[libvirt] [python PATCH] build: make it easier to backport event ids

[Daniel P. Berrange]

On Thu, Jan 15, 2015 at 08:52:44AM -0700, Eric Blake wrote:
> On 01/15/2015 03:51 AM, Jiri Denemark wrote:
> > On Thu, Jan 15, 2015 at 10:42:39 +0000, Daniel Berrange wrote:
> >> On Wed, Jan 14, 2015 at 08:08:16PM -0700, Eric Blake wrote:
> >>> In some cases, it is very easy for downstream distros to backport
> >>> enum values without requiring a .so bump.  Keying the conditional
> >>> code off of the upstream version where the enum value was added
> >>> is not ideal, because downstream then has to patch that the feature
> >>> is available in their build that still reports an earlier version
> >>> number.  For example, if RHEL 7 backports events from 1.2.11 into
> >>> a build based on 1.2.8, building the python bindings would warn:
> >>>
> 
> >>> +    if name.startswith('VIR_DOMAIN_EVENT_ID_') or name.startswith('VIR_NETWORK_EVENT_ID_'):
> >>> +        event_ids.append(name)
> >>
> >> If you use brackets you can break the line without confusing
> >> python eg
> >>
> >>     if (name.startswith('VIR_DOMAIN_EVENT_ID_') or
> >>         name.startswith('VIR_NETWORK_EVENT_ID_')):
> 
> Thanks.
> 
> 
> >>> +        # Write define wrappers around event id enums
> >>> +        include.write("\n")
> >>> +        for event_id in event_ids:
> >>> +            include.write("#define %s %s\n" % (event_id, event_id))
> >>
> >> I'm not seeing the point of adding this define ?
> > ...
> >>> -#if LIBVIR_CHECK_VERSION(0, 10, 0)
> >>> +#ifdef VIR_DOMAIN_EVENT_ID_BALLOON_CHANGE
> > 
> > Without that define, you can't check for the events using ifdef,
> > C preprocessor doesn't see enum items.
> 
> Correct.  Creating self-referential macros is the canonical way to
> convert enums into something that can be probed at preprocessor time for
> using conditional compilation.  Since we have one macro for every enum
> that we read from the xml of the libvirt we are targetting, we can make
> the bindings support exactly the set of enum values that were backported
> into the particular libvirt, without having to worry about backporting
> crossing version number checks.

Sigh. In a sane world we would just fix the CPP to honour enums
directly ;-P

> So does this count as an ACK with the long line fixed?

Sure, ACK

Regards,
Daniel
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