Convenience class to package together the different steps and fields needed
for reading a log entry for undoing. Is used for both txn aborts and
Set up an UndoReader when doing an undo or txn partial rollback for a
Never returns null. The DB ID of the LN must be present in
undoDatabases, or a fatal exception is thrown.
"DB not found during non-recovery undo/rollback, id=" + dbId);
final long lsn = undoLsn;
Set up an UndoReader when doing a recovery partial rollback. In that
case, we have a file reader positioned at the pertinent log entry.
This method calls DbTree.getDb. The caller is responsible for calling
DbTree.releaseDb on the db field.
Null is returned if the DB ID of the LN has been deleted.