ascynio readexactly() should raise ValueError if passed length <= 0 in argument.

Now, it return empty string, which is just error hiding. Why not to raise ValueError ? 

Returning empty string is error prone. This behaviour was not changed since initial commit of asyncio, so probably was not done intentionally.

I'm okay with raising ValueError for length<0; but for length==0 the
behavior is very much intentional -- reading zero bytes is sometimes a
useful end case that otherwise you would have to work around in the
caller.

result of readexactly(0) is indistinguishable from EOF, so no one should try to use that as I think.

What is the useful case when zero bytes should be read ?

read(0) on a regular stream and os.read(fd, 0) also return an empty string without error. This is not something we should keep debating.

Okay, https://github.com/python/asyncio/pull/298 is ready

This issue is now resolved (see https://github.com/python/asyncio/pull/298)