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Asynchronous implementation of MemorySource<TSource>.CopyToAsync #265
Description
Overview
Currently, the stream returned by AsStream() implements CopyToAsync which calls the provided CopyTo implementation, which calls destination.Write. This causes unexpected synchronous I/O.
Due to the synchronous I/O, an exception is thrown if this method is used in ASP.NET Core to write to the response stream:
System.InvalidOperationException: Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpResponseStream.Write(Byte[] buffer, Int32 offset, Int32 count)
at System.IO.Stream.Write(ReadOnlySpan`1 buffer)
at Microsoft.Toolkit.HighPerformance.Streams.MemoryStream`1.CopyTo(Stream destination, Int32 bufferSize)
at Microsoft.Toolkit.HighPerformance.Streams.MemoryStream`1.CopyToAsync(Stream destination, Int32 bufferSize, CancellationToken cancellationToken)
API breakdown
Note: These are internal APIs
In order to implement this in a performant way, I'm thinking of adding a property Memory to ISpanOwner:
internal interface ISpanOwner
{
/// <summary>
/// Gets a <see cref="Memory{T}"/> instance wrapping the underlying memory area.
/// </summary>
Memory<byte> Memory { get; }
}And then implementing CopyToAsync like this:
partial class MemoryStream<TSource>
{
/// <inheritdoc/>
public sealed override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
{
if (cancellationToken.IsCancellationRequested)
{
return Task.FromCanceled(cancellationToken);
}
try
{
MemoryStream.ValidateDisposed(this.disposed);
Memory<byte> source = this.source.Memory.Slice(this.position);
this.position += source.Length;
return destination.WriteAsync(source, cancellationToken).AsTask();
}
catch (OperationCanceledException e)
{
return Task.FromCanceled(e.CancellationToken);
}
catch (Exception e)
{
return Task.FromException(e);
}
}
}Usage example
public async Task ProcessRequestAsync(HttpContext context)
{
var memory = await _service.GetDataAsync();
var stream = memory.AsStream();
await stream.CopyToAsync(context.Response.Body);
}Breaking change?
No
Alternatives
Not applicable
Additional context
Wasn't sure at first if this counted as a bug, upon closer inspection it looks a bit more likely, so I may have chosen the wrong template.
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Activity
[-]Add `CopyToAsync` implementation to `MemorySource<TSource>`[/-][+]Asynchronous implementation of `MemorySource<TSource>.CopyToAsync`[/+]