChunk = byteString;
        await stream.RequestStream.WriteAsync(personImageMessage).ConfigureAwait(false);
    }
    await stream.RequestStream.CompleteAsync().ConfigureAwait(false);
    if (!cancellationToken.IsCancellationRequested)
    {
        var uploadPersonImageResult = await stream.ResponseAsync.ConfigureAwait(false);
        // Process answer...
    }
}

And this is the server code:

public override async Task<TransferStatusMessage> UploadPersonImage(
    IAsyncStreamReader<PersonImageMessage> requestStream, ServerCallContext context)
{
    TransferStatusMessage transferStatusMessage = new TransferStatusMessage();
    transferStatusMessage.Status = TransferStatus.Success;
    try
    {
        await Task.Run(
            async () =>
            {
                CancellationToken cancellationToken = context.CancellationToken;
                await using (Stream fs = File.OpenWrite(ImageFileName))
                {
                    await foreach (PersonImageMessage personImageMessage in
                        requestStream.ReadAllAsync(cancellationToken).ConfigureAwait(false))
                    {
                        fs.Write(personImageMessage.ImageChunk.ToByteArray());
                    }
                }
            }).ConfigureAwait(false);
    }
    // Is thrown on cancellation -> ignore...
    catch (OperationCanceledException)
    {
        transferStatusMessage.Status = TransferStatus.Cancelled;
    }
    catch (RpcException rpcEx)
    {
        if (rpcEx.StatusCode == StatusCode.Cancelled)
        {
            transferStatusMessage.Status = TransferStatus.Cancelled;
        }
        else
        {
            _logger.LogError($"Exception while processing image file '{ImageFileName}'. Exception: '{requestStream}'");
            transferStatusMessage.Status = TransferStatus.Failure;
        }
    }
    // Delete incomplete file
    if (transferStatusMessage.Status != TransferStatus.Success)
    {
        File.Delete(ImageFileName);
    }
    return transferStatusMessage;
}

Everything works fine. I want to cancel the upload in between sending the chunks. Now, CompleteAsync() is called and the server thinks the data transfer ended successfully. I'm looking for a way to trigger the cancellation in the server (i.e. the CancellationToken in the ServerCallContext) via the client. As a workaround I could add a flag to PersonImageMessage, something like 'upload_cancelled', to tell the server that the transfer is aborted. But there must be a built-in mechanism.

Does somebody know the trick?

cactuaroid · Accepted Answer · 2020-05-18 14:00:38Z

1

You can pass cancellationToken to your Client stream RPC method. Then the server can receive it as context.cancellationToken.

var stream = Client.UploadPersonImage(cancellationToken: cancellationToken);

If the client cancels, stream.RequestStream.CompleteAsync() should not be called.

answered May 18, 2020 at 14:00

cactuaroid

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