pre_ci.yml

# When a branch is used to open a PR in the branch's own repository, GitHub will
# trigger 'on: push' workflows against the head commit of the PR, and trigger
# 'on: pull_request' workflows against a merge commit between the head commit
# and the PR's base branch (typically master).
#
# Often times those 2 sets of jobs end up running against identical trees. This
# is the case if there have been no commits on the base branch since when the
# PR's head branch was forked from the base branch.
#
# The code below detects this common scenario and provides a way to bypass CI
# from running redundantly on the merge commit.

name: pre_ci

on:
  workflow_call:
    outputs:
      continue:
        description: "whether to execute the workflow"
        value: ${{jobs.pre_ci.outputs.continue}}

permissions:
  contents: read

jobs:
  pre_ci:
    runs-on: ubuntu-latest
    outputs:
      continue: ${{steps.decision.outputs.continue}}

    steps:
      - id: is_local_pull_request
        run: echo value=true >> $GITHUB_OUTPUT
        if: github.event_name == 'pull_request'
          && github.event.pull_request.head.repo.full_name == github.event.pull_request.base.repo.full_name

      - uses: actions/checkout@v3
        with:
          fetch-depth: 2
        if: steps.is_local_pull_request.outputs.value

      - id: is_noop_merge
        run: if git diff HEAD^2 --quiet; then echo value=true >> $GITHUB_OUTPUT; fi
        if: steps.is_local_pull_request.outputs.value

      - id: decision
        run: echo continue=true >> $GITHUB_OUTPUT
        if: |
          !steps.is_local_pull_request.outputs.value || !steps.is_noop_merge.outputs.value