[Libguestfs] [PATCH nbdkit] python: Implement can_extents + extents.

[Richard W.M. Jones]

On Mon, Aug 10, 2020 at 07:28:30PM +0300, Nir Soffer wrote:
> On Mon, Aug 10, 2020 at 1:51 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> > + def extents(h, count, offset, flags):
> > +   # return a list of (offset, length, type) tuples:
> > +   return [ (off1, len1, type1), (off2, len2, type2), ... ]
> 
> Better to accept an iterable of tuples. This allows the plugin to do:
> 
>     def extents(h, count, offset, flags):
>         for extent in client.extents():
>             if extent.offset > offset:
>                 return
>             yield extent.start, extent.length, type
> 
> The logic is just an example to illustrate the benefit of using
> generators. The plugin does not have to build a list of extent,
> and nbdkit also may not need this list. This may have performance
> benefits, and may make the code nicer.

Yes, I think we should do this, and I believe the change is
backwards compatible too (just as well because I just pushed
this into RHEL ...)

...
> > +    /* We expect a list of extents to be returned.  Each extent is a
> > +     * tuple (offset, length, type).
> > +     */
> > +    if (!PyList_Check (r)) {
> 
> This will work only with the list. We want to check if we got a type
> we can iterated on. I don't know the APi needed but I'm sure this is
> supported by the C API.

I wonder how it works w.r.t. threads ...  Does it use another thread
or does it just have some kind of suspension which is evaluated when
requested?

This seems to be the API so I'll have a play around with it:

https://docs.python.org/3/c-api/iter.html

> Returning extent tuples looks much better than the C version in file plugin.

Certainly easier to use :-/

Rich.

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