Re: Calling 'select' from emacs_gnutls_pull

[Eli Zaretskii <eliz@gnu.org>]

> From: Ted Zlatanov <tzz@lifelogs.com>
> Date: Sat, 16 Feb 2013 10:55:45 -0500
> 
> ...and there are comments accumulated over the years in that
> neighborhood.  Claudio Bley contributed much of the original code here,
> including the piece you're referring to.  The history is at
> 
> http://comments.gmane.org/gmane.emacs.devel/136816

I've (re-)read that thread, but didn't find any rationale for calling
'select'.  Claudio, could you perhaps chime in?

> I think you're correct, but remember the GnuTLS library is calling these
> functions.

But GnuTLS calls these functions because Emacs called
'emacs_gnutls_read', right?  Which seems to be just a fancy way of
reading from a socket, as far as Emacs is concerned, as this fragment
from 'read_process_output', a subroutine of 'wait_reading_process_output',
seems to indicate:

  #ifdef HAVE_GNUTLS
	if (p->gnutls_p)
	  nbytes = emacs_gnutls_read (p, chars + carryover + buffered,
				      readmax - buffered);
	else
  #endif
	  nbytes = emacs_read (channel, chars + carryover + buffered,
			       readmax - buffered);

Here, 'emacs_read' (on Windows) just calls 'sys_read'.  But
'emacs_gnutls_read', via 'emacs_gnutls_pull', in addition also calls
'select', and I wonder why this difference is necessary.

> So I don't know if it would make sense to let the Emacs
> reader thread handle the socket management, because we could end up with
> poor performance or even contention.

We already use the Emacs reader threads for that.  And it works well
enough for sockets.  In a nutshell, the reader thread does blocking
reads from the socket, while 'sys_read' (called by Emacs as 'read')
emulates non-blocking reads, by looking at the status set by the
reader thread.

> if I can help simulate this condition or test some possible
> solutions, I'll be glad to assist.

Thanks.  It would be good if you (or someone else) could describe a
simple setup for reading stuff via GnuTLS from some URL, which I could
then try on my machine under a debugger.