epair: deduplicate interface allocation code #1

[melifaro]

Summary

Simplify epair_clone_create() and epair_clone_destroy() by factoring out epair_softc allocation / desctruction and ifp setup/teardown into separate functions.

[melifaro]

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[zlei]

sys/net/if_epair.c

495

I think epair_alloc_sc() will prevent potential naming pollution.
Although it is modified with static but still a dedicated prefix epair_ looks consistent with current naming.

667

if (sca == NULL ^ scb == NULL), then free_sc(sca) or free_sc(scb) may trigger null pointer dereference.

[kp]

sys/net/if_epair.c

667

Good catch!

A simple if (sc == NULL) return; in free_sc() should work, and matches the regular free() idiom.

[melifaro]

sys/net/if_epair.c

667

Yep, that's a good one!
It was my initial intention but I forgot to actually add it :-)