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

.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_already_deleted.sh
 
   e2e-tests:
     name: E2E Tests

tests/test_merged_branch_already_deleted.sh

@@ -0,0 +1,94 @@
+#!/bin/bash
+#
+# Someone can click "Delete branch" on the merged PR while the action is still
+# running, so `git push origin :<merged>` fails with "remote ref does not
+# exist". The deletion is the last step, after the children are updated and
+# retargeted, so the run must still end green.
+
+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" "$1"
+}
+
+MOCK_DIR=$(mktemp -d)
+TEST_REPO=$(mktemp -d)
+cd "$TEST_REPO"
+echo "Created test repo at $TEST_REPO"
+
+log_cmd git init -b main
+log_cmd git config user.email "test@example.com"
+log_cmd git config user.name "Test User"
+
+echo "line" > file.txt
+log_cmd git add file.txt
+log_cmd git commit -m "Initial commit"
+simulate_push main
+
+log_cmd git checkout -b feature1
+echo "f1" >> file.txt
+log_cmd git add file.txt
+log_cmd git commit -m "Add feature 1"
+simulate_push feature1
+
+log_cmd git checkout -b feature2
+echo "f2" >> file.txt
+log_cmd git add file.txt
+log_cmd git commit -m "Add feature 2"
+simulate_push feature2
+
+# Squash feature1 into main
+log_cmd git checkout main
+log_cmd git merge --squash feature1
+log_cmd git commit -m "Add feature 1 (#1)"
+SQUASH=$(git rev-parse HEAD)
+simulate_push main
+
+# A git that refuses branch deletions, like the remote does once the branch is gone
+cat > "$MOCK_DIR/failing_git.sh" <<'MOCK'
+#!/bin/bash
+if [[ "$1" == "push" && "$3" == :* ]]; then
+    echo "error: unable to delete '${3#:}': remote ref does not exist" >&2
+    exit 1
+fi
+exec "$MOCK_GIT" "$@"
+MOCK
+chmod +x "$MOCK_DIR/failing_git.sh"
+
+set +e
+OUT=$(env \
+    SQUASH_COMMIT="$SQUASH" \
+    MERGED_BRANCH=feature1 \
+    PR_NUMBER=1 \
+    TARGET_BRANCH=main \
+    MOCK_GIT="$SCRIPT_DIR/mock_git.sh" \
+    GH="$SCRIPT_DIR/mock_gh.sh" \
+    GIT="$MOCK_DIR/failing_git.sh" \
+    "$SCRIPT_DIR/../update-pr-stack.sh" 2>&1)
+RC=$?
+set -e
+echo "$OUT"
+
+if [[ "$RC" -ne 0 ]]; then
+    echo "❌ The run must succeed when the merged branch is already deleted (exit $RC)"
+    exit 1
+fi
+if ! grep -q "Could not delete 'feature1'" <<<"$OUT"; then
+    echo "❌ Expected a warning about the failed deletion"
+    exit 1
+fi
+if ! grep -q "gh pr edit 2 --base main" <<<"$OUT"; then
+    echo "❌ The child PR must still be retargeted"
+    exit 1
+fi
+ACTUAL_DIFF=$(git diff main...feature2 | grep '^[+-]' | grep -v '^[+-][+-][+-]')
+if [[ "$ACTUAL_DIFF" != "+f2" ]]; then
+    echo "❌ Diff main...feature2 should show only feature2's change, got:"
+    echo "$ACTUAL_DIFF"
+    exit 1
+fi
+
+echo "✅ Already-deleted merged branch: warned, not failed"

update-pr-stack.sh

@@ -152,6 +152,16 @@ 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 branch can already be gone by then: a human clicking "Delete branch" right
+# after the merge races this run, and "delete branch on merge" wins it outright.
+# Everything that matters is done at that point, so a failed delete is a warning,
+# not a red run.
+delete_merged_branch() {
+    try git push origin ":$MERGED_BRANCH" \
+        || echo "⚠️ Could not delete '$MERGED_BRANCH' (may already be 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() {
@@ -381,8 +391,7 @@ 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 origin ":$MERGED_BRANCH"
+        delete_merged_branch
         return 0
     fi
 
@@ -427,10 +436,9 @@ 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.
+    # Keep the branch for reference while conflicted PRs remain.
     if [[ "${#CONFLICTED_TARGETS[@]}" -eq 0 ]]; then
-        run git push origin ":$MERGED_BRANCH"
+        delete_merged_branch
     else
         echo "⚠️ Keeping branch '$MERGED_BRANCH' - still referenced by conflicted PRs: ${CONFLICTED_TARGETS[*]}"
     fi