ifnet renaming interacts badly with vlans

[John Baldwin]

So first the problem I am trying to solve.  I would like to give meaningful 
aliases to interfaces and vlans on interfaces.  For example, suppose I would 
I have em0 with a vlan of 112, but I would like to call em0, "foo" and 
em0.112, "foo.bar".  One can do this if you do things in this order:

ifconfig em0 name foo
ifconfig foo.112 create
ifconfig foo.112 name foo.bar

However, our rc.d scripts want to create all cloned interfaces before applying 
renames, so to attempt to do this in /etc/rc.conf you would use something 
like:

cloned_interfaces="em0.112"
ifconfig_em0_name="foo"
ifconfig_em0_112_name="foo.bar"

However, this ends up doing a sequence more like:

ifconfig em0.112 create
ifconfig em0 name foo
ifconfig em0.112 name foo.bar

Unfortuantely, this sequence breaks em0.112 at the second step.  The reason is 
that when em0 is renamed, the kernel actaully fires off event handlers 
claiming that the em0 ifnet has been detached and that a new foo ifnet has 
been attached.  When this happens, the em0.112 vlan ifnet is orphaned.

I think this is a bug in how renames are handled.  Specifically, I don't think 
renaming an interface should always have the same semantics as if I had 
physically removed an interface and then later added it back.  One 
alternative would be to instead broadcast a single "rename" event when an 
interface is renamed and things that cared about interface names (such as 
firewall rules) could still honor those events, but other subsystems might 
choose to ignore said events (e.g. vlan devices).  Another alternative might 
be to use two different events ('ifnet_removed_name' and 'ifnet_added_name').  
Subsystems that care about the name changing could reuse their existing 
detached/attached handlers for those two events without needing to change the 
event handlers themselves.  A separate issue if the eventhandlers were 
changed is if we should use a different routing socket message when an 
interface is renamed or if it we should still claim that an interface has 
gone away and then come back.  Given that we don't flush addresses from an 
interface when the name changes or even the ifindex it seems a bit 
disingenous to claim that the interface has detached and then reattached.

-- 
John Baldwin