Keep commits pushed to a branch after it was merged

.github/workflows/tests.yml

@@ -21,6 +21,7 @@ jobs:
           bash tests/test_conflict_absorbed_resolution.sh
           bash tests/test_merge_commit_merge.sh
           bash tests/test_rebase_merge.sh
+          bash tests/test_merged_branch_moved.sh
 
   e2e-tests:
     name: E2E Tests

action.yml

@@ -52,6 +52,7 @@ runs:
         ACTION_MODE: ${{ steps.check.outputs.mode }}
         SQUASH_COMMIT: ${{ github.event.pull_request.merge_commit_sha }}
         MERGED_BRANCH: ${{ github.event.pull_request.head.ref }}
+        MERGED_TIP: ${{ github.event.pull_request.head.sha }}
         TARGET_BRANCH: ${{ github.event.pull_request.base.ref }}
         PR_BRANCH: ${{ github.event.pull_request.head.ref }}
         PR_NUMBER: ${{ github.event.pull_request.number }}

tests/test_conflict_absorbed_resolution.sh

@@ -81,6 +81,7 @@ COMMENT_FILE=$(mktemp)
 OUT=$(env \
     SQUASH_COMMIT="$SQUASH_COMMIT" \
     MERGED_BRANCH=feature1 \
+    MERGED_TIP=$(git rev-parse feature1) \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
     GH="$SCRIPT_DIR/mock_gh.sh" \
@@ -93,7 +94,7 @@ if ! grep -q "Mock: gh pr comment 2" <<<"$OUT"; then
     echo "❌ Expected a conflict comment on the child PR"
     exit 1
 fi
-if grep -q "push origin :feature1" <<<"$OUT"; then
+if grep -q "origin :feature1" <<<"$OUT"; then
     echo "❌ The merged branch must be kept while the child is conflicted"
     exit 1
 fi

tests/test_merge_commit_merge.sh

@@ -50,6 +50,7 @@ FEATURE2_BEFORE=$(git rev-parse feature2)
 OUT=$(env \
     SQUASH_COMMIT="$MERGE_COMMIT" \
     MERGED_BRANCH=feature1 \
+    MERGED_TIP=$(git rev-parse feature1) \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
     GH="$SCRIPT_DIR/mock_gh.sh" \
@@ -69,7 +70,7 @@ fi
 # Children must be retargeted before the merged branch is deleted (deleting a
 # PR's base branch closes the PR).
 EDIT_LINE=$(grep -n "pr edit 2 --base main" <<<"$OUT" | cut -d: -f1 | head -1 || true)
-DELETE_LINE=$(grep -n "push origin :feature1" <<<"$OUT" | cut -d: -f1 | head -1 || true)
+DELETE_LINE=$(grep -n "origin :feature1" <<<"$OUT" | cut -d: -f1 | head -1 || true)
 if [[ -z "$EDIT_LINE" ]]; then
     echo "❌ Child PR must be retargeted onto main"
     exit 1

