MONGOID-5531 adds commit and rollback transactions to Mongoid, targeting mongoid 9.0. As a result, the corresponding PR was not merged onto the 8.1-stable branch. However, a recent commit to address MONGOID-5658 contains code that defines commit and rollback callbacks. This code was merged onto the 8.1-stable branch. The code seems to originate from MONGOID-5531's PR (#5519).

I'm not sure why this was included or whether it's relevant to the fix related to embedded callbacks, but I believe it was erroneous to define these callbacks when the corresponding code that actually executes the callbacks was not added. For our use case, we use a gem that runs callbacks on an after_commit callback if it's defined, and if not, runs callbacks on the after_save callback. Previously, it always ran on the after_save callback because the after_commit callback was not defined on Mongoid models. When we upgraded to mongoid 8.1.3, the callback stopped running, because when the gem saw that after_commit callbacks were available, it set up its callback to trigger on the after_commit callback, which is never actually run by mongoid 8.1.3 (since it's lacking the rest of MONGOID-5531's code that actually enables the callbacks).

I recognize that this may not be especially problematic for many people, but as I said above, I feel that it doesn't really make sense to define callback registration methods for callbacks you don't run, so I would consider this a bug.