Give up cleanly when the recorded target branch is gone

The missing-target backstop returned silently, leaving the PR labeled with
no explanation and no path to ever resume. It now posts a comment and drops
the label like the other two dead-end cases, routed through a shared
abandon_resume helper.

Author: Phlogistique

tests/test_conflict_resolution_resume.sh

@@ -136,5 +136,20 @@ label_line=$(grep -n "remove-label" "$CALLS" | head -1 | cut -d: -f1)
 [[ "$base_line" -lt "$label_line" ]] || fail "C: base edit must come before label removal"
 ok "C: resume retargets base then removes label"
 
+# ---------------------------------------------------------------------------
+echo "### Scenario D: recorded target branch is gone -> give up cleanly"
+setup_repo
+MOCK_LABELS="autorestack-needs-conflict-resolution"
+MOCK_BASE="parent"   # matches marker -> not a manual retarget
+MOCK_COMMENTS_FILE="$WORK/comments.txt"
+{ echo "### conflict"; echo; marker parent ghost-target "$SQUASH"; } > "$MOCK_COMMENTS_FILE"
+run_resume
+
+grep -q "remove-label autorestack-needs-conflict-resolution" "$CALLS" || fail "D: label not removed"
+grep -q "gh pr comment" "$CALLS" || fail "D: no explanatory comment posted"
+grep -q -- "--base" "$CALLS" && fail "D: base must NOT be edited"
+[[ "$(git -C "$ORIGIN" rev-parse child)" == "$CHILD_BEFORE" ]] || fail "D: child was pushed"
+ok "D: missing target detected, no branch mutation, label removed"
+
 echo
 echo "All conflict-resume tests passed 🎉 ($PASS scenarios)"

update-pr-stack.sh

@@ -207,6 +207,16 @@ has_sibling_conflicts() {
     return 1  # No siblings with same base
 }
 
+# Give up on resuming the stack update: tell the user why on the PR, then drop
+# the conflict label so this action stops re-triggering. Used for the dead-end
+# cases where we cannot or must not finish automatically.
+abandon_resume() {
+    local PR_BRANCH="$1"
+    local MESSAGE="$2"
+    echo "$MESSAGE" | log_cmd gh pr comment "$PR_BRANCH" -F -
+    log_cmd gh pr edit "$PR_BRANCH" --remove-label "$CONFLICT_LABEL"
+}
+
 # Continue processing after user manually resolved conflicts
 continue_after_resolution() {
     check_env_var "PR_BRANCH"
@@ -229,9 +239,7 @@ continue_after_resolution() {
     MARKER=$(read_state_marker "$PR_BRANCH")
     if [[ -z "$MARKER" ]]; then
         echo "⚠️ No autorestack state marker on $PR_BRANCH; cannot resume safely. Removing the label."
-        { echo "ℹ️ autorestack could not find its state marker on this PR, so it will not update the stack automatically. If this PR still needs its base updated, do it manually."; } \
-            | log_cmd gh pr comment "$PR_BRANCH" -F -
-        log_cmd gh pr edit "$PR_BRANCH" --remove-label "$CONFLICT_LABEL"
+        abandon_resume "$PR_BRANCH" "ℹ️ autorestack could not find its state marker on this PR, so it will not update the stack automatically. If this PR still needs its base updated, update its base manually."
         return
     fi
 
@@ -249,15 +257,17 @@ continue_after_resolution() {
     CURRENT_BASE=$(gh pr view "$PR_BRANCH" --json baseRefName --jq '.baseRefName')
     if [[ "$CURRENT_BASE" != "$RECORDED_BASE" ]]; then
         echo "⚠️ Base of $PR_BRANCH changed manually ($RECORDED_BASE -> $CURRENT_BASE); not updating the stack."
-        { echo "ℹ️ The base branch of this PR was changed manually, so autorestack stepped back and will not update it automatically."; } \
-            | log_cmd gh pr comment "$PR_BRANCH" -F -
-        log_cmd gh pr edit "$PR_BRANCH" --remove-label "$CONFLICT_LABEL"
+        abandon_resume "$PR_BRANCH" "ℹ️ The base branch of this PR was changed manually, so autorestack stepped back and will not update it automatically."
         return
     fi
 
-    # Defense in depth: never act on a target branch that no longer exists.
+    # Defense in depth: never act on a target branch that no longer exists. The
+    # action checks out with full history (fetch-depth: 0), so a missing origin
+    # ref means the branch is really gone, not just unfetched; no future resume
+    # can succeed, so give up cleanly rather than stranding the PR under the label.
     if ! git rev-parse --verify --quiet "origin/$NEW_TARGET" >/dev/null; then
-        echo "⚠️ Recorded target branch '$NEW_TARGET' no longer exists; leaving $PR_BRANCH untouched."
+        echo "⚠️ Recorded target branch '$NEW_TARGET' no longer exists; abandoning resume of $PR_BRANCH."
+        abandon_resume "$PR_BRANCH" "ℹ️ The branch this PR was being retargeted onto (\`$NEW_TARGET\`) no longer exists, so autorestack stepped back. If this PR still needs its base updated, update its base manually."
         return
     fi