if_clone: Allow maxunit to be zero

[zlei]

Summary

Some drivers, e.g. if_enc(4), only allow one instance to be created, but
the KPI ifc_attach_cloner() treat zero as not limited, aka IF_MAXUNIT .

Introduce a new flag IFC_F_LIMITUNIT to indicate that the provided
maxunit is limited and should be respected.

Consumers should use the new flag if there is an intended limit.

MFC after: 2 weeks

[glebius]

Shouldn't that be done to if_clone_addreq_v2, too?

sys/net/if_clone.c

533–535

Do we really need the old behavior at all? There are no consumers in tree that initialize maxunit.
So could be:

maxunit = (req->flags & IFC_F_LIMITUNIT) ? req->maxunit : IF_MAXUNIT;

[zlei]

Added asserting on the compatibility of layout of struct if_clone_addreq and struct if_clone_addreq_v2 .

[zlei]

In D45757#1043796, @glebius wrote:

Shouldn't that be done to if_clone_addreq_v2, too?

The layout of struct if_clone_addreq_v2 is compatible with that of struct if_clone_addreq . i.e., maxunit of if_clone_addreq_v2 is the same with if_clone_addreq . I added a compile assert to reveal that.

[zlei]

sys/net/if_clone.c

533–535

Do we really need the old behavior at all? There are no consumers in tree that initialize maxunit.
So could be:

maxunit = (req->flags & IFC_F_LIMITUNIT) ? req->maxunit : IF_MAXUNIT;

That is exactly same with my first revision. Well later I decided to MFC this change and I worried 3rd party drivers may have already used the new KPI so I restored the old behavior.

IMO the breakage should be trivial to fix. But it seems neither RELNOTES nor UPDATING is the right place to warn developers. Any suggestions ?

[zlei]

Removed compatibility for old behavior.

[zlei]

Friendly ping @glebius .

[glebius]

Sorry for delay!

[zlei]

In D45757#1044984, @glebius wrote:

Sorry for delay!

Do not worry. I'm just a little eager to have this change in ;)