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

[ronnie sahlberg]

On Fri, 30 Sept 2022 at 11:28, Andrew Bartlett via samba-technical
<samba-technical at lists.samba.org> 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.
>
> Technically even after our server OS has got the FIN, there is
> potentially data in the read buffer (so a read() might not return 0
> anyway), but perhaps most of the time that would be 0.
>
> So how can we detect this?  Can we at least put a timeout on a writev()
> call via tsocket et al?  If so, how do we do that?

Do I understand this right?
Your process has a socket that is in CLOSE_WAIT state. When the process tries to
writev() to this socket it gets -EAGAIN and not a more "hard" error?

I wonder if getsockopt(fd, SOL_SOCKET, SO_ERROR,  could be useful here
or would that also return EAGAIN.


>
> Mailing list threads:
>
> https://lists.samba.org/archive/samba/2022-September/241869.html
> https://lists.samba.org/archive/samba/2022-September/241873.html
>
> Andrew Bartlett
> --
> Andrew Bartlett (he/him)       https://samba.org/~abartlet/
> Samba Team Member (since 2001) https://samba.org
> Samba Team Lead, Catalyst IT   https://catalyst.net.nz/services/samba
>
> Samba Development and Support, Catalyst IT - Expert Open Source
> Solutions
>
>