The collection scan stage of index builds is not guaranteed to perform reads inside of a transaction. This is not normally problematic, because WiredTiger still guarantees snapshot isolation. The flag to ignore prepare conflicts with "ignore_prepare=true" is set on the call to "begin_transaction, so If no transaction is open, prepare conflicts will not be ignored.

The cursor used by the collection scan stage opens a transaction implicitly by a call to getSession(). The WriteUnitOfWork::commit() during index build collection scans ends the current WT transaction, and inactivates the RecoveryUnit. Any subsequent cursor operation will use a valid cursor, but not inside of a transaction, and not ignoring prepare conflicts.