On Wed, Aug 01, 2007 at 02:45:57PM +0100, Richard W.M. Jones wrote:
> Daniel Veillard wrote:
> >>+typedef enum {
> >>+ /* Driver supports V1-style virDomainMigrate, ie.
> >>domainMigratePrepare/
> >>+ * domainMigratePerform/domainMigrateFinish.
> >>+ */
> >>+ VIR_DRV_FEATURE_MIGRATION_V1 = 1,
> >>+
> >>+ /* Driver is not local. */
> >>+ VIR_DRV_FEATURE_REMOTE = 2,
> >>+} virDrvFeature;
> >
> > Probably best done with defines than enums, as you want to be able to
> > compose
> >them the fact it comes from a type doesn't help, and I have learnt the
> >hard way
> >that enums sucks in C in general.
>
> What in particular? I'm not particularly concerned either way since
> enums are basically just as unsafe as #defines in C, but it is nice for
> the library user to be able to connect the argument prototype
> ("virDrvFeature feature") to the list of permitted types.
>
> >>+typedef int
> >>+ (*virDrvSupportsFeature) (virConnectPtr conn, virDrvFeature
> >>feature);
> >
> > I would rather use , int features) where you OR the features, allows
> >to know in one call if you get what you want or not.
>
> In the case where you've got multiple different migration possibilities
> (VIR_DRV_FEATURE_MIGRATE_V1 & VIR_DRV_FEATURE_MIGRATE_V2) then this
> saves you one remote call in the legacy case (VIR_DRV_FEATURE_MIGRATE_V2
> not supported so we have to do a second remote call to check
> VIR_DRV_FEATURE_MIGRATE_V1).
You seems to think it's useful only for migrate. I want that to be
usable for a lot more. You talk to a remote server, you want to know if
it supports any of the entry point in the driver table, how do you do this ?
> On the other hand, it complicates the interface. You need to return an
> array rather than a single int. (OK, so you can return a bit array, but
> now the feature list had better always be <= 32 entries long). And in
> the case where someone queries VIR_DRV_FEATURE_MIGRATE_V1 &
> VIR_DRV_FEATURE_REMOTE you need to have two different drivers answering
> a single request.
>
> I think this complicates things unnecessarily ...
I think this would be a waste to design it with a single narrowly focused
usage in mind, when it can be far more generally useful.
Daniel
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