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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: Thu, 20 Oct 2016 10:03:56 +0100
User-agent: Mutt/1.7.0 (2016-08-17)

On Thu, Oct 20, 2016 at 10:55:52AM +0200, Markus Armbruster wrote:
> "Dr. David Alan Gilbert" <address@hidden> writes:
> > * Daniel P. Berrange (address@hidden) wrote:
> >> 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.
> >
> > 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?
> Well, that would be similar to how QMP used to work.
> Back when the design of the QMP monitor was hammered out, we discussed
> how to do errors.
> Anthony argued for passing around error objects.  I pointed out the
> enormous amount of work this would require: every call chain from the
> monitor to an error needs to be modified, with ripple effects throughout
> So I proposed a shortcut: have a function that reports the error, except
> when in QMP context store it in the monitor instead.  That way, you need
> to touch only places reporting errors, not every call chains leading to
> one.
> Sadly, that function couldn't be error_report() back then, because
> Anthony insisted on rich error objects, against my opposition.  To
> support them, we invented a new function, in commit 8204a91.  Code still
> had to be converted to this new function.  But it was the least
> laborious solution given the rich error object requirement.
> Anthony reluctantly accepted "store errors in monitor" as a transitional
> interface, mostly because we needed to get QMP off the ground fast, and
> passing around error objects would have slowed command conversion to a
> crawl.  I hoped the transitional interface would turn out to be quite
> practical, and remain.
> Rich errors turned out to be a dead end, and we abandoned them after a
> bit over two years (commit de253f1).
> The "store error in the monitor" turned out to be a dead end, too.  They
> lingered in the tree for a long time, until commit 4629ed1.  My memory
> is foggy on why exactly they didn't work out, but reasons include:
> * What if code attempts to store multiple errors?  We initially made
>   that an assertion failure, but quickly had to relax that so that
>   subsequent errors are silently ignored (commit 27a749f).  That's
>   differently suboptimal.
> * Failure remains difficult to see in the code.  Before QMP, a monitor
>   command handler didn't return status to the monitor core, it simply
>   reported it to the human user, possibly buried deep down in some call
>   chain.  Only if something up the chain needed to know, we additionally
>   propagated failure up the chain in ad hoc ways.  Making error
>   propagation the only way to fail commands made failure more obvious in
>   the code.
> * Plumbing errors to the correct monitor is easy only in the
>   (synchronous) monitor command handler.  If the handler kicks off some
>   background job, you can't store them in the monitor even if you know
>   which monitor kicked off the job, because that could interfere with
>   another handler's execution!  You'd have to find some other place to
>   store, and create some other code to examine that store and do what
>   needs to be done.  Whatever that may be: could be sending the error in
>   an asynchronous event, could be retaining for a later command to
>   report synchronously.  But then propagating errors up the call chain
>   starts to look more appealing than it used to.

Our code has increasingly converted to propagate errors up the call
chain, but having a mix of different error reporting approaches
is increasingly causing pain.

eg a function which propagates errors wants to call into a function
whicih uses error_report - there's no nice way to propagate the error
since it has already been reported.  If the function then wants to
explicitly ignore the error, then that's impossible too,since it has
already been reported.  Add in our code which doesn't use error_report
and instead returns errno values, such as the block layer, and it gets
even worse because if that calls a function which propagates an error,
it has to throw away that useful error and return a useless invented
errno value :(

IMHO continuing to convert code to propagate errors is the only way
out of this swamp, because it provides the greatest flexibility to
the callers of said functions to decide how to deal with the error.

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