On Mon, Jul 21, 2025 at 03:05:59PM +0100, Daniel P. Berrangé wrote:
> On Mon, Jul 21, 2025 at 07:24:23PM +0530, Arun Menon wrote:
> > Hi,
> > Thank you for the review.
> >
> > On Mon, Jul 21, 2025 at 02:32:48PM +0100, Daniel P. Berrangé wrote:
> > > On Mon, Jul 21, 2025 at 04:59:28PM +0530, Arun Menon wrote:
> > > > - We need to have good error reporting in the callbacks in
> > > > VMStateDescription struct. Specifically pre_save, post_save,
> > > > pre_load and post_load callbacks.
> > > > - It is not possible to change these functions everywhere in one
> > > > patch, therefore, we introduce a duplicate set of callbacks
> > > > with Error object passed to them.
> > > > - So, in this commit, we implement 'errp' variants of these callbacks,
> > > > introducing an explicit Error object parameter.
> > > > - This is a functional step towards transitioning the entire codebase
> > > > to the new error-parameterized functions.
> > > > - Deliberately called in mutual exclusion from their counterparts,
> > > > to prevent conflicts during the transition.
> > > > - New impls should preferentally use 'errp' variants of
> > > > these methods, and existing impls incrementally converted.
> > > > The variants without 'errp' are intended to be removed
> > > > once all usage is converted.
> > > >
> > > > Signed-off-by: Arun Menon <arme...@redhat.com>
> > > > ---
> > > > include/migration/vmstate.h | 11 +++++++++++
> > > > migration/vmstate.c | 47
> > > > +++++++++++++++++++++++++++++++++++++++------
> > > > 2 files changed, 52 insertions(+), 6 deletions(-)
> > > >
> > >
> > > > diff --git a/migration/vmstate.c b/migration/vmstate.c
> > > > index
> > > > 288b57e1ed778cce21247b64d5e97dfef41ad586..d96908d12ccffaef421e5d399a48e26cada2cb77
> > > > 100644
> > > > --- a/migration/vmstate.c
> > > > +++ b/migration/vmstate.c
> > >
> > > > @@ -524,7 +548,12 @@ int vmstate_save_state_v(QEMUFile *f, const
> > > > VMStateDescription *vmsd,
> > > > if (ret) {
> > > > error_setg(errp, "Save of field %s/%s failed",
> > > > vmsd->name, field->name);
> > > > - if (vmsd->post_save) {
> > > > + if (vmsd->post_save_errp) {
> > > > + ret = vmsd->post_save_errp(opaque, &local_err);
> > > > + if (ret < 0) {
> > > > + error_propagate(errp, local_err);
> > > > + }
> > >
> > > This is still broken. 'errp' is already set a few lines earlier, so you
> > > can't propagate a new error over the top
> >
> > I was wondering that we should preserve the first error that was
> > encountered.
> > So even if local_err was set, and in case errp already has an error, then
> > it will
> > be a no-op and local_err will be freed.
>
> We know that 'local_err' is definitely set when 'post_save_errp' is called,
> because there's a call to 'error_setg' right above it.
mmm, error_setg() above that sets errp, local_err is set only of
post_save_errp() has
errors. Do we want both the erros to be propagated? or is it okay to propagate
the first
error that was encountered.
>
>
>
> > > > + } else if (vmsd->post_save) {
> > > > vmsd->post_save(opaque);
> > > > }
>
> ... pre-existing mistake not checking return value of
> post_save.
>
> > > > return ret;
> > > > @@ -552,7 +581,13 @@ int vmstate_save_state_v(QEMUFile *f, const
> > > > VMStateDescription *vmsd,
> > > >
> > > > ret = vmstate_subsection_save(f, vmsd, opaque, vmdesc, errp);
> > > >
> > > > - if (vmsd->post_save) {
> > > > + if (vmsd->post_save_errp) {
> > > > + int ps_ret = vmsd->post_save_errp(opaque, &local_err);
> > > > + if (!ret && ps_ret) {
> > > > + ret = ps_ret;
> > > > + error_propagate(errp, local_err);
> > > > + }
> > >
> > > Again, propagating over the top of an existing error
> >
> > Sorry, correct me if I am wrong.
> > Since we have 'if (!ret && ps_ret)' ,
> > if vmstate_subsection_save() fails, the above condition will not hold true.
> > Only if the first function call vmstate_subsection_save() is successful and
> > the second one
> > post_save_errp() fails then we try to propagate, again hoping to preserve
> > the first error.
>
> Opps, yes, you're right - I missed the 'ps_ret' check. That means this
> code is a memory leak when 'ret' is non-zero, as nothing frees 'local_err'
> in that case.
Yes, maybe I can null check local_err and error_free() it.
>
> >
> > >
> > > > + } else if (vmsd->post_save) {
> > > > int ps_ret = vmsd->post_save(opaque);
> > > > if (!ret && ps_ret) {
> > > > ret = ps_ret;
> > > >
> > > > --
> > > > 2.50.0
> > > >
> > > >
> > >
> > > With regards,
> > > Daniel
> > > --
> > > |: https://berrange.com -o-
> > > https://www.flickr.com/photos/dberrange :|
> > > |: https://libvirt.org -o-
> > > https://fstop138.berrange.com :|
> > > |: https://entangle-photo.org -o-
> > > https://www.instagram.com/dberrange :|
> > >
> >
> > Regards,
> > Arun Menon
> >
>
> With regards,
> Daniel
> --
> |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :|
> |: https://libvirt.org -o- https://fstop138.berrange.com :|
> |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
>
Regards,
Arun
