When I create a brand new CodeBuild project, it allows me to select an IAM Service Role, and when I check the box "Allow AWS CodeBuild to modify this service role so it can be used with this build project", AWS modifies that Service Role with a custom policy that's specific to this role.
But If after creating that CodeBuild project I want to attach a different service role to it, I keep getting the below message saying "The policy was not attached ot role [x]"
I'm pretty sure I'm missing a permission somewhere, but I'm not sure where.
Edit with more troubleshooting data:
If I uncheck the box "Allow aws [...]" It allows me to update the CodeBuild project configuration, but all subsequent builds fail at startup. This is expected.
If I try to re-add the original service role I added to this project when I created it, it lets me add it without any problems.