Add rollback of Pool setup when starting a pool fails

[jbaublitz]

Related to #3274

[mulkieran]

There was one tiny thing I noticed a while back when glancing at it several days ago. Doing a serious pass now.

[mulkieran]

This seems to me to hang together very well. I have a few requests for comments, but that's it.

[mulkieran]

Addressing a problem with behavior when some filesystems can be torn down but not others.

[jbaublitz]

@mulkieran I think the most recent commit should resolve that.

[mulkieran]

I think it would be better if the teardown of a thin device was also idempotent. When we stop pools, removal is handled correctly, which is good. But we may find another way to get ourselves into the same phantom filesystem problem that we had beforee. I think that tearing down filesystems could use the same approach as teardown for the other devices, and be made idempotent that way.

[mulkieran]

I think it would be better if the teardown of a thin device was also idempotent. When we stop pools, removal is handled correctly, which is good. But we may find another way to get ourselves into the same phantom filesystem problem that we had beforee. I think that tearing down filesystems could use the same approach as teardown for the other devices, and be made idempotent that way.

I would also be willing to defer consideration of this proposal until later. It may not gain us much during any remotely normal operation.