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Currently, if a command wants to do asynchronous IO, a callback is registered in the libvirtd eventloop to handle writes and reads. However, there's a race in virCommandWait. The eventloop may already be executing the callback, while virCommandWait is mangling internal state of virCommand. To deal with it, we need to either introduce locking or spawn a separate thread where we poll() on stdio from child. The former, however, requires to unlock all mutexes held, as the event loop may execute other callbacks which tries to lock one of the mutexes, deadlock and thus never wake us up. So it's safer to spawn a separate thread.
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>