[Libguestfs] [PATCH nbdkit v2 02/10] python: Add various constants to the API.

[Richard W.M. Jones]

On Fri, Nov 22, 2019 at 11:26:55PM +0200, Nir Soffer wrote:
> On Fri, Nov 22, 2019 at 10:52 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> >
> > On Fri, Nov 22, 2019 at 10:05:15PM +0200, Nir Soffer wrote:
> > > On Fri, Nov 22, 2019 at 9:54 PM Richard W.M. Jones <rjones at redhat.com> wrote:
> > > >
> > > > These are accessible from the plugin by:
> > > >
> > > >   import nbdkit
> > > >
> > > >   if flags & nbdkit.FLAG_MAY_TRIM:
> > > >   &c.
> > >
> > > Nice way to expose the flags!
> > >
> > > > Many (all?) of these are not yet useful for plugins, some will never
> > > > be useful, but they only consume a tiny bit of memory and it's nice to
> > > > have the complete set available for future use.
> > > > ---
> > > >  plugins/python/python.c | 27 +++++++++++++++++++++++++++
> > > >  1 file changed, 27 insertions(+)
> > > >
> > > > diff --git a/plugins/python/python.c b/plugins/python/python.c
> > > > index d65ac45..69cf4e9 100644
> > > > --- a/plugins/python/python.c
> > > > +++ b/plugins/python/python.c
> > > > @@ -231,6 +231,33 @@ create_nbdkit_module (void)
> > > >      nbdkit_error ("could not create the nbdkit API module");
> > > >      exit (EXIT_FAILURE);
> > > >    }
> > > > +
> > > > +  /* Constants corresponding to various flags. */
> > > > +  PyModule_AddIntConstant (m, "THREAD_MODEL_SERIALIZE_CONNECTIONS",
> > > > +                           NBDKIT_THREAD_MODEL_SERIALIZE_CONNECTIONS);
> > >
> > > This can fail and needs cleanup. Very unlikely and a lot of code in
> > > the standard library have
> > > this issue.
> > >
> > > In sanlock we do:
> > >
> > >     if (PyModule_AddIntConstant(m, "LSFLAG_ADD", SANLK_LSF_ADD))
> > >         return -1;
> > >
> > > And the caller decref the module object and return NULL.
> > >
> > > See
> > > https://github.com/nirs/sanlock/blob/53f7f0284c084ac2e4542fd1f71d0406075adb5d/python/sanlock.c#L1846
> > > https://github.com/nirs/sanlock/blob/53f7f0284c084ac2e4542fd1f71d0406075adb5d/python/sanlock.c#L1763
> >
> > Yes I can make this change.  It's worth it in case there is some
> > subtle Python API change in future that we need to be aware of.
> >
> > We can in fact exit if load() fails so I guess we don't need to bother
> > doing the PY_DECREF for this case.
> 
> exit can hide the original error if we are not careful, so it is best avoided.
> 
> It is nice to decref and return NULL, the user code will fail with
> nice traceback
> about (likely) MemoryError.
> 
> And it does not cost much to do the right thing.

This is called before we enter any Python code so I don't think that
will work.  (In general Python is embedded in C in nbdkit, so
everything works in reverse to usual Python/C extensions.)

Rich.

> >
> > Thanks,
> >
> > Rich.
> >
> > > > +  PyModule_AddIntConstant (m, "THREAD_MODEL_SERIALIZE_ALL_REQUESTS",
> > > > +                           NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS);
> > > > +  PyModule_AddIntConstant (m, "THREAD_MODEL_SERIALIZE_REQUESTS",
> > > > +                           NBDKIT_THREAD_MODEL_SERIALIZE_REQUESTS);
> > > > +  PyModule_AddIntConstant (m, "THREAD_MODEL_PARALLEL",
> > > > +                           NBDKIT_THREAD_MODEL_PARALLEL);
> > > > +
> > > > +  PyModule_AddIntConstant (m, "FLAG_MAY_TRIM",  NBDKIT_FLAG_MAY_TRIM);
> > > > +  PyModule_AddIntConstant (m, "FLAG_FUA",       NBDKIT_FLAG_FUA);
> > > > +  PyModule_AddIntConstant (m, "FLAG_REQ_ONE",   NBDKIT_FLAG_REQ_ONE);
> > > > +  PyModule_AddIntConstant (m, "FLAG_FAST_ZERO", NBDKIT_FLAG_FAST_ZERO);
> > > > +
> > > > +  PyModule_AddIntConstant (m, "FUA_NONE",       NBDKIT_FUA_NONE);
> > > > +  PyModule_AddIntConstant (m, "FUA_EMULATE",    NBDKIT_FUA_EMULATE);
> > > > +  PyModule_AddIntConstant (m, "FUA_NATIVE",     NBDKIT_FUA_NATIVE);
> > > > +
> > > > +  PyModule_AddIntConstant (m, "CACHE_NONE",     NBDKIT_CACHE_NONE);
> > > > +  PyModule_AddIntConstant (m, "CACHE_EMULATE",  NBDKIT_CACHE_EMULATE);
> > > > +  PyModule_AddIntConstant (m, "CACHE_NATIVE",   NBDKIT_CACHE_NATIVE);
> > > > +
> > > > +  PyModule_AddIntConstant (m, "EXTENT_HOLE",    NBDKIT_EXTENT_HOLE);
> > > > +  PyModule_AddIntConstant (m, "EXTENT_ZERO",    NBDKIT_EXTENT_ZERO);
> > > > +
> > > >    return m;
> > > >  }
> > > >
> > > > --
> > > > 2.23.0
> > > >
> >
> > --
> > Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
> > Read my programming and virtualization blog: http://rwmj.wordpress.com
> > virt-p2v converts physical machines to virtual machines.  Boot with a
> > live CD or over the network (PXE) and turn machines into KVM guests.
> > http://libguestfs.org/virt-v2v
> >

-- 
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