name: History Check

# Rejects PRs whose branch has no common ancestor with main.
#
# In May 2026 PR #25045 was merged from a branch that had been disconnected
# from main's history (likely an accidental `git checkout --orphan` or
# `.git/` re-init).  GitHub's merge UI does not refuse merges of unrelated
# histories, so the PR landed cleanly with the intended one-file change —
# but its parent-less root commit (413990c94) got grafted into main as a
# second root, and ~1500 files' worth of `git blame` history collapsed
# onto that single commit.
#
# This check catches the failure mode by requiring `git merge-base` between
# the PR head and main to be non-empty.

on:
  pull_request:
    branches: [main]

permissions:
  contents: read

jobs:
  check-common-ancestor:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5  # v4
        with:
          fetch-depth: 0  # full history both sides for merge-base

      - name: Reject PRs with no common ancestor on main
        run: |
          # `git merge-base` exits non-zero AND prints nothing when the two
          # commits share no ancestor.  We check both conditions explicitly
          # so the failure message is clear regardless of which signal fires
          # first.
          if ! BASE=$(git merge-base origin/main HEAD 2>/dev/null) || [ -z "$BASE" ]; then
            echo ""
            echo "::error::This PR has no common ancestor with main."
            echo ""
            echo "Your branch's history is disconnected from main.  Common causes:"
            echo "  - the branch was created with 'git checkout --orphan'"
            echo "  - '.git/' was re-initialized at some point during the work"
            echo "  - the branch was force-pushed from an unrelated repository"
            echo ""
            echo "Merging an unrelated-history PR grafts a parent-less root commit"
            echo "into main and collapses git blame for every file in that snapshot."
            echo "Reference: PR #25045 caused this and re-rooted blame on ~1500"
            echo "files to a single orphan commit."
            echo ""
            echo "To fix, rebase your changes onto current main:"
            echo "  git fetch origin main"
            echo "  git checkout -b fix-branch origin/main"
            echo "  # re-apply your changes (cherry-pick, copy files, etc.)"
            echo "  git push -f origin fix-branch"
            exit 1
          fi
          echo "::notice::Common ancestor with main: $BASE"