Do not fail the run when the merged branch is already deleted

.github/workflows/tests.yml

@@ -22,6 +22,7 @@ jobs:
           bash tests/test_merge_commit_merge.sh
           bash tests/test_rebase_merge.sh
           bash tests/test_merged_branch_moved.sh
+          bash tests/test_merged_branch_already_deleted.sh
 
   e2e-tests:
     name: E2E Tests

tests/test_merged_branch_already_deleted.sh

@@ -0,0 +1,54 @@
+#!/bin/bash
+#
+# Someone can click "Delete branch" on the merged PR while the action is still
+# running. The deletion is the action's last step, so the run must still end
+# green.
+
+set -ueo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+ROOT_DIR="$(cd "$SCRIPT_DIR/.." && pwd)"
+
+fail() { echo "❌ $1"; exit 1; }
+
+WORK=$(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
+git fetch -q origin
+
+# The branch disappears from the remote after the action's checkout.
+git --git-dir="$ORIGIN" update-ref -d refs/heads/parent
+
+cat > "$MOCK_DIR/gh" <<'MOCK'
+#!/bin/bash
+if [[ "$1" == "api" ]]; then echo "2 child"; fi
+MOCK
+chmod +x "$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" >"$MOCK_DIR/out.log" 2>&1 || RC=$?
+
+[[ "$RC" -eq 0 ]] || { cat "$MOCK_DIR/out.log"; fail "the run must succeed when the merged branch is already deleted"; }
+grep -q "'parent' was already deleted" "$MOCK_DIR/out.log" || fail "expected a warning about the deleted branch"
+git fetch -q origin
+git merge-base --is-ancestor "$SQUASH_COMMIT" origin/child || fail "child must be re-parented onto main"
+
+echo "✅ Already-deleted merged branch: warned, not failed"

update-pr-stack.sh

@@ -153,6 +153,21 @@ post_conflict_comment() {
     run gh pr edit "$PR_NUMBER" --add-label "$CONFLICT_LABEL"
 }
 
+# Deleting a PR's base branch closes the PR, so callers must retarget first.
+# The lease keeps commits pushed to the branch after the merge: they are in
+# neither the squash commit nor the children. The branch can also be gone
+# already: a human clicking "Delete branch" right after the merge races this run.
+# Everything that matters is done at that point, so only that case stays green.
+delete_merged_branch() {
+    try git push --force-with-lease="$MERGED_BRANCH:MERGED_TIP" origin ":$MERGED_BRANCH" && return 0
+    local RC=0
+    try git ls-remote --exit-code --heads origin "refs/heads/$MERGED_BRANCH" >/dev/null || RC=$?
+    if [[ "$RC" -ne 2 ]]; then
+        die "could not delete '$MERGED_BRANCH'"
+    fi
+    echo "⚠️ '$MERGED_BRANCH' was already deleted"
+}
+
 # Args: head branch, base branch, PR number. git commands use the branch; gh
 # commands use the number, since a head branch can carry several PRs.
 update_direct_target() {
@@ -389,8 +404,7 @@ main() {
             [[ -n "$BRANCH" ]] || continue
             run gh pr edit "$NUMBER" --base "$TARGET_BRANCH"
         done <<<"$CHILDREN"
-        # Deleting a PR's base branch closes the PR, so the retargets come first.
-        run git push --force-with-lease="$MERGED_BRANCH:MERGED_TIP" origin ":$MERGED_BRANCH"
+        delete_merged_branch
         return 0
     fi
 
@@ -435,12 +449,9 @@ main() {
         run gh pr edit "$NUMBER" --base "$TARGET_BRANCH"
     done
 
-    # 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.
+    # Keep the branch for reference while conflicted PRs remain.
     if [[ "${#CONFLICTED_TARGETS[@]}" -eq 0 ]]; then
-        run git push --force-with-lease="$MERGED_BRANCH:MERGED_TIP" origin ":$MERGED_BRANCH"
+        delete_merged_branch
     else
         echo "⚠️ Keeping branch '$MERGED_BRANCH' - still referenced by conflicted PRs: ${CONFLICTED_TARGETS[*]}"
     fi