Details
Description
3.4 is checking acceptedEpoch/currentEpoch files against the snap/log files in datadir. These files are new in 3.4. If they don't exist the server will create them, however if they do exist the server will validate them.
As a result if a user
1) upgrades from 3.3 to 3.4 this is fine
2) downgrades from 3.4 to 3.3 this is also fine (3.3 ignores these files)
3) however, 3.4->3.3->3.4 fails because 3.4 will see invalid *Epoch files in the datadir (as 3.3 would have ignored them, applying changes to snap/log w/o updating them)