I was writing a patch recently and in order to test it, I needed to make changes to the test bundles. I ended up making the same change across several files, and missing one (thanks to the reviewer for noticing that!).
Some of the other projects I'm involved with use symlinks to a base bundle with overlays:
./tests$ ls -l bundles/ total 20 -rw-rw---- 2 xav xav 5046 Sep 29 14:56 base.yaml lrwxrwxrwx 1 xav xav 9 Sep 29 10:01 bionic.yaml -> base.yaml lrwxrwxrwx 1 xav xav 9 Sep 29 10:01 focal.yaml -> base.yaml drwxrwx--x 2 xav xav 4096 Sep 29 10:01 overlays lrwxrwxrwx 1 xav xav 9 Sep 29 10:01 xenial.yaml -> base.yaml ./tests$ ls bundles/overlays/ bionic.yaml.j2 focal.yaml.j2 local-charm-overlay.yaml.j2 xenial.yaml.j2
This means that I can edit base.yaml just once, and if a change is specific to any of the particular bundles there's a place for that in the individual overlays. When we have bundles for each release going back to Mitaka, this could be quite an effort saver.
I gather there's been some discussion around this already, and would be keen to see if there's a reason folks might avoid that pattern?