New function: Supply a timestamp for a transaction
virtual Status RecoveryUnit::setTimestamp(SnapshotName timestamp);

• This function may only be called in a unit of work. (Cannot set timestamps without an open transaction.)
• It is presumed that only transactions that perform writes will call this function, to assign timestamps. MongoDB aborts all read transactions anyway, so assigning timestamps for such transactions would have no effect.
• If this function is called multiple times in a WriteUnitOfWork, the timestamps may only move forward (or stay the same). An error will be returned if setTimestamp() attempts to assign a timestamp that is sooner than a previously assigned timestamp for a transaction.

New function: Use a timestamp to read at a point in time
virtual Status RecoveryUnit::selectSnapshot(SnapshotName timestamp);

• This function may optionally be called prior to calling getCursor() on a RecordStore.
• This function could be called again after calling getCursor(); this would be useful to, say, change to a more recent snapshot at query yield time. This is the current behavior of read-concern-majority, for example.
• This function cannot be called while a transaction is active: invariant(!inATransaction)
• The data returned from the cursor will reflect data from transactions that have committed with write timestamps up to and including time. If a transaction had multiple timestamps assigned to individual writes, such a transaction could be sliced by the read operation. Slicing is only expected to occur for transactions that were committed by the replication machinery for oplog application.
• The body of this function will set a member variable for the snapshot time in the RecoveryUnit. At the next begin_transaction, it will use the stored time as the read_timestamp parameter.