This commit includes a test rollback_after_enabling_majority_reads.js that documents the behavior of entering ROLLBACK immediately after upgrading enableMajorityReadConcern to true. This can fassert if it attempts to roll back operations that were included in the last stable checkpoint when enableMajorityReadConcern was false. If this happens, the user can restart the server with enableMajorityReadConcern=false to complete the rollback, then upgrade again to enableMajorityReadConcern=true.
It might not be necessary to include this in our user-facing documentation, but I would like to ensure that TSEs know. I will inform the TSE organization of this issue.
We have coverage for toggling enableMajorityReadConcern in disable_majority_reads_restart.js. However, we do not have coverage for rollbacks after toggling. We should add test coverage to determine what the behavior is and decide whether the behavior is acceptable.