A “clean build fence” is used to indicate to the build that a particular commit should invalidate incremental builds on either side of the change. fx build and fint build both call force_clean_if_needed.py which will clobber the build if it detects a difference between the set of fences which was used for a particular build and those which are generated by the current checkout.
Note: Adding a new fence will clobber any existing existing incremental caches, both on builders and on developer machines. As a result of the widespread impact, new fences must be approved by the build team after investigating other options for making a change.
Add a new print("...") statement to the print_fences function in get_fences.py, including a bug number and short explanation as to why it's needed.