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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* 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?

Dave

> > >                           Beyond the short term we have no choice but
> > > to plumb in error_setg throughout, otherwise QMP will continue to
> > > have useless error reporting in this area of code.
> > 
> > Actually, no choice but propagate errors up the stack until they reach a
> > spot that knows how to report them.  error_setg() is *one* way to do
> > that.  Its prime advantage is ability to carry an error message.  Its
> > disadvantage is boilerplate.  Use it as needed.  Just don't convert back
> > and forth between Error and other representations while propagating up
> > the stack.
> 
> Regards,
> Daniel
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK