Poudriere error: cannot rollback ..., there is a snapshot ... more recent ....

[George Hartzell]

I have a FreeBSD 12.1p3 (just updated) root-on-zfs system that's
having trouble doing Poudriere builds.

It started off life as a FreeBSD 12.1 system and has been building
it's own set of packages using Poudriere successfully since its
inception.

I few updates ago I started seeing errors in the Poudriere builds.

I'm trying to run one now and am seeing the errors again.  Here's an
example:

[...]
[00:30:24] [02] [00:00:00] Building devel/py-more-itertools at py37 | py37-more-itertools-8.2.0
cannot rollback 'zroot/poudriere/jails/12_1-ports_and_hartzell-ref/02': there is a snapshot or bookmark more recent than 'zroot/poudriere/jails/12_1-ports_and_hartzell-ref/02 at prepkg'
[00:30:26] [02] [00:00:02] Error: Unable to rollback zroot/poudriere/jails/12_1-ports_and_hartzell-ref/02 to prepkg
=>> Error: Unable to rollback zroot/poudriere/jails/12_1-ports_and_hartzell-ref/02 to prepkg
[...]

After crashing, I can hit ^T and see:

[...]
load: 1.01  cmd: sh 36519 [piperd] 1994.36r 0.08u 0.13s 0% 3188k
[12_1-ports_and_hartzell] [2020-03-20_09h30m34s] [parallel_build:] Queued: 149 Built: 72  Failed: 1   Skipped: 4   Ignored: 0   Tobuild: 72   Time: 00:33:31
        [01]:                                                       crashed
        [02]: devel/p5-IO-stringy       | p5-IO-stringy-2.111       package         (00:00:00 / 00:00:06)
[00:33:33] Logs: /usr/local/poudriere/data/logs/bulk/12_1-ports_and_hartzell/2020-03-20_09h30m34s
[...]

a bit later, that builder seems to recover and continue sucessfully
with new jobs:

[...]
load: 3.40  cmd: sh 36519 [piperd] 2282.32r 0.09u 0.15s 0% 3188k
[12_1-ports_and_hartzell] [2020-03-20_09h30m34s] [parallel_build:] Queued: 149 Built: 80  Failed: 3   Skipped: 10  Ignored: 0   Tobuild: 56   Time: 00:38:19
        [01]: lang/cython at py27          | py27-cython-0.29.15       build           (00:01:00 / 00:01:12)
        [02]: devel/icu                 | icu-66.1,1                build           (00:03:10 / 00:03:24)
[00:38:21] Logs: /usr/local/poudriere/data/logs/bulk/12_1-ports_and_hartzell/2020-03-20_09h30m34s
[...]

I'd like to get it working normally again.

Any ideas what might be going wrong?

g.