Invalid cross-device link when world migration starts (ZFS with a dataset for the dimension targeted)

Expected behavior

Migrate world to new format

Observed/Actual behavior

Instantly hits a cross-device link error due to trying to link a file outside the filesystem boundaries.

Steps/models to reproduce

1. Make a dataset or filesystem mountpoint for mcserver/world (or any depth down to: world/dimensions/minecraft/overworld for example)
2. Install into mcserver/ using a papermc version from earlier this year (Say, a release for 1.21.11)
3. Run it
4. Stop it once some world generates
5. Upgrade
6. Run it
7. Migration fails due to a hardlink attempt across different filesystems

Plugin and Datapack List

None

Paper version

26.2-38-dev/26.2@58e1f84

Previous version: 26.1.2-61-8dea6f1 (MC: 26.1.2)

(It was also 1.21.11 earlier)

Other

I think this niche problem could be solved by Paper retrying this preparation step with a traditional copy of the world data rather than making a link. Though this would not be very efficient on a CoW filesystem, it would avoid the crash. Or just checking first whether or not the copy operation would cross filesystem boundaries and choosing to not do a link. Or doing the entire conversion operation in the same directory as the world data instead of a few directories up? Not sure of the best approach.

Other 2

The dataset layout is show below and is why this problem happens in my case. I did this layout for granular control, rollback options and testing various compression options. (I'd recommend anyone else to just install CoreProtect and be done with it but I wanted to run my storage tests.)

minecraft@WanA ~ $ zfs list -r tank/minecraft/survival
NAME                                                                     USED  AVAIL  REFER  MOUNTPOINT
tank/minecraft/survival                                                 20.9G  45.6G  6.29G  /minecraft/survival
tank/minecraft/survival/logs                                            22.1M  45.6G  4.92M  /minecraft/survival/logs
tank/minecraft/survival/world                                           12.8G  45.6G  2.52M  /minecraft/survival/world
tank/minecraft/survival/world/dimensions                                7.33G  45.6G   200K  /minecraft/survival/world/dimensions
tank/minecraft/survival/world/dimensions/minecraft                      7.33G  45.6G   312K  /minecraft/survival/world/dimensions/minecraft
tank/minecraft/survival/world/dimensions/minecraft/overworld            6.18G  45.6G   924M  /minecraft/survival/world/dimensions/minecraft/overworld
tank/minecraft/survival/world/dimensions/minecraft/the_end               257M  45.6G  71.9M  /minecraft/survival/world/dimensions/minecraft/the_end
tank/minecraft/survival/world/dimensions/minecraft/the_nether            925M  45.6G   283M  /minecraft/survival/world/dimensions/minecraft/the_nether
tank/minecraft/survival/world/playerdata                                4.05M  45.6G   208K  /minecraft/survival/world/playerdata

Start by locating the world-migration preparation path and the hardlink operation involved when the world contains separately mounted ZFS datasets. Reproduce the upgrade with the dataset layout and world-generation steps described above. Done means migration completes without a cross-device-link failure while preserving the migrated world data.