/// <summary>Copies a source stream to a LimitMemoryStream by writing directly to the LimitMemoryStream's buffer.</summary>
/// <param name="source">The source stream from which to copy.</param>
/// <param name="destination">The destination LimitMemoryStream to write to.</param>
/// <param name="bufferSize">The size of the buffer to allocate if one needs to be allocated.</param>
/// <param name="disposeSource">Whether to dispose of the source stream after the copy has finished successfully.</param>
private static async Task CopyAsyncToPreSizedLimitMemoryStream(Stream source, HttpContent.LimitMemoryStream destination, int bufferSize, bool disposeSource)
{
// When a LimitMemoryStream is constructed to represent a response with a particular ContentLength, its
// Capacity is set to that amount in order to pre-size it. We can take advantage of that in this copy
// by handing the destination's pre-sized underlying byte[] to the source stream for it to read into
// rather than creating a temporary buffer, copying from the source into that, and then copying again
// from the buffer into the LimitMemoryStream.
long capacity = destination.Capacity;
Debug.Assert(capacity > 0, "Caller should have checked that there's capacity");
// Get the initial length of the stream. When the length of a LimitMemoryStream is increased, the newly available
// space is zero-filled, either due to allocating a new array or due to an explicit clear. As a result, we can't
// write into the array directly and then increase the length to the right size afterward, as doing so will overwrite
// all of the data newly written. Instead, we need to increase the length to the capacity, write in our data, and
// then subsequently trim back the length to the end of the written data.
long startingLength = destination.Length;
if (startingLength < capacity)
{
destination.SetLength(capacity);
}
int bytesRead;
try
{
// Get the LimitMemoryStream's buffer.
ArraySegment<byte> entireBuffer;
bool gotBuffer = destination.TryGetBuffer(out entireBuffer);
Debug.Assert(gotBuffer, "Should only be in CopyAsyncToMemoryStream if we were able to get the buffer");
Debug.Assert(entireBuffer.Offset == 0, "LimitMemoryStream's are only constructed with a 0-offset");
Debug.Assert(entireBuffer.Count == entireBuffer.Array.Length, $"LimitMemoryStream's buffer count {entireBuffer.Count} should be the same as its length {entireBuffer.Array.Length}");
// While there's space remaining in the destination buffer, do another read to try to fill it.
// Each time we read successfully, we update the position of the destination stream to be
// at the end of the data read.
int spaceRemaining = (int)(entireBuffer.Array.Length - destination.Position);
while (spaceRemaining > 0)
{
// Read into the buffer
bytesRead = await source.ReadAsync(entireBuffer.Array, (int)destination.Position, spaceRemaining).ConfigureAwait(false);
if (bytesRead == 0)
{
DisposeSource(disposeSource, source);
return;
}
destination.Position += bytesRead;
spaceRemaining -= bytesRead;
}
}
finally
{
// Now that we're done reading directly into the buffer, if we previously increased the length
// of the stream, set it be at the end of the data read.
if (startingLength < capacity)
{
destination.SetLength(destination.Position);
}
}
// A typical case will be that we read exactly the amount requested. This means that the next
// read will likely return 0 bytes, but we need to try to do the read to know that, which means
// we need a buffer to read into. Use a cached single-byte array to do a read for 1-byte.
// Ideally this read returns 0, and we're done.
byte[] singleByteArray = RentCachedSingleByteArray();
bytesRead = await source.ReadAsync(singleByteArray, 0, 1).ConfigureAwait(false);
if (bytesRead == 0)
{
ReturnCachedSingleByteArray(singleByteArray);
DisposeSource(disposeSource, source);
return;
}
// The read actually returned data, which means there was more data available then
// the capacity of the LimitMemoryStream. This is likely an error condition, but
// regardless we need to finish the copy. First, we write out the byte we read...
await destination.WriteAsync(singleByteArray, 0, 1).ConfigureAwait(false);
ReturnCachedSingleByteArray(singleByteArray);
// ...then we fall back to doing the normal read/write loop.
await CopyAsyncAnyStreamToAnyStreamCore(source, destination, bufferSize, disposeSource).ConfigureAwait(false);
}