Re: [PATCH 1/5] qapi: allow for g_autoptr(Error) usage

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

On Tue, Jul 23, 2024 at 01:36:32PM +0200, Markus Armbruster wrote:
> Daniel P. Berrangé <berrange@redhat.com> writes:
> 
> > While common error propagation practice does not require manually
> > free'ing of local 'Error' objects, there are some cases where this
> > is needed. One example is where the 'Error' object is only used
> > for providing info to a trace event probe. Supporting g_autoptr
> > avoids the need to manually call 'error_free'.
> >
> > Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
> > ---
> >  include/qapi/error.h | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/include/qapi/error.h b/include/qapi/error.h
> > index 71f8fb2c50..6e429809d8 100644
> > --- a/include/qapi/error.h
> > +++ b/include/qapi/error.h
> > @@ -437,6 +437,8 @@ Error *error_copy(const Error *err);
> >   */
> >  void error_free(Error *err);
> >  
> > +G_DEFINE_AUTOPTR_CLEANUP_FUNC(Error, error_free);
> > +
> >  /*
> >   * Convenience function to assert that *@errp is set, then silently free it.
> >   */
> 
> The Error interface is designed for a certain way of using it: an Error
> object flows from the spot detecting the error to a spot handling it.
> Failure to handle the error is a memory leak.  Our tooling can help with
> tracking these down.
> 
> The interface tries to make the intended use easy: functions that report
> an error consume the Error object.  Explicit error_free() should only
> needed when you handle an error in some other way.
> 
> When such an explicit error_free() is needed on all paths to return,
> then replacing it with auto-freeing is nice.  But what if it isn't?
> 
> Say we add a new error path and use error_report_err(err) there.  This
> has always been just fine.  No more: if @err is auto-freed, this is a
> double-free.  We have to also add err = NULL.  Feels like a trap for
> developers to me.
> 
> Your use of auto-freeing is in the next patch.  It's this pattern:
> 
>     g_autoptr(Error) err = NULL;
> 
>     if (!frobnicate(args, &err)) {
>         trace_frobnicate_err(..., error_get_pretty(err));
>     }
> 
> You want to report the error to a trace point.  That's perfectly
> legitimate.  The problem is that this kind of error reporting function
> does not free, unlike the ones provided by qapi/error.h.
> 
> We could extend tracing to accept Error values, so that
> 
>         trace_frobnicate_err(..., err);
> 
> does free.  Doesn't seem worthwhile unless we find quite a few more uses
> for it.

That is awkward because the trace calls expand to nothing at all
when tracing is disabled, so we can't rely on them to free any
args.


> If we conclude we want to provide auto-free as an option, we at least
> need to point out the trap in a comment.  A bit of a pain to write, and
> whether people will read, understand, and remember it is uncertain.
> 
> My gut feeling right now: stick to the design, and free manually.  If
> you think my gut is wrong, tell me.

I'll drop this since there's only one place benefitting right now.

With regards,
Daniel
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