A resumed index build goes through this if-else-if statement in the index build code wherein mutual setup code sets the PrepareConflictBehavior::kIgnoreConflictsAllowWrites flag on the RecoveryUnit. Setting that flag avoids a possible invariant check that the index build thread is killable (to avoid prepare conflict deadlocks).

The drain phase does not execute the if-else-if code, it is the undefined else situation. For that else situation (drain phase) we still need to set the PrepareConflictBehavior::kIgnoreConflictsAllowWrites flag on the RecoveryUnit. I recommend pulling the mutual code up, so it's more clear that the flag is set and more difficult to accidentally forget.