public async override ValueTask <ReadResult> ReadAsync(CancellationToken cancellationToken = default)
{
// ReadAsync needs to handle some common situations:
// 1. Base64 requires are least 4 bytes to decode content. If less than 4 bytes are returned
// from the inner reader then repeatedly call the inner reader until 4 bytes are available.
// 2. It is possible that ReadAsync is called many times without consuming the data. We don't
// want to decode the same base64 content over and over. ReadAsync only decodes new content
// and appends it to a sequence.
var innerResult = await _inner.ReadAsync(cancellationToken);
if (innerResult.Buffer.IsEmpty)
{
_currentDecodedBuffer = innerResult.Buffer;
_currentInnerBuffer = innerResult.Buffer;
return(innerResult);
}
// Minimum valid base64 length is 4. Read until we have at least that much content
while (innerResult.Buffer.Length - _currentInnerBufferRead < 4)
{
if (innerResult.IsCompleted)
{
// If the reader completes with less than 4 bytes then the base64 isn't valid..
throw new InvalidOperationException("Unexpected end of data when reading base64 content.");
}
if (innerResult.IsCanceled)
{
// Cancelled before we have enough data to decode. Return a cancelled result with no data.
_currentDecodedBuffer = ReadOnlySequence <byte> .Empty;
_currentInnerBuffer = innerResult.Buffer;
return(new ReadResult(
ReadOnlySequence <byte> .Empty,
innerResult.IsCanceled,
innerResult.IsCompleted));
}
// Attempt to get more data
_inner.AdvanceTo(innerResult.Buffer.Start, innerResult.Buffer.End);
innerResult = await _inner.ReadAsync(cancellationToken);
}
// Limit result to complete base64 segments (multiples of 4)
var newResultLength = innerResult.Buffer.Length - _currentInnerBufferRead;
var newResultValidLength = (newResultLength / 4) * 4;
var buffer = innerResult.Buffer.Slice(_currentInnerBufferRead, newResultValidLength);
// The content can contain multiple fragments of base64 content
// Check for padding, and limit returned data to one fragment at a time
var paddingIndex = PositionOf(buffer, (byte)'=');
if (paddingIndex != null)
{
buffer = buffer.Slice(0, ((paddingIndex.Value / 4) + 1) * 4);
}
// Copy the buffer data to a new array.
// Need a copy that we own because it will be decoded in place.
var decodedBuffer = buffer.ToArray();
var status = Base64.DecodeFromUtf8InPlace(decodedBuffer, out var bytesWritten);
if (status == OperationStatus.Done || status == OperationStatus.NeedMoreData)
{
_currentInnerBuffer = innerResult.Buffer.Slice(0, _currentInnerBufferRead + decodedBuffer.Length);
_currentInnerBufferRead = _currentInnerBuffer.Length;
// Update decoded buffer. If there have been multiple reads with the same content then
// newly decoded content will be appended to the sequence.
if (_currentDecodedBuffer.IsEmpty)
{
// Avoid creating segments for single segment sequence.
_currentDecodedBuffer = new ReadOnlySequence <byte>(decodedBuffer, 0, bytesWritten);
}
else if (_currentDecodedBuffer.IsSingleSegment)
{
var start = new MemorySegment <byte>(_currentDecodedBuffer.First);
// Append new content to end.
var end = start.Append(decodedBuffer.AsMemory(0, bytesWritten));
_currentDecodedBuffer = new ReadOnlySequence <byte>(start, 0, end, end.Memory.Length);
}
else
{
var start = (MemorySegment <byte>)_currentDecodedBuffer.Start.GetObject() !;
var end = (MemorySegment <byte>)_currentDecodedBuffer.End.GetObject() !;
// Append new content to end.
end = end.Append(decodedBuffer.AsMemory(0, bytesWritten));
_currentDecodedBuffer = new ReadOnlySequence <byte>(start, 0, end, end.Memory.Length);
}
return(new ReadResult(
_currentDecodedBuffer,
innerResult.IsCanceled,
innerResult.IsCompleted));
}
throw new InvalidOperationException("Unexpected status: " + status);
}