[libvirt] [PATCH v3 2/4] qemu: support passing pre-opened UNIX socket listen FD

[Daniel P. Berrangé]

On Thu, May 24, 2018 at 04:50:36PM -0500, Eric Blake wrote:
> On 05/17/2018 08:40 AM, 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.
> > 
> 
> > +++ b/tests/qemuxml2argvmock.c
> 
> > +int
> > +qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev ATTRIBUTE_UNUSED)
> > +
> > +{
> > +    /* We need to return an FD number for a UNIX listener socket,
> > +     * which will be given to QEMU via a CLI arg. We need a fixed
> > +     * number to get stable tests. This is obviously not a real
> > +     * FD number, so when virCommand closes the FD in the parent
> > +     * it will get EINVAL, but that's (hopefully) not going to
> > +     * be a problem....
> > +     */
> > +    return 1729;
> 
> Is it worth asserting that 1729 is not an open fd (perhaps by fcntl(1729,
> F_GETFD) == -1) because of something strange in the test environment?

Sure I'll add

diff --git a/tests/qemuxml2argvmock.c b/tests/qemuxml2argvmock.c
index 36d25dfc3f..56a6d4892b 100644
--- a/tests/qemuxml2argvmock.c
+++ b/tests/qemuxml2argvmock.c
@@ -228,5 +228,7 @@ qemuOpenChrChardevUNIXSocket(const virDomainChrSourceDef *dev ATTRIBUTE_UNUSED)
      * it will get EINVAL, but that's (hopefully) not going to
      * be a problem....
      */
+    if (fcntl(1729, F_GETFD) != -1)
+        abort();
     return 1729;
 }

Regards,
Daniel
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