Head vs base ahead error

[rhaynes]

https://github.com/jitterbit/get-changed-files/blob/master/src/main.ts#L79

The base branch that github reports can be anywhere between the initial commit on the base branch where head branched off to the latest commit on the base branch (or any commit in between). This seems to happen most frequently when new commits are pushed after a PR is already open. This is fine for repos that require a rebase or merge on the base branch before merging a PR, but otherwise the line above occasionally throws an error. If this line were changed to core.info instead of core.setFailed all changed files are reported correctly so it's probably unnecessary to consider this failed when that check doesn't pass.

[sean-krail]

Hi @rhaynes, thanks for opening this issue.

The base branch that github reports can be anywhere between the initial commit on the base branch where head branched off to the latest commit on the base branch (or any commit in between). This seems to happen most frequently when new commits are pushed after a PR is already open.

This is true for the base commit's branch ref, but not for the base commit's SHA. The action should be using the base commit's SHA which doesn't change even if the base branch ref gets updated after the PR is already open. Here's where it gets the base commit's SHA:

get-changed-files/src/main.ts

Lines 29 to 36 in b17fbb0

	case 'pull_request':
	base = context.payload.pull_request?.base?.sha
	head = context.payload.pull_request?.head?.sha
	break
	case 'push':
	base = context.payload.before
	head = context.payload.after
	break

I may be wrong though. I'll try to find some time over the next couple weeks to reproduce/verify this.

[rhaynes]

Hi @sean-krail thanks for following up! I think I've figured out what's going on. You're correct the base sha doesn't change once the PR is open. The problem seems to be that the base sha is assigned when the PR is opened and is whatever sha is currently at for the base branch (as opposed to the commit sha when the branch was created off the base branch). In other words:

1. Create two branches off the same base commit and make one commit to each branch
2. PR and merge one of the two branches.
3. PR the second branch. The script should fail b/c the base sha will be the sha after the first branch was merged, but the head commit on the second branch will be behind that merge commit.

If you rebase on master for the 2nd PR, it's not an issue, but if the merge process for your repo is to merge branches into master without rebasing it throws the error above in the case described.

In any case, the script correctly determines the diff between the base branch and head commit even though the head is behind the base commit so it seems like this error is unnecessary and the script would work as expected without it.