Re: kern/60782: SIGIO says socket ready for send before send does

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

> Date: Thu, 24 Sep 2026 21:17:18 -0000 (UTC)
> From: mlelstv%serpens.de@localhost (Michael van Elst)
> 
> Sending SIGIO and waking up a blocked writer is done at the same time,
> but a blocked writer just blocks again without noticable issue.

I think the basic issue is that the wakeup path in recv(2) that leads
to SIGIO is simply not conditional on anything about the buffer state.
I bet nobody noticed this because it doesn't cause an actual return
from poll -- just another pass through polscan or whatever, which
might hurt performance but doesn't hurt correctness.

Perhaps sorwakeup/sowwakeup should just check soreadable/sowritable
before calling sowakeup.

> poll(2) checks sbspace(sb) >= sb->sb_lowat. So it signals a writable
> descriptor at a different time. If sbspace(sb) > 0 a send smaller
> than sb_lowat may even succeed when, according to poll(2) it's not
> yet possible.
> 
> 
> Maybe changing the test in sbspace to:
> 
> 	if (sb->sb_hiwat <= sb->sb_cc)
> 		return 0;
> 	if (sb->sb_mbmax <= sb->sb_mbcnt) 
> 		return 1;
> 
> already helps.

Why would changing sbspace affect this?  I think it is not used
anywhere in the decision of whether to send SIGIO or not.  (But maybe
I missed something -- I only took a cursory skim at this part while
lifting the SIGIO rock and finding a whole colony of bugs under it!)