Make memory pool plugable

src/Microsoft.AspNetCore.Http/StreamPipeWriter.cs

@@ -25,12 +25,12 @@ public class StreamPipeWriter : PipeWriter, IDisposable
  private List<CompletedBuffer> _completedSegments;
  private Memory<byte> _currentSegment;
  private IMemoryOwner<byte> _currentSegmentOwner;
+ private MemoryPool<byte> _pool;
  private int _position;
 
  private CancellationTokenSource _internalTokenSource;
  private bool _isCompleted;
  private ExceptionDispatchInfo _exceptionInfo;
-
  private object lockObject = new object();
 
  private CancellationTokenSource InternalTokenSource
@@ -60,6 +60,7 @@ public StreamPipeWriter(Stream writingStream, int minimumSegmentSize, MemoryPool
  {
  _minimumSegmentSize = minimumSegmentSize;
  _writingStream = writingStream;
+ _pool = pool ?? MemoryPool<byte>.Shared;
  }
 
  /// <inheritdoc />
@@ -252,7 +253,7 @@ private void AddSegment(int sizeHint = 0)
  }
 
  // Get a new buffer using the minimum segment size, unless the size hint is larger than a single segment.
- _currentSegmentOwner = MemoryPool<byte>.Shared.Rent(Math.Max(_minimumSegmentSize, sizeHint));
+ _currentSegmentOwner = _pool.Rent(Math.Max(_minimumSegmentSize, sizeHint));
  _currentSegment = _currentSegmentOwner.Memory;
  _position = 0;
  }