Skip PRs merged with a merge commit

.github/workflows/tests.yml

@@ -17,6 +17,7 @@ jobs:
           bash tests/test_rebase_workflow.sh
           bash tests/test_mixed_workflows.sh
           bash tests/test_conflict_resolution_resume.sh
+          bash tests/test_merge_commit_skip.sh
 
   e2e-tests:
     name: E2E Tests

README.md

@@ -124,7 +124,7 @@ jobs:
 
 ### Notes
 
-* Currently only supports squash merges
+* Currently only supports squash merges; PRs merged with a merge commit are detected and skipped (history isn't rewritten, so stacked PRs stay valid as-is)
 * If a merge hits a conflict, you'll need to resolve it manually; pushing the resolution automatically continues the stack update
 * Very large stacks might hit GitHub rate limits
 

tests/test_merge_commit_skip.sh

@@ -0,0 +1,69 @@
+#!/bin/bash
+#
+# A PR merged with a merge commit (not squashed) must be left alone: history is
+# not rewritten, so stacked children stay valid and need no synthetic merge.
+
+set -ueo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+source "$SCRIPT_DIR/../command_utils.sh"
+
+simulate_push() {
+    log_cmd git update-ref "refs/remotes/origin/$1" "$1"
+}
+
+TEST_REPO=$(mktemp -d)
+cd "$TEST_REPO"
+echo "Created test repo at $TEST_REPO"
+
+log_cmd git init -b main
+log_cmd git config user.email "test@example.com"
+log_cmd git config user.name "Test User"
+
+echo "line" > file.txt
+log_cmd git add file.txt
+log_cmd git commit -m "Initial commit"
+simulate_push main
+
+log_cmd git checkout -b feature1
+echo "f1" >> file.txt
+log_cmd git add file.txt
+log_cmd git commit -m "Add feature 1"
+simulate_push feature1
+
+log_cmd git checkout -b feature2
+echo "f2" >> file.txt
+log_cmd git add file.txt
+log_cmd git commit -m "Add feature 2"
+simulate_push feature2
+
+# Merge feature1 into main with a real merge commit
+log_cmd git checkout main
+log_cmd git merge --no-ff --no-edit feature1
+MERGE_COMMIT=$(git rev-parse HEAD)
+simulate_push main
+
+FEATURE2_BEFORE=$(git rev-parse feature2)
+
+OUT=$(env \
+    SQUASH_COMMIT="$MERGE_COMMIT" \
+    MERGED_BRANCH=feature1 \
+    TARGET_BRANCH=main \
+    GH="$SCRIPT_DIR/mock_gh.sh" \
+    GIT="$SCRIPT_DIR/mock_git.sh" \
+    "$SCRIPT_DIR/../update-pr-stack.sh" 2>&1)
+echo "$OUT"
+
+if ! grep -q "merged with a merge commit" <<<"$OUT"; then
+    echo "❌ Expected the merge-commit skip message"
+    exit 1
+fi
+if grep -q "pr edit" <<<"$OUT"; then
+    echo "❌ No PR must be retargeted"
+    exit 1
+fi
+if [[ "$(git rev-parse feature2)" != "$FEATURE2_BEFORE" ]]; then
+    echo "❌ feature2 must not be modified"
+    exit 1
+fi
+echo "✅ Merge-commit merge skipped, stack untouched"

update-pr-stack.sh

@@ -301,6 +301,12 @@ main() {
 
     log_cmd git update-ref SQUASH_COMMIT "$SQUASH_COMMIT"
 
+    # A merge-commit merge does not rewrite history; stacked PRs stay valid.
+    if git rev-parse --verify --quiet SQUASH_COMMIT^2 >/dev/null; then
+        echo "✓ '$MERGED_BRANCH' was merged with a merge commit, not squashed; nothing to do"
+        return 0
+    fi
+
     # Find all PRs directly targeting the merged PR's head
     INITIAL_NUMBERS=()
     INITIAL_TARGETS=()