Re: [PATCH v2 6/7] qemufile: Always return a verbose error

[Peter Xu <peterx@redhat.com>]

On Wed, Jul 05, 2023 at 06:54:37PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > There're a lot of cases where we only have an errno set in last_error but
> > without a detailed error description.  When this happens, try to generate
> > an error contains the errno as a descriptive error.
> >
> > This will be helpful in cases where one relies on the Error*.  E.g.,
> > migration state only caches Error* in MigrationState.error.  With this,
> > we'll display correct error messages in e.g. query-migrate when the error
> > was only set by qemu_file_set_error().
> >
> > Signed-off-by: Peter Xu <peterx@redhat.com>
> > ---
> >  migration/qemu-file.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > index acc282654a..419b4092e7 100644
> > --- a/migration/qemu-file.c
> > +++ b/migration/qemu-file.c
> > @@ -156,15 +156,24 @@ void qemu_file_set_hooks(QEMUFile *f, const QEMUFileHooks *hooks)
> >   *
> >   * Return negative error value if there has been an error on previous
> >   * operations, return 0 if no error happened.
> > - * Optional, it returns Error* in errp, but it may be NULL even if return value
> > - * is not 0.
> >   *
> > + * If errp is specified, a verbose error message will be copied over.
> >   */
> >  int qemu_file_get_error_obj(QEMUFile *f, Error **errp)
> >  {
> > +    if (!f->last_error) {
> > +        return 0;
> > +    }
> > +
> > +    /* There is an error */
> >      if (errp) {
> > -        *errp = f->last_error_obj ? error_copy(f->last_error_obj) : NULL;
> > +        if (f->last_error_obj) {
> > +            *errp = error_copy(f->last_error_obj);
> > +        } else {
> > +            error_setg_errno(errp, -f->last_error, "Channel error");
> 
> There are a couple of places that do:
> 
>     ret = vmstate_save(f, se, ms->vmdesc);
>     if (ret) {
>         qemu_file_set_error(f, ret);
>         break;
>     }
> 
> and vmstate_save() can return > 0 on error. This would make this message
> say "Unknown error". This is minor.
> 
> But take a look at qemu_fclose(). It can return f->last_error while the
> function documentation says it should return negative on error.
> 
> Should we make qemu_file_set_error() check 'ret' and always set a
> negative value for f->last_error?

Yeah, maybe we can add a sanity check, but logically it's better we just
fix vmstate_save() to make sure it always returns a <0 error.

It seems to me there're so many hooks in vmstate_save_state_v() that it can
return random things.  What's the one you spot?  If it's an obvious issue
we can fix them.

-- 
Peter Xu