Mark a plan as stale

[grimm26]

I use terragrunt with a custom workflow and I have run into occasions where a user has a PR ready to apply and runs the apply and it fails partway through. Then they immediately do another atlantis apply which goes ahead using the original plan again. This borks the state file because the resources that were created by the first apply are removed. Atlantis should not let a second apply be run on the same plan, and I assume this is enforced with the default workflow but I can't find how that is handled in the code.

The question is how do I do this in my custom workflow? I tried just removing the PLANFILE on a failed apply. This makes a subsequent apply succeed with 0 plans applied. How do I mark the plan as stale and needing to be refreshed?