/// <inheritdoc />
public override async Task <int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
{
using (StreamOperationToken op = StartNewOperation(StreamOperation.Read))
{
BackgroundOperationSlot backgroundOperationSlot = await op.WaitForBackgroundOperationSlotAsync();
Contract.Assume(
internalBuffer.State != FileBuffer.BufferState.Locked,
"Buffer should not be locked, since no background operation is running.");
int bytesReadFromBuffer;
bool fillStarted = ReadBufferAndStartFillIfEmptiedButUsable(backgroundOperationSlot, buffer, offset, count, out bytesReadFromBuffer);
if (bytesReadFromBuffer == 0)
{
if (fillStarted)
{
backgroundOperationSlot = await op.WaitForBackgroundOperationSlotAsync();
}
// We just ran a fill and waited for it (usability may be updated on its completion) or we were unable to start a fill.
// In either case, we should now respond to usability. We don't have any bytes read, and so we are now in some sense at the position
// where the unusability occurs, rather than behind it (e.g. we should quietly exhaust the buffer before complaining about EOF).
switch (backgroundOperationSlot.Usability)
{
case StreamUsability.Usable:
Contract.Assume(
fillStarted,
"ReadBufferAndStartFillIfEmptiedButUsable should have started a fill, since the stream is usable");
Analysis.IgnoreResult(
ReadBufferAndStartFillIfEmptiedButUsable(backgroundOperationSlot, buffer, offset, count, out bytesReadFromBuffer));
Contract.Assume(
bytesReadFromBuffer > 0,
"Usable stream implies that the completed fill obtained bytes. Zero bytes returned from ReadFile implies failure.");
break;
case StreamUsability.EndOfFileReached:
Contract.Assume(
internalBuffer.State == FileBuffer.BufferState.Empty,
"EndOfFileReached usability coincides with a totally-failed fill (nothing read)");
break;
case StreamUsability.Broken:
throw backgroundOperationSlot.ThrowExceptionForBrokenStream();
default:
throw Contract.AssertFailure("Unhandled StreamUsability");
}
}
// We've satisfied a read request for 'bytesReadFromBuffer' bytes, which advances our virtual file position.
op.AdvancePosition(bytesReadFromBuffer);
return(bytesReadFromBuffer);
}
}