Re: [Qemu-devel] [Qemu-block] [PATCH for-2.10 01/16] block: Add PreallocMode to BD.bdrv_truncate()

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Wed, Mar 22, 2017 at 05:50:59PM +0100, Max Reitz wrote:
> On 22.03.2017 17:28, Stefan Hajnoczi wrote:
> > On Mon, Mar 20, 2017 at 04:07:16PM +0100, Max Reitz wrote:
> >> On 20.03.2017 11:18, Stefan Hajnoczi wrote:
> >>> On Mon, Mar 13, 2017 at 10:39:46PM +0100, Max Reitz wrote:
> >>>> diff --git a/block/iscsi.c b/block/iscsi.c
> >>>> index ab559a6f71..5d6265c4a6 100644
> >>>> --- a/block/iscsi.c
> >>>> +++ b/block/iscsi.c
> >>>> @@ -2060,11 +2060,16 @@ static void iscsi_reopen_commit(BDRVReopenState *reopen_state)
> >>>>      }
> >>>>  }
> >>>>  
> >>>> -static int iscsi_truncate(BlockDriverState *bs, int64_t offset, Error **errp)
> >>>> +static int iscsi_truncate(BlockDriverState *bs, int64_t offset,
> >>>> +                          PreallocMode prealloc, Error **errp)
> >>>>  {
> >>>>      IscsiLun *iscsilun = bs->opaque;
> >>>>      Error *local_err = NULL;
> >>>>  
> >>>> +    if (prealloc != PREALLOC_MODE_OFF) {
> >>>> +        return -ENOTSUP;
> >>>> +    }
> >>>> +
> >>>>      if (iscsilun->type != TYPE_DISK) {
> >>>>          return -ENOTSUP;
> >>>>      }
> >>>
> >>> Nevermind what I said about adding a BiteSizedTasks entry:
> >>>
> >>> The missing errp usage is not in qemu.git/master yet.  Please fix up
> >>> your bdrv_truncate() errp patch to use errp in all cases, e.g.
> >>> error_setg("Unable to truncate non-disk LUN").
> >>
> >> The thing is that I wasn't comfortable doing that for all block drivers.
> >> I mean, I can take another look but I'd rather have vague error messages
> >> ("truncation failed: #{strerror}") than outright wrong ones because I
> >> didn't know what error message to use.
> >>
> >> Of course you could argue that this may probably come out during review
> >> but that implies that every submaintainer for every block driver would
> >> actually come out for review...
> > 
> > I'm worried about errp being set in only a subset of error cases.
> > 
> > This is likely to cause bugs if callers use if (local_err).  Grepping
> > through the codebase I can see instances of:
> 
> Yes, but the generic bdrv_truncate() will always set errp if the driver
> hasn't done so.

I prefer consistently setting errp to make patch review easy (no
checking all callers to see how they behave), making it safe to
copy-paste the code elsewhere, etc.

It's safer to be consistent, can produce more detailed error messages,
and the cost of writing error_setg() calls is low.  But it's a matter of
style, you've pointed out the code is technically correct right now.

Stefan

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