I'm pretty sure this used to work at one time, perhaps long ago. It has been failing recently because if you call disk_open() with dev->d_partition set to -1 when d_slice refers to a bsd slice, it assumes you want it to open the first partition within that slice. When you then pass that open dev instance to ptable_open(), it tries to read the start of the 'a' partition and decides there is no recognizable partition type there. This restores the old functionality by resetting d_offset to the start of the raw slice after disk_open() returns. For good measure, d_partition is also set back to -1, although that doesn't currently affect anything. I would have preferred to make disk_open() avoid such rude assumptions and if you ask for partition -1 you get the raw slice. But the commit history shows that someone already did that once (r239058), and had to revert it (r239232), so I didn't even try to go down that road.
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