geom mirror now rebuilding on every reboot?

[Tom Uffner]

not quite the same issue, but i tried to update a system with gmirror from

FreeBSD eris.uffner.com 10.0-CURRENT FreeBSD 10.0-CURRENT #210: Thu Mar 15 
16:41:18 EDT 2012     tom at eris.uffner.com:/usr/obj/usr/src/sys/ERIS  i386

to -current from Aug 4th, and booting now fails into mountroot> with

GEOM_RAID: Promise: Array Promise Created
GEOM_RAID: Promise: Disk ad4 state changed from NONE to SPARE
GEOM_MIRROR: Cannot open consumer ad4 (error=1)
GEOM_MIRROR: Cannot add disk ad4 to gm0 (error=1)
GEOM_MIRROR: Device gm0 destroyed
GEOM_RAID: Promise: Array Promise Created
GEOM_RAID: Promise: Disk ad6 state changed from NONE to SPARE
GEOM_MIRROR: Cannot open consumer ad6 (error=1)
GEOM_MIRROR: Cannot add disk ad6 to gm0 (error=1)
GEOM_MIRROR: Device gm0 destroyed

nor will it boot from the underlying partitions (ad4s1a, ad6s1a), even if i
unplug one of the drives. the partitions & metadata are not corrupt and work
just fine if i revert to the previous kernel.

i haven't had time yet to track down the commit where this breaks. i was just
wondering if anyone knew offhand what is going on & how to fix it.

tom