read_some() with null_buffers blocks in non-blocking mode

[Gennady Proskurin <gpr@…>]

I want to poll socket for readability (the same as calling POSIX poll or select with zero timeout), to just know if it is readable:

1. call sock.non_blocking(true)
2. call read_some with null_buffers()
3. it blocks

If I change null_buffers() to some read buffer, all works fine, would_block error is returned. The correct behaviour, I think, is to return immediately with corresponding error (no error if socket is readable, would_block if it is not).

At least tcp socket and posix::stream_descriptor are affected, not tested for other sockets. Also not tested write_some(), not sure if it is affected.

test program attached

[chris_kohlhoff]

(In [74818]) Make sure the synchronous null_buffers operations obey the user's non_blocking setting. Refs #5756

[chris_kohlhoff]

(In [74863]) Merge from trunk...

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