Re: [PATCH 2/3] qemu: support passing pre-opened UNIX socket listen FD

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On Mon, Mar 26, 2018 at 03:19:36PM +0200, Ján Tomko wrote:
> On Wed, Mar 14, 2018 at 05:33:05PM +0000, Daniel P. Berrangé wrote:
> > There is a race condition when spawning QEMU where libvirt has spawned
> > QEMU but the monitor socket is not yet open. Libvirt has to repeatedly
> > try to connect() to QEMU's monitor until eventually it succeeds, or
> > times out. We use kill() to check if QEMU is still alive so we avoid
> > waiting a long time if QEMU exited, but having a timeout at all is still
> > unpleasant.
> > 
> > With QEMU 2.12 we can pass in a pre-opened FD for UNIX domain or TCP
> > sockets. If libvirt has called bind() and listen() on this FD, then we
> > have a guarantee that libvirt can immediately call connect() and
> > succeed without any race.
> > 
> > Although we only really care about this for the monitor socket and agent
> > socket, this patch does FD passing for all UNIX socket based character
> > devices since there appears to be no downside to it.
> > 
> > We don't do FD passing for TCP sockets, however, because it is only
> > possible to pass a single FD, while some hostnames may require listening
> > on multiple FDs to cover IPv4 and IPv6 concurrently.
> > 
> > Signed-off-by: Daniel P. Berrangé <berrange@xxxxxxxxxx>
> > ---
> > src/qemu/qemu_command.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 56 insertions(+), 2 deletions(-)
> > 
> > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> > index fa0aa5d5c3..291aad13cf 100644
> > --- a/src/qemu/qemu_command.c
> > +++ b/src/qemu/qemu_command.c
> > @@ -5010,8 +5010,62 @@ qemuBuildChrChardevStr(virLogManagerPtr logManager,
> >         break;
> > 
> >     case VIR_DOMAIN_CHR_TYPE_UNIX:
> > -        virBufferAsprintf(&buf, "socket,id=%s,path=", charAlias);
> > -        virQEMUBuildBufferEscapeComma(&buf, dev->data.nix.path);
> > +#ifndef WIN32
> 
> Why the ifdef?
> 
> Do we even build QEMU driver on WIN32?

Good point - this was just habit for any UNIX sockets code.

Regards,
Daniel
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