> This is a hairy issue

Indeed.

Performing partial read/write may sound imperfect, but using buffered I/O around non-blockind FD is definitely not a good idea.
Also, the advantage of the current approach is that at least, no data is ever lost (and changing the behavior to raise a BlockingIOError might break some code out there in the wild).

Note that Java's BufferedInputStream and ReadableByteChannel also return partial reads.

So I'm somewhat inclined to keep the current behavior (it would however probably be a good idea to update the documentation to warn about this limitation, though).