Re: [PATCH 2/2] qemu_process: Start QEMU for caps probing more robustly

When probing QEMU capabilities, we look at whatever <emulator/> was specified in the domain XML and execute it with couple of arguments (-daemonize being one of them) Then, we use virCommandSetErrorBuffer() to read stderr of the child process hoping to read possible error message just before the process daemonized itself. Well, this works as long as the emulator binary behaves. If the binary is evil and basically does the following: #!/bin/bash sleep 1h then virCommandRun() called from qemuProcessQMPLaunch() doesn't return for whole hour (because it's stuck in reading stderr of the child process). This behavior of ours is very suboptimal. The solution is to not rely on the binary behaving correctly on -daemonize argument but to daemonize the process ourselves (via virCommandDaemonize()) and then wait for the monitor to show up with a timeout. This in turn means, that we can no longer use virCommandSetErrorBuffer() but we can do the equivalent with virCommandSetErrorFD() and a bit of code. Sure, this doesn't shield us from malicious binaries 100% but helps preventing depletion of worker threads.