This is discussion thread regarding the documentation at: Migrate to Drupal 9 on Pantheon
I found a few areas that needed some adjustment to work on my workstation on MacOS.
Everything appeared to go well until I needed to move my composerify changes to the Dev environment. I ran into a permissions issue and needed to change permissions of the sites default directory at web/sites/default. This can be found at: Move composerify to the main dev branch.
I added step 2
1) Retrieve the most recent commit hash from the local composerify branch:
$ git log —format=”%H” -n 1
This will give you a commit hash like fd3636f58f5b275b998bb1c9267bff8808353840.
2) Change Permissions of the sites default directory
$ chmod ug+w web/sites/default
3) Reset the master branch to match that commit then force push that to the Dev environment:
$ git checkout master
$ git reset —hard fd3636f58f5b275b998bb1c9267bff8808353840 #USE YOUR OWN COMMIT HASH
$ git push —force origin master
Now the site’s Dev environment has a Drupal 9 codebase.