Re: [Xen-devel] [PATCH v7 02/11] error: auto propagated local_err

[Eric Blake <eblake@xxxxxxxxxx>]

On 2/21/20 3:42 AM, Vladimir Sementsov-Ogievskiy wrote:

> > > +#define ERRP_AUTO_PROPAGATE()                                  \
> > > +    g_auto(ErrorPropagator) _auto_errp_prop = {.errp = errp};  \
> > > +    errp = ((errp == NULL || *errp == error_fatal)             \
> > > +            ? &_auto_errp_prop.local_err : errp)
> > > +
> > >   /*
> > >    * Special error destination to abort on error.
> > >    * See error_setg() and error_propagate() for details.
> > *errp == error_fatal tests *errp == NULL, which is not what you want.
> > You need to test errp == &error_fatal, just like error_handle_fatal().
> Oops, great bug) And nobody noticed before) Of course, you are right.
Sorry I missed it in my earlier looks.

> > Superfluous parenthesis around the first operand of ?:.
> >
> > Wouldn't
> >
> >     #define ERRP_AUTO_PROPAGATE()                                  \
> >         g_auto(ErrorPropagator) _auto_errp_prop = {.errp = errp};  \
> >         if (!errp || errp == &error_fatal) {                       \
> >             errp = &_auto_errp_prop.local_err;                     \
> >         }
> >
> > be clearer?
> Hmm, notation with "if" will allow omitting ';' after macro invocation, 
> which seems not good..
Then wrap it:

g_auto(ErrorPropagator) _auto_errp_prop = {.errp = errp}; \
do { \
  if (!errp || errp == &error_fata) {
    errp = &_auto_errp_prop.local_err; \
  } \
while (0)


> And if I'm not wrong we've already discussed it somewhere in previous 
> versions.
The original use of ?: stems from my suggestion on an earlier revision 
when we were still trying to pack everything into two consecutive 
declaration lines, rather than a declaration and a statement (as ?: is 
necessary for conditionals in declarations).  But since then, we decided 
to go with a statement (we require a C99 compiler, so declaration after 
statement is supported by our compiler, even if our coding style 
currently avoids it where possible), so as long as we support 
statements, we might as well go with a legible statement instead of 
insisting on the compact ?: form.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org


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