Github Actions: Git diff master...HEAD fatal: no merge base

Question

Some context

Long story short: I am trying to run jest --changedSince=master when I open up a new Pull Request. The jest changedSince flag runs git diff master...HEAD in the background. This results in an error which I can't seem to get my head around.

Current situation

To debug this I have a Github Action which has a step which closely resembles the following:

runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - run: |
        git fetch --no-tags --depth=1 origin master
        git checkout -t origin/master
        git checkout pull/1/merge
        git diff master...HEAD

This results in the following error (Which is the same error returned by Jest):

fatal: refs/remotes/origin/master...HEAD: no merge base

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What I have tried

The problem does not occur when I run this locally (imagine that). I do realise that the biggest issue here is most likely the fact that the actions/checkout@2 action does not fetch the entire repository. That's why I added the git fetch & git checkout -t origin/master and pull/1/merge command.

I did attempt to run git branch -a to debug if this worked as expected, these were the results:

* (HEAD detached at pull/1/merge)
  master
  remotes/origin/master
  remotes/pull/1/merge

All the refs which are required to run git diff master...HEAD seem to be available in the action.

I have also tried to check if the actual ref is returned correctly: I ran git show-ref master inside the action which returns:

<commit-sha> refs/heads/master
<commit-sha> refs/remotes/origin/master

I am out of ideas on what could cause this issue, any ideas are appreciated greatly!

Answer 1

If you want to explore the history of master and HEAD (as in : find the merge base for these two commits), you can't limit your clone/fetch actions to a shallow clone -- not with depth=1 at least.

Try setting fetch-depth: 0 (or perhaps fetch-depth: 1000 # a high enough number) in the config of your action/checkout@v2 (as described in the project's Readme), and drop the --depth=1 (or set it to a higher value) when you run git fetch origin master.

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Another way could be to get this kind of info through the GitHub API, instead of querying the local clone.