tests/test_merged_branch_moved.sh

@@ -0,0 +1,61 @@
+#!/bin/bash
+#
+# A commit pushed to the merged branch after GitHub squash-merged it is in
+# neither the squash commit nor the children: the child must not absorb it, and
+# the merged branch must not be deleted.
+
+set -ueo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+
+fail() { echo "❌ $1"; exit 1; }
+ok() { echo "✅ $1"; }
+
+WORK=$(mktemp -d)
+ORIGIN=$(mktemp -d)
+MOCK_DIR=$(mktemp -d)
+git init -q --bare "$ORIGIN"
+git init -q -b main "$WORK"
+cd "$WORK"
+git config user.email t@e.com && git config user.name t
+git remote add origin "$ORIGIN"
+
+echo base > f.txt && git add f.txt && git commit -qm initial
+git checkout -q -b parent
+echo parent > p.txt && git add p.txt && git commit -qm parent
+MERGED_TIP=$(git rev-parse parent)
+git checkout -q -b child
+echo child > c.txt && git add c.txt && git commit -qm child
+git checkout -q main
+git merge -q --squash parent && git commit -qm "Squash merge parent"
+SQUASH_COMMIT=$(git rev-parse main)
+git push -q origin main parent child
+
+# Someone pushes to the parent branch after the merge, before the action runs.
+git checkout -q parent
+echo late > late.txt && git add late.txt && git commit -qm late
+LATE=$(git rev-parse parent)
+git push -q origin parent
+git checkout -q main
+git fetch -q origin
+
+cat > "$MOCK_DIR/gh" <<'EOF'
+#!/bin/bash
+if [[ "$1" == "api" ]]; then echo "2 child"; fi
+EOF
+chmod +x "$MOCK_DIR/gh"
+
+RC=0
+env SQUASH_COMMIT="$SQUASH_COMMIT" MERGED_BRANCH=parent MERGED_TIP="$MERGED_TIP" \
+    TARGET_BRANCH=main PR_NUMBER=1 GH="$MOCK_DIR/gh" \
+    bash "$ROOT_DIR/update-pr-stack.sh" >"$MOCK_DIR/out.log" 2>&1 || RC=$?
+
+git fetch -q origin
+git merge-base --is-ancestor "$SQUASH_COMMIT" origin/child || fail "child must be re-parented onto main"
+git merge-base --is-ancestor "$LATE" origin/child && fail "child must not absorb the late commit"
+ok "child re-parented without absorbing the late commit"
+
+[[ "$(git rev-parse origin/parent 2>/dev/null)" == "$LATE" ]] || fail "parent branch must be kept at the late commit"
+[[ "$RC" -ne 0 ]] || fail "the run must fail when it cannot delete the merged branch"
+ok "parent branch kept, run failed"

tests/test_mixed_workflows.sh

@@ -90,6 +90,7 @@ run_update_pr_stack() {
       env \
       SQUASH_COMMIT=$SQUASH_COMMIT \
       MERGED_BRANCH=feature1 \
+      MERGED_TIP=$(git rev-parse feature1) \
       PR_NUMBER=1 \
       TARGET_BRANCH=main \
       GH="$SCRIPT_DIR/mock_gh.sh" \

tests/test_rebase_merge.sh

@@ -59,6 +59,7 @@ simulate_push main
 OUT=$(env \
     SQUASH_COMMIT="$MERGE_COMMIT" \
     MERGED_BRANCH=feature1 \
+    MERGED_TIP=$(git rev-parse feature1) \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
     GH="$SCRIPT_DIR/mock_gh.sh" \
@@ -75,7 +76,7 @@ if ! grep -q "Mock: gh pr edit 2 --base main" <<<"$OUT"; then
     echo "❌ The child PR must be retargeted to main"
     exit 1
 fi
-if ! grep -q "push origin :feature1" <<<"$OUT"; then
+if ! grep -q "origin :feature1" <<<"$OUT"; then
     echo "❌ The merged branch must be deleted"
     exit 1
 fi

tests/test_rebase_workflow.sh

@@ -85,6 +85,7 @@ run_update_pr_stack() {
     env \
     SQUASH_COMMIT=$SQUASH_COMMIT \
     MERGED_BRANCH=feature1 \
+    MERGED_TIP=$(git rev-parse feature1) \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
     GH="$SCRIPT_DIR/mock_gh.sh" \

tests/test_update_pr_stack.sh

@@ -82,6 +82,7 @@ run_update_pr_stack() {
     env \
     SQUASH_COMMIT=$SQUASH_COMMIT \
     MERGED_BRANCH=feature1 \
+    MERGED_TIP=$(git rev-parse feature1) \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
     GH="$SCRIPT_DIR/mock_gh.sh" \
@@ -95,7 +96,7 @@ run_update_pr_stack
 # the retarget (deleting a PR's base branch closes the PR).
 push_line=$(grep -n "git push origin feature2" "$RUN_LOG" | head -1 | cut -d: -f1 || true)
 edit_line=$(grep -n "pr edit 2 --base main" "$RUN_LOG" | head -1 | cut -d: -f1 || true)
-delete_line=$(grep -n "git push origin :feature1" "$RUN_LOG" | head -1 | cut -d: -f1 || true)
+delete_line=$(grep -n "origin :feature1" "$RUN_LOG" | head -1 | cut -d: -f1 || true)
 if [[ -n "$push_line" && -n "$edit_line" && -n "$delete_line" \
       && "$push_line" -lt "$edit_line" && "$edit_line" -lt "$delete_line" ]]; then
     echo "✅ Ordering: push head, then retarget base, then delete merged branch"
@@ -220,6 +221,7 @@ FAIL_LOG="$TEST_REPO/update_fail_run.log"
 if log_cmd env \
     SQUASH_COMMIT=$SQUASH_COMMIT \
     MERGED_BRANCH=feature1 \
+    MERGED_TIP=$(git rev-parse feature1) \
     PR_NUMBER=1 \
     TARGET_BRANCH=main \
     MOCK_PR_LIST_FAIL=1 \
@@ -230,7 +232,7 @@ if log_cmd env \
     cat "$FAIL_LOG"
     exit 1
 fi
-if grep -q "git push origin :feature1" "$FAIL_LOG"; then
+if grep -q "origin :feature1" "$FAIL_LOG"; then
     echo "❌ merged branch must not be deleted when the children cannot be listed"
     cat "$FAIL_LOG"
     exit 1

