>>   max(buffer_size, n-avail)
> 
> I mimicked the original logic rather than rethink the algorithm. I'm not totally
> sure what motivates the original logic but the purpose seems to be that
> non-blocking streams can return at least a few bytes rather than an empty string
> when less than N bytes are ready at OS level.

IIUC, a read of the full requested size would achieve exactly that: on a
non-blocking stream (IIUC), a read will always return
min(bytes_available, bytes_requested).

> Hmm, what do you mean by "if the stream is unbuffered"? Unbuffered streams
> should use the raw unbuffered objects (e.g. FileIO) rather than wrap them with
> BufferedReader.

IIUC, io.open will always return a BufferedReader, potentially with
buffer_size=0 for unbuffered IO. This case must be supported.