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Re: [libvirt] [PATCH v2 11/38] Introduce virStreamSkip



On Fri, May 05, 2017 at 01:25:34PM +0200, Michal Privoznik wrote:
> On 05/04/2017 11:29 PM, John Ferlan wrote:
> > 
> > 
> > On 04/20/2017 06:01 AM, Michal Privoznik wrote:
> >> This API can be used to tell the other side of the stream to skip
> > 
> > s/can be/is  (unless it can be used for something else ;-))
> > 
> >> some bytes in the stream. This can be used to create a sparse
> >> file on the receiving side of a stream.
> >>
> >> It takes just one argument @length, which says how big the hole
> >> is. Since our streams are not rewindable like regular files, we
> >> don't need @whence argument like seek(2) has.
> > 
> > lseek is an implementation detail...  However, it could be stated that
> > the skipping would be from the current point in the file forward by some
> > number of bytes.  It's expected to be used in conjunction with code that
> > is copying over the real (or non-zero) data and should be considered an
> > optimization over sending zere data segments.
> > 
> >>
> >> Signed-off-by: Michal Privoznik <mprivozn redhat com>
> >> ---
> >>  include/libvirt/libvirt-stream.h |  3 +++
> >>  src/driver-stream.h              |  5 ++++
> >>  src/libvirt-stream.c             | 57 ++++++++++++++++++++++++++++++++++++++++
> >>  src/libvirt_public.syms          |  1 +
> >>  4 files changed, 66 insertions(+)
> >>
> > 
> > While it would be unused for now, should @flags be added.  Who knows
> > what use it could have, but avoids a new Flags API, but does cause a few
> > other wording changes here.
> 
> Ah sure. We should have @flags there. Good point.
> 
> > 
> > Perhaps it's just me - but "Skip" and "HoleSize" just seem awkward.
> > Would "virStreamSetSkip" and "virStreamGetSkip" be more apropos? (or
> > s/Skip/HoleSize/ - ewww). Names would then follow our more recent
> > function naming guidelines. I think I dislike the HoleSize much more
> > than the Skip.
> 
> SetSkip and GetSkip sound wrong to me instead :D
> 
> > 
> >> diff --git a/include/libvirt/libvirt-stream.h b/include/libvirt/libvirt-stream.h
> >> index bee2516..4e0a599 100644
> >> --- a/include/libvirt/libvirt-stream.h
> >> +++ b/include/libvirt/libvirt-stream.h
> >> @@ -50,6 +50,9 @@ int virStreamRecvFlags(virStreamPtr st,
> >>                         size_t nbytes,
> >>                         unsigned int flags);
> >>  
> >> +int virStreamSkip(virStreamPtr st,
> >> +                  unsigned long long length);
> > 
> > Was there consideration for using 'off_t' instead of ULL? I know it's an
> > implementation detail of virFDStreamData and lseek() usage, but it does
> > hide things... IDC either way.
> 
> The problem with off_t is that it is signed type, while ULL is unsigned.
> There's not much point in sending a negative offset, is there?
> Moreover, we use ULL for arguments like offset (not sure really why).
> Frankly, I don't really know why. Perhaps some types don't exist everywhere?

If anything, we would use  size_t, for consistency with the Send/Recv
methods. 

Ultimately though we have a fixed size data type on the wire (64-bit),
so mapping to unsigned long long makes sense from that POV, as off_t
can in theory be 32-bit in old platforms - similarly how we aim to
avoid 'long int' (except for historical mistakes).

Regards,
Daniel
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