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

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] chardev's and fd's in monitors
Date: Wed, 19 Oct 2016 13:21:58 +0100
User-agent: Mutt/1.7.0 (2016-08-17)

On Wed, Oct 19, 2016 at 02:16:05PM +0200, Markus Armbruster wrote:
> "Daniel P. Berrange" <address@hidden> 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.

> >                           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.

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