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Re: [PATCH] use qemudReadLogOutput (was: Re: [libvirt] can't install fedora guest)



On Sat, Feb 07, 2009 at 06:05:51PM +0100, Guido G?nther wrote:
> On Fri, Feb 06, 2009 at 04:48:09PM +0000, Daniel P. Berrange wrote:
> > That will cause libvirtd to spin 100% cpu forever, if a guest fails
> > to start up. eg disk to mis-configured disk
> > 
> > The core problem here, is that ret == 0 has 2 possible implications
> > 
> >  - QEMU has exited, and no more data will be written
> >  - QEMU is still starting up, and we have read all the data
> >    written so far, but more may arrive soon.
> > 
> > The current code there is correct for the first scenario, but even
> > removing it, is not entirely correct for the 2nd scenario. If we
> > hit ret == 0, and QEMU is still running, we shouldn't spin 100%
> > CPU in read - we should poll() to wait for more data.
> > 
> > As a quick fix though, we could probably detect whether QEMU has exited
> > by doing  'kill(vm->pid, 0)' and check for errno == ESRCH - indicates
> > that the process no longer exists.
> > 
> > eg,
> > 
> >    } else if (ret == 0) {
> >        if (kill(vm->pid, 0) == -1) {
> >           if (errno == ERSCH)
> >              return 0;
> >           else
> >              return -1;
> >        } else {
> >          continue; /* should really go into poll() at this point */
> >        }
> >    } else {
> The problem is that we're using this function for two purposes: To read
> from a logfile where poll()'ing on POLLIN always returns "data readable"
> which leaves us spinning (and EOF might get in our way) and also using
> it on the monitor PTY itself where it works as expected. Why not simply
> introduce qemudReadLogOutput? This at least allows us to usleep while
> waiting for the log file to fill up with data. I couldn't make libvirtd
> spin with this patch anymore. O.k. to apply?

Ah of course, I forgot poll()  isn't particularly useful on regular
files. For a non-spinning version we'd need to integrate with inotify
to monitor changes. ACK to your proposed patch for now.

Daniel
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