questions about --retry-on-change and --ref

With "--retry-on-change 0", will the resulting dar backup record
anything about a file that changed while being read?  

If the file being changed stops changing prior to the dar backup
completing, but late enough for that backup to abandon it, will a
subsequent dar incremental backup using this one as --ref pick up the
changes?

If one dar archive is itself still being written while being used as a
--ref argument for another, what will happen?  I am triggering a dar
backup (with --retyr-on-cahnge 0) after a user logs out, and there is a
small possibility that the user will log in and out again before the
first backup completes, and that the second backup will --ref the
first. I am wondering if dar itself will fail gracefully in this case,
or whether I have to use some mechanism to prevent any such overlap.