The patch is inspired from MySQL. Instead of using a single String to
hold the current active debug_sync signal, use a Hash_set to store
LEX_STRINGS. This patch ensures that a signal can not be lost, by being
overwritten by another thread via set DEBUG_SYNC = '... SIGNAL ...';
All signals are kepts "alive" until they are consumed by a wait event.
This requires updating test cases that assume the GLOBAL signal is never
consumed.
Follow-up work needed:
Port the additional syntax that allows one to set multiple signals
and also conditionally deactivate signals when waiting.
On deadlock transaction is rolled back (and trx->state is cleared) but
SELECT continued the loop because evaluate_join_record() ignored the
error status returned from lower join evaluation. val_int() does not
return error status so it is checked by thd->is_error().
Test case was created by Thirunarayanan Balathandayuthapani
<thiru@mariadb.com>
