Buffered Stream - Synchronous operations are disallowed in ASP.NET Core 3.0

Question

I had an REST API targeting AspNetCore 2.2 with an endpoint that allows download of some big json files. After migrating to AspNetCore 3.1 this code stopped working:

    try
    {
        HttpContext.Response.StatusCode = (int)HttpStatusCode.OK;
        HttpContext.Response.Headers.Add("Content-Type", "application/json");

        using (var bufferedOutput = new BufferedStream(HttpContext.Response.Body, bufferSize: 4 * 1024 * 1024))
        {
            await _downloadService.Download(_applicationId, bufferedOutput);
        }
    }
    catch (Exception ex)
    {
        _logger.LogError(ex, ex.Message);                
    }

This is the download method, that created the json that I want to return on the HttpContext.Response.Body:

    public async Task Download(string applicationId, Stream output, CancellationToken cancellationToken = default(CancellationToken))
    {       
        using (var textWriter = new StreamWriter(output, Constants.Utf8))
        {
            using (var jsonWriter = new JsonTextWriter(textWriter))
            {
                jsonWriter.Formatting = Formatting.None;
                await jsonWriter.WriteStartArrayAsync(cancellationToken);

                //write json...
                await jsonWriter.WritePropertyNameAsync("Status", cancellationToken);
                await jsonWriter.WriteValueAsync(someStatus, cancellationToken); 

                await jsonWriter.WriteEndArrayAsync(cancellationToken);
            }
        }
    }

Now I get this exception: "Synchronous operations are disallowed in ASP.NET Core 3.0" How can I change this code to work without using AllowSynchronousIO = true;

Answer 1

AllowSynchronousIO option is disabled by default from .Net core 3.0.0-preview3 in (Kestrel, HttpSys, IIS in-process, TestServer) because these APIs are source of thread starvation and application hangs.

The option can be overridden on a per request for a temporary migration :

var allowSynchronousIoOption = HttpContext.Features.Get<IHttpBodyControlFeature>();
if (allowSynchronousIoOption != null)
{
    allowSynchronousIoOption.AllowSynchronousIO = true;
}

You can find more info and follow the ASP.NET Core Issue Tracker : AllowSynchronousIO disabled in all servers