[libvirt] [PATCH v2 2/4] qemu: support passing pre-opened UNIX socket listen FD

[Peter Krempa]

On Fri, Apr 20, 2018 at 10:18:18 -0400, John Ferlan wrote:
> 
> 
> On 04/18/2018 01:30 PM, 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.
> > 
> > Reviewed-by: John Ferlan <jferlan at redhat.com>
> > Signed-off-by: Daniel P. Berrangé <berrange at redhat.com>
> > ---
> >  src/qemu/qemu_command.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 52 insertions(+), 2 deletions(-)
> > 
> 
> This one is now affected by Peter's recent xml2argv adjustment for:
> 
> tests/qemuxml2argvdata/disk-drive-write-cache.x86_64-latest.args
> 
> which now fails w/ :
> 
> 253) QEMU XML-2-ARGV disk-drive-write-cache.x86_64-latest
> ... libvirt: QEMU Driver error : Unable to bind to UNIX socket path
> '/tmp/lib/domain--1-QEMUGuest1/monitor.sock': No such file or directory
> FAILED

I'm kind of glad I've added this test :)

> because the qemuProcessCreatePretendCmd doesn't allow this code to
> distinguish that it shouldn't attempt a bind/listen <sigh>
> 
> I "think" this may require moving the socket setup code into
> qemuProcessLaunch or somehow passing a flag into BuildCommandLine to
> tell it not to do anything "real".

Yeah. The command line generator should be inert to the host, so this
needs to be moved to the preparation steps.
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