MDEV-21953 deadlock between BACKUP STAGE BLOCK_COMMIT and parallel repl.

The issue was:
T1, a parallel slave worker thread, is waiting for another worker thread to
commit. While waiting, it has the MDL_BACKUP_COMMIT lock.
T2, working for mariabackup, is doing BACKUP STAGE BLOCK_COMMIT and blocks
all commits.
This causes a deadlock as the thread T1 is waiting for can't commit.

Fixed by moving locking of MDL_BACKUP_COMMIT from ha_commit_trans() to
commit_one_phase_2()

Other things:
- Added a new argument to ha_comit_one_phase() to signal if the
  transaction was a write transaction.
- Ensured that ha_maria::implicit_commit() is always called under
  MDL_BACKUP_COMMIT. This code is not needed in 10.5
- Ensure that MDL_Request values 'type' and 'ticket' are always
  initialized. This makes it easier to check the state of the MDL_Request.
- Moved thd->store_globals() earlier in handle_rpl_parallel_thread() as
  thd->init_for_queries() could use a MDL that could crash if store_globals
  where not called.
- Don't call ha_enable_transactions() in THD::init_for_queries() as this
  is both slow (uses MDL locks) and not needed.

1 files changed, 1 insertions, 1 deletions

diff --git a/sql/xa.cc b/sql/xa.cc
index b08ab973cc5..15116ba9366 100644
--- a/sql/xa.cc
+++ b/sql/xa.cc
@@ -582,7 +582,7 @@ bool trans_xa_commit(THD *thd)
     {
       DEBUG_SYNC(thd, "trans_xa_commit_after_acquire_commit_lock");
 
-      res= MY_TEST(ha_commit_one_phase(thd, 1));
+      res= MY_TEST(ha_commit_one_phase(thd, 1, 1));
       if (res)
         my_error(ER_XAER_RMERR, MYF(0));
     }