swap_relation_files() calls toast_get_valid_index() to find and lock this index, just before swapping with the rebuilt TOAST index. The latter function releases the lock before returning. Potential for mischief is low; a concurrent session can issue ALTER INDEX ... SET (fillfactor = ...), which is not alarming. Nonetheless, changing pg_class.relfilenode without a lock is unconventional. Back-patch to 9.5 (all supported versions), because another fix needs this.
e629a01f69 During heap rebuild, lock any TOAST index until end of transaction.
src/backend/access/common/toast_internals.c | 4 ++--
src/backend/commands/cluster.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)