Stop the resume from crash-looping when the kept parent branch is gone

tests/test_conflict_resolution_resume.sh

@@ -187,5 +187,35 @@ grep -q -- "--base" "$CALLS" && fail "E: base must NOT be edited"
 [[ "$(git -C "$ORIGIN" rev-parse child)" == "$CHILD_BEFORE" ]] || fail "E: child was pushed"
 ok "E: malformed marker dies, PR untouched, label kept"
 
+# ---------------------------------------------------------------------------
+echo "### Scenario F: recorded base branch is gone -> give up cleanly, no crash"
+setup_repo
+# Advance main with the squash commit so the child is not already up to date and
+# the resume would actually reach the merge step.
+git checkout -q main
+echo squash > s.txt && git add s.txt && git commit -qm squash
+SQUASH2=$(git rev-parse main)
+git push -q origin main
+git checkout -q child
+# The kept parent branch was deleted (auto-delete head branches left enabled, or
+# manual deletion). Before the up-front check, the resume tried to merge the
+# missing ref, failed `git merge --abort` because no merge was in progress, and
+# exited nonzero after re-posting a misleading conflict comment and the label,
+# repeating on every push.
+git push -q origin ":parent"
+MOCK_LABELS="autorestack-needs-conflict-resolution"
+PR_BASE="parent"   # matches marker -> not a manual retarget
+MOCK_COMMENTS_FILE="$WORK/comments.txt"
+{ echo "### conflict"; echo; marker parent main "$SQUASH2"; } > "$MOCK_COMMENTS_FILE"
+run_resume
+
+grep -q "EXIT=" "$WORK/out.log" && fail "F: script exited nonzero: $(cat "$WORK/out.log")"
+grep -q "remove-label autorestack-needs-conflict-resolution" "$CALLS" || fail "F: label not removed"
+grep -q -- "add-label" "$CALLS" && fail "F: conflict label must NOT be re-added"
+grep -q "gh pr comment" "$CALLS" || fail "F: no explanatory comment posted"
+grep -q -- "--base" "$CALLS" && fail "F: base must NOT be edited"
+[[ "$(git -C "$ORIGIN" rev-parse child)" == "$CHILD_BEFORE" ]] || fail "F: child was pushed"
+ok "F: missing base branch detected, no crash, label removed"
+
 echo
 echo "All conflict-resume tests passed 🎉 ($PASS scenarios)"

update-pr-stack.sh

@@ -142,6 +142,16 @@ list_child_prs() {
     log_cmd gh pr list --base "$MERGED_BRANCH" --json number,headRefName --jq '.[] | "\(.number) \(.headRefName)"'
 }
 
+# A failed git merge does not always leave a merge in progress: when the ref to
+# merge does not exist ("not something we can merge"), there is no MERGE_HEAD,
+# and `git merge --abort` itself fails ("There is no merge to abort"). Only
+# abort when a merge is actually in progress.
+abort_merge_if_in_progress() {
+    if git rev-parse --verify --quiet MERGE_HEAD >/dev/null; then
+        log_cmd git merge --abort
+    fi
+}
+
 # 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() {
@@ -167,14 +177,14 @@ update_direct_target() {
     if ! log_cmd git merge --no-edit "origin/$MERGED_BRANCH"; then
         CONFLICTS+=("origin/$MERGED_BRANCH")
         BASE_MERGE_CLEAN=false
-        log_cmd git merge --abort
+        abort_merge_if_in_progress
     fi
     # Only try merging the pre-squash target state if it's not already
     # included in the merged branch — otherwise the first merge covers it.
     if ! git merge-base --is-ancestor SQUASH_COMMIT~ "origin/$MERGED_BRANCH"; then
         if ! log_cmd git merge --no-edit SQUASH_COMMIT~; then
             CONFLICTS+=( "$(git rev-parse SQUASH_COMMIT~)  # $TARGET_BRANCH just before $MERGED_BRANCH was merged" )
-            log_cmd git merge --abort
+            abort_merge_if_in_progress
         fi
     fi
 
@@ -332,6 +342,16 @@ continue_after_resolution() {
         return
     fi
 
+    # Same check for the old base: the resume re-merges origin/$OLD_BASE, so if
+    # that branch is gone (auto-delete head branches left enabled, or deleted
+    # manually) the merge can never succeed and the label would re-trigger a
+    # failing run on every push. Give up cleanly instead.
+    if ! git rev-parse --verify --quiet "origin/$OLD_BASE" >/dev/null; then
+        echo "⚠️ Recorded base branch '$OLD_BASE' no longer exists; abandoning resume of $PR_BRANCH."
+        abandon_resume "$PR_NUMBER" "ℹ️ The branch this PR was based on (\`$OLD_BASE\`) no longer exists, so autorestack stepped back. If this PR still needs its base updated, update its base manually."
+        return
+    fi
+
     # The squash-merge run pushed the base merge and asked the user to resolve the
     # pre-squash merge, but it never recorded the squash itself. Finish that now:
     # re-run the same merge sequence as the squash-merge path. With the user's