The existing, but non working, function is called PetscOptionsInsertFile_YAML. I just kept its name without checking the petsc developer style guide. My mistake.

The attached patch fixes this issue.

"There is a limit of 128,000 char on the file length." … What? The old function had a (temporary) limit of 128,000 characters *per line* but both approaches should be using Petscgetline().