Re: [Qemu-devel] [PATCH 3/9] block: bdrv_reopen_prepare(): use error_setg_file_open()

[Luiz Capitulino <lcapitulino@redhat.com>]

On Mon, 10 Jun 2013 15:54:10 +0200
Kevin Wolf <kwolf@redhat.com> wrote:

> Am 10.06.2013 um 15:21 hat Luiz Capitulino geschrieben:
> > On Mon, 10 Jun 2013 10:43:47 +0200
> > Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > 
> > > On Fri, Jun 07, 2013 at 03:52:29PM -0400, Luiz Capitulino wrote:
> > > > Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
> > > > ---
> > > >  block.c | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > 
> > > > diff --git a/block.c b/block.c
> > > > index 79ad33d..c78f152 100644
> > > > --- a/block.c
> > > > +++ b/block.c
> > > > @@ -1291,8 +1291,7 @@ int bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
> > > >              if (local_err != NULL) {
> > > >                  error_propagate(errp, local_err);
> > > >              } else {
> > > > -                error_set(errp, QERR_OPEN_FILE_FAILED,
> > > > -                          reopen_state->bs->filename);
> > > > +                error_setg_file_open(errp, errno, reopen_state->bs->filename);
> > > 
> > > Looking closer, my suggestion was wrong too.
> > > 
> > > I think QERR_OPEN_FILE_FAILED is simply the wrong error here.  We don't
> > > know that the error occurred when trying to open a file.
> > 
> > Right.
> > 
> > > errno does not necessarily contain the error value!
> > 
> > There are two ways to fix it (and they're not mutually exclusive):
> > 
> >  1. We could review all bdrv_reopen_prepare() methods and make sure they
> >     set errno on failure
> > 
> >  2. We set errno=0 before calling the bdrv_reopen_prepare() method and if
> >     there's an error and if errno != 0 we use it, otherwise we set a generic
> >     "failed to prepare to reopen image" error
> > 
> > Option 1 goes a bit beyond the time I'd like to spent on this series.
> > Option 2 is quite reasonable.
> > 
> > What do you think?
> 
> errno is definitely not reliable here and won't be. You must use -ret if
> you want a meaningful error message.
> 
> I think Stefan's point was more that "Could not open" might not be the
> right message for a reopen, so we'd have to use error_setg_errno()
> directly with a different message here, like "Could not prepare reopen
> for '%s'".

Ok, but Stefan also said that -ret is not reliable. And after quickly
checking the code I see he's right, as there are methods that return -1.

I think the safest thing to do is to have a generic error message for
for this now and in the future we should propagate errp or return -errno.