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Currently the lxc controller process itself is responsible for daemonizing itself into the background and writing out its pid file. The lxc driver would fork the controller and then attempt to connect to the lxc monitor. This connection would only succeed after the controller has backgrounded itself, setup cgroups and written its pid file, so startup was race free. The problem is that we need to delay create of the cgroups to much later, such that we can tell systemd the container init pid when we create the cgroups. If we delay cgroup creation though the current synchronization won't work. A second problem is that the controller needs the XML config of the guest. Currently we write out the plain virDomainDefPtr XML before starting the controller, and then later replace it with the full virDomainObjPtr status XML. This is kind of gross and also means that the controller doesn't get a record of the live XML config right away. This means it doesn't have a record of the veth device names either and so can't give that info to systemd when creating the cgroups. To address this we change the startup sequencing. The goal is that we want to get the PID as soon as possible, before the LXC controller even starts. So we stop letting the LXC controller daemonize itself, and instead use virCommand's built-in capabilities. This daemonizes and writes the PID before LXC controller is exec'd. So the driver can read the PID as soon as virCommandRun returns. It is no longer safe to connect to the monitor or detect the cgroups though. Fortunately the LXC controller already has a second point of synchronization. Immediately before its event loop starts running, it performs a handshake with the driver. So we move the opening of the monitor connection and cgroup detection after this synchronization point.
LibVirt : simple API for virtualization
Libvirt is a C toolkit to interact with the virtualization capabilities
of recent versions of Linux (and other OSes). It is free software
available under the GNU Lesser General Public License. Virtualization of
the Linux Operating System means the ability to run multiple instances of
Operating Systems concurrently on a single hardware system where the basic
resources are driven by a Linux instance. The library aim at providing
long term stable C API initially for the Xen paravirtualization but
should be able to integrate other virtualization mechanisms if needed.
Daniel Veillard <veillard@redhat.com>