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, 4 insertions, 1 deletions

diff --git a/sql/mdl.h b/sql/mdl.h
index b084670e5c6..123ffbcade6 100644
--- a/sql/mdl.h
+++ b/sql/mdl.h
@@ -122,6 +122,8 @@ public:
 */
 
 enum enum_mdl_type {
+  /* This means that the MDL_request is not initialized */
+  MDL_NOT_INITIALIZED= -1,
   /*
     An intention exclusive metadata lock (IX). Used only for scoped locks.
     Owner of this type of lock can acquire upgradable exclusive locks on
@@ -592,12 +594,13 @@ public:
   */
   MDL_request& operator=(const MDL_request &)
   {
+    type= MDL_NOT_INITIALIZED;
     ticket= NULL;
     /* Do nothing, in particular, don't try to copy the key. */
     return *this;
   }
   /* Another piece of ugliness for TABLE_LIST constructor */
-  MDL_request() {}
+  MDL_request(): type(MDL_NOT_INITIALIZED), ticket(NULL) {}
 
   MDL_request(const MDL_request *rhs)
     :type(rhs->type),