Name the pull request's branch, and abort a conflicting merge first

The Action named its local copy of the pull request pr-<number>, so the
comment read "git merge pr-1". It now uses the pull request's own
branch name, falling back to pr-<number> when that name is the base's
or one git won't take.

For a merge that will conflict, pre-flight advised resetting away a
merge commit that git never makes, since it stops at the conflict. It
now says git merge --abort puts everything back, and how to undo the
merge once it's resolved and committed.

Signed-off-by: Jacob Stopak <jacob@initialcommit.io>

Author: initialcommit-io

integrations/github-action/action.yml

@@ -54,35 +54,44 @@ runs:
       run: python -m pip install --quiet "$PACKAGE"
 
     - name: Fetch the pull request and its base
+      id: fetch
       shell: bash
       env:
         BASE: ${{ github.event.pull_request.base.ref }}
         NUMBER: ${{ github.event.pull_request.number }}
+        HEAD_REF: ${{ github.event.pull_request.head.ref }}
       run: |
         set -euo pipefail
-        # The pull request's commits as a branch git-sim can name. The base is
-        # fetched apart from it: on pull_request_target the checkout is the base
-        # branch itself, which git refuses to fetch into.
-        git fetch -q origin "+refs/pull/$NUMBER/head:refs/heads/pr-$NUMBER"
+        # The pull request's commits as a local branch named like its own, so
+        # the report says "merge feature"; pr-<number> when that name can't be
+        # used (a fork's main into main, or a name git won't take)
+        BRANCH="$HEAD_REF"
+        if [ -z "$BRANCH" ] || [ "$BRANCH" = "$BASE" ] || ! git check-ref-format --branch "$BRANCH" > /dev/null 2>&1; then
+          BRANCH="pr-$NUMBER"
+        fi
+        # The base is fetched apart from it: on pull_request_target the checkout
+        # is the base branch itself, which git refuses to fetch into.
+        git fetch -q origin "+refs/pull/$NUMBER/head:refs/heads/$BRANCH"
         git fetch -q origin "$BASE"
         git checkout -q -B "$BASE" "origin/$BASE"
+        echo "branch=$BRANCH" >> "$GITHUB_OUTPUT"
 
     - name: Pre-flight and simulate
       id: sim
       shell: bash
       env:
         MODE: ${{ inputs.mode }}
-        NUMBER: ${{ github.event.pull_request.number }}
+        BRANCH: ${{ steps.fetch.outputs.branch }}
         BASE: ${{ github.event.pull_request.base.ref }}
         git_sim_auto_open: "false"
       run: |
         set -euo pipefail
         mkdir -p git-sim-out
         if [ "$MODE" = "rebase" ]; then
-          git checkout -q "pr-$NUMBER"
+          git checkout -q "$BRANCH"
           CMD=(rebase "$BASE")
         else
-          CMD=(merge "pr-$NUMBER")
+          CMD=(merge "$BRANCH")
         fi
         git-sim preflight --markdown "${CMD[@]}" > git-sim-out/report.md || true
         PAGE=$(git-sim -d --media-dir git-sim-out --output-only-path "${CMD[@]}" 2>/dev/null | tail -n 1 || true)

src/git_sim/preflight.py

@@ -757,6 +757,7 @@ def _analyze_merge(repo: git.Repo, args: List[str], report: PreflightReport) ->
         report.marks[c.hexsha] = "INCOMING"
 
     # Deterministic conflict detection via git's own merge machinery (git >= 2.38).
+    conflicts = False
     if not ff:
         mt = ["git", "merge-tree", "--write-tree", "--name-only", "--no-messages"]
         if unrelated_ok:
@@ -767,6 +768,7 @@ def _analyze_merge(repo: git.Repo, args: List[str], report: PreflightReport) ->
         else:
             # the merged tree's id, then one conflicted path per line
             conflicted = list(dict.fromkeys(line.strip() for line in out.splitlines()[1:] if line.strip()))
+            conflicts = True
             report.escalate(Risk.CAUTION)
             report.warnings.append(
                 f"Merge WILL conflict in {len(conflicted)} file(s): "
@@ -783,6 +785,15 @@ def _analyze_merge(repo: git.Repo, args: List[str], report: PreflightReport) ->
         report.recovery.append(
             f"A fast-forward only moves the branch: git reset --hard ORIG_HEAD puts it back on {_short_sha(head)}"
         )
+    elif conflicts:
+        # git stops at the conflict before committing, so there is no merge
+        # commit to reset away yet
+        report.recovery.append(
+            "It stops at the conflict without making a merge commit: git merge --abort puts everything back"
+        )
+        report.recovery.append(
+            f"Once you resolve and commit it, undo the merge with: git reset --hard ORIG_HEAD (back to {_short_sha(head)})"
+        )
     else:
         report.recovery.append(
             f"Undo the merge commit with: git reset --hard ORIG_HEAD (back to {_short_sha(head)})"

tests/unit_tests/test_preflight.py

@@ -93,6 +93,9 @@ def test_merge_conflict_detected(repo):
     report = analyze("git merge feature", str(repo))
     assert any("conflict" in w.lower() for w in report.warnings)
     assert any("file1.txt" in w for w in report.warnings)
+    # git stops at the conflict with no merge commit, so --abort comes first
+    assert "git merge --abort" in report.recovery[0]
+    assert "resolve and commit" in report.recovery[1] and "ORIG_HEAD" in report.recovery[1]
 
 
 def conflict_on_main(repo):