How to detect a client-closed connection during a write from our LDAP server?

[ronnie sahlberg]

On Fri, 30 Sept 2022 at 21:12, Isaac Boukris via samba-technical
<samba-technical at lists.samba.org> wrote:
>
> On Fri, 2022-09-30 at 14:26 +1300, Andrew Bartlett via samba-technical
> wrote:
> > I've been trying to chase down the CPU spins reported by our users in
> > the writev() codepath from our LDAP server.
> >
> > A private mail the the strace output shows the sockets are in
> > CLOSE_WAIT state, returning EAGAIN over and over (after a call to
> > epoll() each time).  That alone would be enough to keep things
> > spinning.
> >
> > But they are being shut down, however our LDAP server won't be
> > triggering a read any time soon, it is waiting to flush the response
> > out.
>
> Why wouldn't it? I mean why does the read waits for the write? if epoll
> says so then we should read, then we may detect that the client closed
> it end and may decide to give up the writes.
>
> Technically, I think there is no problem to write to a socket after the
> peer sent us FIN,

I think possibly the problem is that IF the tcp tx buffer is full, you
might get -EAGAIN instead of a better error
if the socket is only half-closed.

> it just mean he closed its end and won't write no
> more, but we can still write until we close or he/we reset (as is the
> case with HTTP 1.0).
>