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?
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3266
Virtualization: qemu.org | libvirt.org
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