Actions: pull_request event checks out PR head, not merge commit

[bilderbuchi]

Description

tl;dr: When using a CI configuration set up with on: pull_request, this fires on PR updates, but checks out not the merge commit, but the PR HEAD.
This is unexpected, because

• This behaviour differs from Github's
• This makes it harder to do Continuous Integration, as the PR does not get tested as integrated into the target branch.

Observed behaviour

As documented, Gitea Actions implement the pull_request workflow trigger event, and it is "compatible with Github".
Githubs docs, linked from the above page, say that GITHUB_SHA will be the "Last merge commit on the GITHUB_REF branch" and GITHUB_REF will be the "PR merge branch refs/pull/:prNumber/merge".

I have a workflow configured with

on: 
  push:
    branches:
      - main
  pull_request:

and use actions/checkout@v3.

On a pull request update, in the "Set up job" log of the Workflow run, I unexpectedly see "GITHUB_REF:refs/pull/61/head" and a GITHUB_SHA entry that corresponds to the PR HEAD (i.e. the last pushed commit), and not the merge commit, as documented.

Expected behaviour

A pull_request event builds the merge commit of the PR commit that triggered the event, or some other way to continuously build the integrated branch. Otherwise a PR could test green, but break the tests after merging.

Gitea Version

1.20.3

Can you reproduce the bug on the Gitea demo site?

Could not check

Log Gist

No response

Screenshots

No response

Git Version

No response

Operating System

No response

How are you running Gitea?

This is a self-hosted instance, but I'm not running it so I can't offer more details.

Database

None

[lunny]

@Zettat123 Could you have a look at this?

[Zettat123]

Thanks for the bug report. The reason of this bug is Gitea doesn't have refs/pull/:prNumber/merge references.

refs/pull/:prNumber/merge is a reference pointing to the merge commit between the PR's base reference and head reference. It is created by Github. Gitea currently has no such reference as refs/pull/:prNumber/merge

There has been a proposal(#13134) to add this kind of reference. This bug can only be fixed when Gitea supports such a reference. I'll update the documentation later to explain this difference between Gitea Actions and Github Actions.

[bilderbuchi]

Thank you. That's a pity, as it also increases the conceptual distance between Gitea and Github CICD at a pretty base level. If indeed

the merge commit id is stored in the pull_request table.

#13134 does not sound like a complicated enhancement?

[Zettat123]

the merge commit id is stored in the pull_request table.

#13134 does not sound like a complicated enhancement?

The refs/pull/:prNumber/merge reference in GitHub is different from the merge commit id in Gitea's pull_request table. In GitHub, refs/pull/:prNumber/merge is a reference to an intermediate state where the head branch is hypothetically merged to the base branch.

However, in Gitea, the commit id in the pull_request table is the actual commit id and the id can only be set after the head branch is actually merged into the base branch. So we cannot get the id before the PR is merged.

[bilderbuchi]

I'm not sure I get the distinction between "hypothetical" and "actual" here -- I'd think that Github actually does the PR merge in the background (maybe as part of the merge conflict checking?), because how else would it be able to derive a correct GITHUB_SHA for "Last merge commit on the GITHUB_REF branch"?
If that's the case, can Gitea do the same thing? e.g. also setting that id in the table as part of the merge conflict checking?