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

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(a)redhat.com&gt; > Signed-off-by: Daniel P. Berrangé <berrange(a)redhat.com&gt; > --- > 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

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".