I've added the TODO item:
When pg_upgrade -j ... is interrupted (for example, ctrl-C from the keyboard) make it cancel the children processes.
The context where this arises is that I want to populate data into a new installation compiled with a patch under review, but immediately get error messages indicating I forgot to install a required extension. I hit ctrl-C so I can fix the problem, but it keeps running anyway.
Cheers,
Jeff