The collection validate cmd must run mutually exclusively with taking WT checkpoints. This requires a mutex being taken in the WT engine implementation code, as well as at the MongoDB command level.

SERVER-42221 will add such a mutex, by putting it in the StorageEngineImpl class and creating a factory class to return a ExclusiveLock on the mutex through the StorageEngine interface.

However, the more ideal solution appears to be to elevate the checkpoint concept into the StorageEngine interface such that we can cover checkpoint by locking a mutex around a StorageEngine call. Then we would not have a mutex trying providing service across many abstraction layers.