Make GCB downstream builder job fail if a commit creation error occurs

[shuyama1]

What kind of contribution is this issue about?

Other (specify in details)

Details

In a recent downstream build job, we've seen downstream generation step silently swallowed a commit creation error Error creating commit: path error running git: fork/exec /usr/bin/git: argument list too long without causing the builder job to fail. GCB link

This behavior may have been intentional, given that certain PRs do not generate any downstream changes and Magician returns errors in such scenarios. However, we need to find a way to distinguish between instances where no downstream commits should be generated and actual generation errors. For the latter, we should ensure the build job fails accordingly.

References

No response

[melinath]

Note from grooming: We could potentially check the error code from making the commit, or check whether there are any diffs prior to attempting to make a commit (so that we don't have to ignore empty commit failures)

[iyabchen]

I can take this ticket.