Trust only our own comments for the state marker (#51)

`read_state_marker` accepted a marker from *any* comment, so on a public
repo anyone able to comment could plant one; if its `base=` matched, the
resume would merge an attacker-chosen commit (fork-pushed objects are
reachable by hash in the repo network) into the branch and push it with
the action's token. Benign variant: a quote-reply of an old conflict
comment resurrects a stale marker, since HTML comments survive quoting
and the newest marker wins.

Fix: filter comments to `viewerDidAuthor` — those posted with the same
token the action runs under — which needs no configured identity. The
resume test's gh mock rejects comment queries without that filter.
Caveat: if the repo switches tokens (e.g. `GITHUB_TOKEN` → App) while a
PR sits in conflict, the old marker is no longer "ours" and the resume
takes the safe abandon path.

Also rejects markers with missing fields instead of passing empty values
to git (a marker missing `squash=` used to crash on `update-ref` and
strand the PR under the label); new scenario E covers it.

Stacked on #50 (same function).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

https://claude.ai/code/session_01JHvKryT4QUpHYdNq9YEQxX

---
_Generated by [Claude
Code](https://claude.ai/code/session_01JHvKryT4QUpHYdNq9YEQxX)_

---------

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: github-actions <github-actions@github.com>

Author: 3 people

tests/test_conflict_resolution_resume.sh

@@ -33,7 +33,11 @@ echo "gh $*" >> "$CALLS"
 if [[ "$1 $2" == "pr view" ]]; then
     case "$*" in
         *--json\ labels*)   printf '%s\n' "${MOCK_LABELS:-}";;
-        *--json\ comments*) cat "${MOCK_COMMENTS_FILE:-/dev/null}";;
+        *--json\ comments*)
+            # The comments file stands for our own comments only, so the query
+            # must restrict itself to those.
+            [[ "$*" == *viewerDidAuthor* ]] || { echo "comments query must filter by viewerDidAuthor" >&2; exit 1; }
+            cat "${MOCK_COMMENTS_FILE:-/dev/null}";;
         *) echo "unhandled pr view: $*" >&2; exit 1;;
     esac
 elif [[ "$1 $2" == "pr comment" ]]; then
@@ -167,5 +171,21 @@ grep -q -- "--base" "$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 "### Scenario E: malformed state marker -> internal error, die without touching the PR"
+setup_repo
+MOCK_LABELS="autorestack-needs-conflict-resolution"
+PR_BASE="parent"
+MOCK_COMMENTS_FILE="$WORK/comments.txt"
+{ echo "### conflict"; echo; echo '<!-- autorestack-state: base=parent target=main -->'; } > "$MOCK_COMMENTS_FILE"
+run_resume
+
+grep -q "EXIT=1" "$WORK/out.log" || fail "E: run must die on a malformed marker"
+grep -q "remove-label" "$CALLS" && fail "E: label must stay on"
+grep -q "gh pr comment" "$CALLS" && fail "E: no PR comment for an internal error"
+grep -q -- "--base" "$CALLS" && fail "E: base must NOT be edited"
+[[ "$(git -C "$ORIGIN" rev-parse child)" == "$CHILD_BEFORE" ]] || fail "E: child was pushed"
+ok "E: malformed marker dies, PR untouched, label kept"
+
 echo
 echo "All conflict-resume tests passed 🎉 ($PASS scenarios)"

update-pr-stack.sh

@@ -44,7 +44,8 @@ format_state_marker() {
 read_state_marker() {
     local PR_NUMBER="$1"
     local BODIES
-    if ! BODIES=$(gh pr view "$PR_NUMBER" --json comments --jq '.comments[].body'); then
+    if ! BODIES=$(gh pr view "$PR_NUMBER" --json comments \
+            --jq '.comments[] | select(.viewerDidAuthor) | .body'); then
         echo "Error: could not read comments of PR #$PR_NUMBER" >&2
         exit 1
     fi
@@ -307,6 +308,11 @@ continue_after_resolution() {
     read -r OLD_BASE NEW_TARGET SQUASH_HASH < <(parse_state_marker "$MARKER")
     echo "Recorded state: base=$OLD_BASE target=$NEW_TARGET squash=$SQUASH_HASH"
 
+    if [[ -z "$OLD_BASE" || -z "$NEW_TARGET" || -z "$SQUASH_HASH" ]]; then
+        echo "Error: malformed state marker on $PR_BRANCH: $MARKER" >&2
+        exit 1
+    fi
+
     # The PR was left based on the merged parent branch. If the payload shows a
     # different base, a human retargeted the PR; the recorded target is stale,
     # so step back before any mutation.