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Avoid cancelling readers before end of stream #53905

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merged 2 commits into from
Jun 10, 2021
Merged

Avoid cancelling readers before end of stream #53905

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6ec7f1a
Isolate reads from writes to avoid cancellation paths
sharwell Jun 6, 2021
833f7e1
Rewrite cancellation to avoid closing the reader
sharwell Jun 7, 2021
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25 changes: 19 additions & 6 deletions src/Workspaces/Remote/Core/BrokeredServiceConnection.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -293,22 +293,33 @@ internal static async ValueTask<TResult> InvokeStreamingServiceAsync<TResult>(
// 6. 'pipe.Writer' is completed
// 7. 'pipe.Reader' is completed
// 8. OperationCanceledException is thrown back to the caller
//
// Since StreamJsonRpc will automatically complete writers when an error occurs (and cancellation is
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// treated as an error), we create a buffered reader to isolate callback actions from this
// implementation.

// Create a separate cancellation token for the reader, which we keep open until after the call to invoke
// completes. If we close the reader before cancellation is processed by the remote call, it might block
// (deadlock) while writing to a stream which is no longer processing data.
using var readerCancellationSource = new CancellationTokenSource();

var pipe = new Pipe();
var writeBufferPipe = new Pipe();
var readBufferPipe = new Pipe();

// Create new tasks that both start executing, rather than invoking the delegates directly
// to make sure both invocation and reader start executing and transfering data.

var writerTask = Task.Run(async () =>
{
var copyTask = writeBufferPipe.Reader.CopyToAsync(readBufferPipe.Writer, cancellationToken);

try
{
await invocation(service, pipe.Writer, cancellationToken).ConfigureAwait(false);
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await invocation(service, writeBufferPipe.Writer, cancellationToken).ConfigureAwait(false);

// Complete the copy and mark the operation complete
await copyTask.NoThrowAwaitable(captureContext: false);
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await readBufferPipe.Writer.CompleteAsync().ConfigureAwait(false);
}
catch (Exception e)
{
Expand All @@ -318,9 +329,11 @@ internal static async ValueTask<TResult> InvokeStreamingServiceAsync<TResult>(
readerCancellationSource.Cancel();

// Ensure that the writer is complete if an exception is thrown
// before the writer is passed to the RPC proxy. Once it's passed to the proxy
// before the writer is passed to the RPC proxy. Once it's passed to the proxy
// the proxy should complete it as soon as the remote side completes it.
await pipe.Writer.CompleteAsync(e).ConfigureAwait(false);
await writeBufferPipe.Writer.CompleteAsync(e).ConfigureAwait(false);
await copyTask.NoThrowAwaitable(captureContext: false);
await readBufferPipe.Writer.CompleteAsync(e).ConfigureAwait(false);

throw;
}
Expand All @@ -333,15 +346,15 @@ internal static async ValueTask<TResult> InvokeStreamingServiceAsync<TResult>(

try
{
return await reader(pipe.Reader, readerCancellationSource.Token).ConfigureAwait(false);
return await reader(readBufferPipe.Reader, readerCancellationSource.Token).ConfigureAwait(false);
}
catch (Exception e) when ((exception = e) == null)
{
throw ExceptionUtilities.Unreachable;
}
finally
{
await pipe.Reader.CompleteAsync(exception).ConfigureAwait(false);
await readBufferPipe.Reader.CompleteAsync(exception).ConfigureAwait(false);
}
}, readerCancellationSource.Token);

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