update-pr-stack.sh

@@ -6,6 +6,7 @@
 # SQUASH_COMMIT - The merged PR's merge_commit_sha: the squash commit, or the
 #                 last copied commit of a rebase merge
 # MERGED_BRANCH - The name of the branch that was merged and will be deleted
+# MERGED_TIP    - The merged PR's head commit, as GitHub merged it
 # TARGET_BRANCH - The name of the branch that the PR was merged into
 # PR_NUMBER - The number of the PR that was merged
 #
@@ -178,15 +179,15 @@ See $GITHUB_SERVER_URL/$GITHUB_REPOSITORY/actions/runs/$GITHUB_RUN_ID"
     fi
 
     # Re-parent the child onto the target in a single merge: merge the squash
-    # commit with the base forced to merge-base(HEAD, origin/$MERGED_BRANCH). That
+    # commit with the base forced to merge-base(HEAD, MERGED_TIP). That
     # drops the merged branch's content (now carried by the target via the squash)
     # while keeping the child's own changes -- the merge equivalent of
     # `git rebase --onto`, done by the vendored git-merge-onto. --absorbed records
     # the merged branch's tip as an extra parent: the head then still descends
     # from its current base (the merged branch) even when that base moved after
     # the head forked, so the PR reads as mergeable until it is retargeted.
     local RC=0
-    try python3 "$SCRIPT_DIR/git-merge-onto" --absorbed -m "$MERGE_MSG" SQUASH_COMMIT "origin/$MERGED_BRANCH" || RC=$?
+    try python3 "$SCRIPT_DIR/git-merge-onto" --absorbed -m "$MERGE_MSG" SQUASH_COMMIT MERGED_TIP || RC=$?
     if [[ "$RC" -eq 0 ]]; then
         return 0
     fi
@@ -365,10 +366,12 @@ main() {
     # Check required environment variables
     check_env_var "SQUASH_COMMIT"
     check_env_var "MERGED_BRANCH"
+    check_env_var "MERGED_TIP"
     check_env_var "TARGET_BRANCH"
     check_env_var "PR_NUMBER"
 
     run git update-ref SQUASH_COMMIT "$SQUASH_COMMIT"
+    run git update-ref MERGED_TIP "$MERGED_TIP"
 
     # A merge-commit merge does not rewrite history: each child's head already
     # contains the merged branch's commits, and the merge commit carries them
@@ -382,7 +385,7 @@ main() {
             run gh pr edit "$NUMBER" --base "$TARGET_BRANCH"
         done <<<"$CHILDREN"
         # Deleting a PR's base branch closes the PR, so the retargets come first.
-        run git push origin ":$MERGED_BRANCH"
+        run git push --force-with-lease="$MERGED_BRANCH:MERGED_TIP" origin ":$MERGED_BRANCH"
         return 0
     fi
 
@@ -429,8 +432,10 @@ main() {
 
     # Deleting a PR's base branch closes the PR, so this must come after the
     # retargets. Keep the branch for reference while conflicted PRs remain.
+    # The lease keeps commits pushed to the branch after the merge: they are in
+    # neither the squash commit nor the children.
     if [[ "${#CONFLICTED_TARGETS[@]}" -eq 0 ]]; then
-        run git push origin ":$MERGED_BRANCH"
+        run git push --force-with-lease="$MERGED_BRANCH:MERGED_TIP" origin ":$MERGED_BRANCH"
     else
         echo "⚠️ Keeping branch '$MERGED_BRANCH' - still referenced by conflicted PRs: ${CONFLICTED_TARGETS[*]}"
     fi