The server-side move cycle guard (getPageBreadCrumbs) and the move UPDATE ran
as two separate, unlocked statements. Two concurrent moves ("A under B" and
"B under A") could each read the same pre-write acyclic snapshot, both pass the
guard, then persist A.parentPageId=B AND B.parentPageId=A — a parent/child
cycle (TOCTOU, #207#7).
Run the cycle check and the UPDATE inside one transaction (executeTx) guarded
by a per-space advisory lock (pg_advisory_xact_lock, held until COMMIT) so all
moves within a space serialize: the second mover blocks until the first commits
and then sees the freshly written parent, so its guard rejects the cycle.
getPageBreadCrumbs gains an optional trx so the check runs on the locked snapshot.
Adds an integration test driving two opposing concurrent movePage calls and
asserting no cycle ever persists and exactly one move is rejected. Updates the
movePage unit-test stubs for the new transactional path.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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