Re: [Qemu-devel] chardev's and fd's in monitors

["Daniel P. Berrange" <berrange@redhat.com>]

On Wed, Oct 19, 2016 at 07:06:16PM +0100, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Wed, Oct 19, 2016 at 02:16:05PM +0200, Markus Armbruster wrote:
> > > "Daniel P. Berrange" <berrange@redhat.com> writes:
> > > 
> > > > On Wed, Oct 19, 2016 at 11:05:53AM +0100, Dr. David Alan Gilbert wrote:
> > > >> 
> > > >> We need a way to be able to report an error without plumbing error_setg
> > > >> up the stack; if you're saying error_report isn't suitable then we
> > > >> should just recommend we switch everything in migration back to
> > > >> fprintf(stderr,
> > > 
> > > In the cases where error_report() isn't suitable, fprintf() is just as
> > > unsuitable for the exact same reasons.
> > > 
> > > > Well both error_report() + fprintf  are broken from POV of anything
> > > > using QMP. error_report() is slightly less broken for HMP,
> > > 
> > > error_report() is not broken at all for HMP code.  The trouble is code
> > > that can't know whether it's running in a context where error_report()
> > > is suitable.
> > > 
> > > >                                                            but doesn't
> > > > help QMP.
> > > 
> > > Correct.
> > > 
> > > > In the short term we should just make error_report be  threadsafe in
> > > > its usage of the monitor.
> > > 
> > > Any problems left once cur_mon is thread-local (which it should be
> > > anyway)?
> > 
> > If we make cur_mon a thread-local, then error_report() is equivalent
> > to fprintf(stderr) for the migration code, since the migration
> > code runs in a different thread thread, and so would always see
> > cur_mon == NULL.
> 
> Yes, that would become safe; it does sound the best fix for the current
> worry.
> 
> If we had that, then why not wire up error_report to pass errors back to QMP
> as well?

You have a problem of context - if you have multiple monitors, how do
you know which to send the error back to if you're not in the event
loop thread, and thus cur_mon is NULL. With Marc-Andre's series which
allows proper async command processing it gets even harder, because
there's potentially many outstanding commands associated with a monitor
and you need to decide which the error should be given to.

Regards,
Daniel
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