Fix basic static checks comparing same commit with itself

[jedcunningham]

The prek command was comparing github.sha with itself, which checked no files. Now uses merge-base to check all files changed in the PR against the base branch.

This ensures static checks are actually ran in PRs like #60168.

[Copilot]

Pull request overview

Fixes a bug in the basic-tests workflow where static checks were comparing the same commit (github.sha) with itself, resulting in no files being checked. The fix introduces a merge-base calculation to properly identify all changed files in a PR by comparing against the base branch.

Key changes:

• Implements merge-base calculation to find the common ancestor between PR and base branch
• Updates static check command to compare merge-base commit against HEAD instead of comparing github.sha with itself
• Increases fetch depth and adds fallback logic to handle cases where merge-base isn't found in shallow clones

---

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[bugraoz93]

Suggested change

	git fetch origin "$BASE_BRANCH" --depth=50
	# Try to find merge-base, deepen if not found
	MERGE_BASE=$(git merge-base "origin/$BASE_BRANCH" HEAD 2>/dev/null || echo "")
	if [ -z "$MERGE_BASE" ]; then
	echo "Merge-base not found with depth=50, fetching more history..."
	git fetch --deepen=450
	git fetch origin "$BASE_BRANCH" --deepen=450
	MERGE_BASE=$(git merge-base "origin/$BASE_BRANCH" HEAD)
	fi
	echo "sha=${MERGE_BASE}" >> $GITHUB_OUTPUT
	env:
	BASE_BRANCH: ${{ github.base_ref || 'main' }}
	git fetch origin "$BASE_BRANCH" --depth=$FETCH_DEPTH
	# Try to find merge-base, deepen if not found
	MERGE_BASE=$(git merge-base "origin/$BASE_BRANCH" HEAD 2>/dev/null || echo "")
	if [ -z "$MERGE_BASE" ]; then
	echo "Merge-base not found with depth=$FETCH_DEPTH, fetching more history..."
	git fetch --deepen=$DEEPEN
	git fetch origin "$BASE_BRANCH" --deepen=$DEEPEN
	MERGE_BASE=$(git merge-base "origin/$BASE_BRANCH" HEAD)
	fi
	echo "sha=${MERGE_BASE}" >> $GITHUB_OUTPUT
	env:
	BASE_BRANCH: ${{ github.base_ref || 'main' }}
	FETCH_DEPTH: 50
	DEEPEN: 450

[bugraoz93]

Small nit, maybe we can make the values parameterised for both --depth and --deepen from env :)

[amoghrajesh]

Wow, thats a strange one

[potiuk]

Strange one .. I wonder when it happened and how it did not work (I am sure it worked before). But it can be done way simpler.

All you need it is to add missing ^ in --from-ref

At the moment you do this check - github.sha already contains merge commit - this is how pull-request workflow works.

https://docs.github.com/en/actions/reference/workflows-and-actions/events-that-trigger-workflows#pull_request

Note that GITHUB_SHA for this event is the last merge commit of the pull request merge branch. If you want to get the commit ID for the last commit to the head branch of the pull request, use github.event.pull_request.head.sha instead.

[potiuk]

Superseded by #60202

[potiuk]

This is precisely why we needed 2 depth - because in pull_request the github.sha is merge and it's parent is the commit that was in the target branch when the merge was created (i.e. last time the PR was rebased).

[potiuk]

I merged my fix and rebased #60168 to verify that it works (it should fail)

[potiuk]

Nice catch @jedcunningham BTW :)

[potiuk]

Yep. Nicely failed now https://github.com/apache/airflow/actions/runs/20776566467/job/59663769292?pr=60168

[jedcunningham]

Nice, I'd considered just adding "^" but somehow I confused myself thinking that would only go 1 commit up - that if we had many commits on the branch it'd only look at the last one. I guess it must squash? Anyways, glad its working.

edit:

github.sha is merge and it's parent is the commit that was in the target branch when the merge was created (i.e. last time the PR was rebased)

Ah, a merge commit. Confusing since it has 2 parents. Apparently the "first" parent is the commit from the base branch, which is why this works. TIL.

[potiuk]

Ah, a merge commit. Confusing since it has 2 parents. Apparently the "first" parent is the commit from the base branch, which is why this works. TIL.

Correct. This is actually what we very heavily bank on (this why I knew it by heart).
Basically whole selective checks code is based on the fact that we have "merge" commit passed as github.sha and it's first parent is base branch.

https://github.com/apache/airflow/blob/main/dev/breeze/src/airflow_breeze/commands/ci_commands.py#L163:

def get_changed_files(commit_ref: str | None) -> tuple[str, ...]:
    if commit_ref is None:
        return ()
    cmd = [
        "git",
        "diff-tree",
        "--no-commit-id",
        "--name-only",
        "-r",
        commit_ref + "^",
        commit_ref,
    ]
    result = run_command(cmd, check=False, capture_output=True, text=True)
    if result.returncode != 0:
        get_console().print(
            f"[warning] Error when running diff-tree command [/]\n{result.stdout}\n{result.stderr}"
        )
        return ()
    changed_files = tuple(result.stdout.splitlines()) if result.stdout else ()
    get_console().print("\n[info]Changed files:[/]\n")
    get_console().print(changed_files)
    get_console().print()
    return changed_files

[potiuk]

BTW. There is also another interesting thing I learned with prek

There is a very nice shortcut to not have to find merge-base.

• git diff main..your_pr_branch
• git diff main...your_pr_branch

Spot the difference .... there are THREE dots in second case. And the ... is actually showing the changes in your branch from the merge-base

And ... (pun intended) there are few other gotchas: https://darekkay.com/blog/git-commit-ranges/#git-diff

For example git diff main...your-branch is not reverse of git diff your-branch...diff as you would suspect. The first one shows your changes since merge-base, the second shows SURPRISE -> all the changes in main (!) since merge-base 😱 😱 😱 😱 😱 😱 😱 😱 😱

Git is wild.

[potiuk]

So basically if you want to have prek to execute on "only your changes" from a branch you have PR to main with - do this (without running any of git merge-base thingies):

prek --from-ref maim

(we have it in our docs)