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, 70 insertions, 47 deletions

diff --git a/sql/handler.cc b/sql/handler.cc
index cc7eedd18f2..5f94f9e893b 100644
--- a/sql/handler.cc
+++ b/sql/handler.cc
@@ -114,7 +114,7 @@ static TYPELIB known_extensions= {0,"known_exts", NULL, NULL};
 uint known_extensions_id= 0;
 
 static int commit_one_phase_2(THD *thd, bool all, THD_TRANS *trans,
-                              bool is_real_trans);
+                              bool is_real_trans, bool rw_trans);
 
 
 static plugin_ref ha_default_plugin(THD *thd)
@@ -133,6 +133,23 @@ static plugin_ref ha_default_tmp_plugin(THD *thd)
   return ha_default_plugin(thd);
 }
 
+#if defined(WITH_ARIA_STORAGE_ENGINE) && MYSQL_VERSION_ID < 100500
+void ha_maria_implicit_commit(THD *thd, bool new_trn)
+{
+  if (ha_maria::has_active_transaction(thd))
+  {
+    int error;
+    MDL_request mdl_request;
+    mdl_request.init(MDL_key::BACKUP, "", "", MDL_BACKUP_COMMIT, MDL_EXPLICIT);
+    error= thd->mdl_context.acquire_lock(&mdl_request,
+                                         thd->variables.lock_wait_timeout);
+    ha_maria::implicit_commit(thd, new_trn);
+    if (!error)
+      thd->mdl_context.release_lock(mdl_request.ticket);
+  }
+}
+#endif
+
 
 /** @brief
   Return the default storage engine handlerton for thread
@@ -1445,10 +1462,6 @@ int ha_commit_trans(THD *thd, bool all)
     DBUG_RETURN(2);
   }
 
-#ifdef WITH_ARIA_STORAGE_ENGINE
-  ha_maria::implicit_commit(thd, TRUE);
-#endif
-
   if (!ha_info)
   {
     /*
@@ -1462,7 +1475,9 @@ int ha_commit_trans(THD *thd, bool all)
       wsrep_commit_empty(thd, all);
     }
 #endif /* WITH_WSREP */
-    DBUG_RETURN(0);
+
+    ha_maria_implicit_commit(thd, TRUE);
+    DBUG_RETURN(error);
   }
 
   DBUG_EXECUTE_IF("crash_commit_before", DBUG_SUICIDE(););
@@ -1475,33 +1490,9 @@ int ha_commit_trans(THD *thd, bool all)
   /* rw_trans is TRUE when we in a transaction changing data */
   bool rw_trans= is_real_trans &&
                  (rw_ha_count > (thd->is_current_stmt_binlog_disabled()?0U:1U));
-  MDL_request mdl_request;
   DBUG_PRINT("info", ("is_real_trans: %d  rw_trans:  %d  rw_ha_count: %d",
                       is_real_trans, rw_trans, rw_ha_count));
 
-  if (rw_trans)
-  {
-    /*
-      Acquire a metadata lock which will ensure that COMMIT is blocked
-      by an active FLUSH TABLES WITH READ LOCK (and vice versa:
-      COMMIT in progress blocks FTWRL).
-
-      We allow the owner of FTWRL to COMMIT; we assume that it knows
-      what it does.
-    */
-    mdl_request.init(MDL_key::BACKUP, "", "", MDL_BACKUP_COMMIT, MDL_EXPLICIT);
-
-    if (!WSREP(thd) &&
-      thd->mdl_context.acquire_lock(&mdl_request,
-                                    thd->variables.lock_wait_timeout))
-    {
-      ha_rollback_trans(thd, all);
-      DBUG_RETURN(1);
-    }
-
-    DEBUG_SYNC(thd, "ha_commit_trans_after_acquire_commit_lock");
-  }
-
   if (rw_trans &&
       opt_readonly &&
       !(thd->security_ctx->master_access & SUPER_ACL) &&
@@ -1541,7 +1532,7 @@ int ha_commit_trans(THD *thd, bool all)
       // Here, the call will not commit inside InnoDB. It is only working
       // around closing thd->transaction.stmt open by TR_table::open().
       if (all)
-        commit_one_phase_2(thd, false, &thd->transaction.stmt, false);
+        commit_one_phase_2(thd, false, &thd->transaction.stmt, false, false);
     }
   }
 #endif
@@ -1561,7 +1552,7 @@ int ha_commit_trans(THD *thd, bool all)
       goto wsrep_err;
     }
 #endif /* WITH_WSREP */
-    error= ha_commit_one_phase(thd, all);
+    error= ha_commit_one_phase(thd, all, rw_trans);
 #ifdef WITH_WSREP
     if (run_wsrep_hooks)
       error= error || wsrep_after_commit(thd, all);
@@ -1613,7 +1604,7 @@ int ha_commit_trans(THD *thd, bool all)
 
   if (!is_real_trans)
   {
-    error= commit_one_phase_2(thd, all, trans, is_real_trans);
+    error= commit_one_phase_2(thd, all, trans, is_real_trans, rw_trans);
     goto done;
   }
 #ifdef WITH_WSREP
@@ -1631,7 +1622,7 @@ int ha_commit_trans(THD *thd, bool all)
   DEBUG_SYNC(thd, "ha_commit_trans_after_log_and_order");
   DBUG_EXECUTE_IF("crash_commit_after_log", DBUG_SUICIDE(););
 
-  error= commit_one_phase_2(thd, all, trans, is_real_trans) ? 2 : 0;
+  error= commit_one_phase_2(thd, all, trans, is_real_trans, rw_trans) ? 2 : 0;
 #ifdef WITH_WSREP
   if (run_wsrep_hooks && (error || (error = wsrep_after_commit(thd, all))))
   {
@@ -1694,16 +1685,6 @@ err:
                 thd->rgi_slave->is_parallel_exec);
   }
 end:
-  if (rw_trans && mdl_request.ticket)
-  {
-    /*
-      We do not always immediately release transactional locks
-      after ha_commit_trans() (see uses of ha_enable_transaction()),
-      thus we release the commit blocker lock as soon as it's
-      not needed.
-    */
-    thd->mdl_context.release_lock(mdl_request.ticket);
-  }
 #ifdef WITH_WSREP
   if (wsrep_is_active(thd) && is_real_trans && !error &&
       (rw_ha_count == 0 || all) &&
@@ -1719,6 +1700,7 @@ end:
 /**
   @note
   This function does not care about global read lock. A caller should.
+  However backup locks are handled in commit_one_phase_2.
 
   @param[in]  all  Is set in case of explicit commit
                    (COMMIT statement), or implicit commit
@@ -1727,7 +1709,7 @@ end:
                    autocommit=1.
 */
 
-int ha_commit_one_phase(THD *thd, bool all)
+int ha_commit_one_phase(THD *thd, bool all, bool rw_trans)
 {
   THD_TRANS *trans=all ? &thd->transaction.all : &thd->transaction.stmt;
   /*
@@ -1753,20 +1735,50 @@ int ha_commit_one_phase(THD *thd, bool all)
     if ((res= thd->wait_for_prior_commit()))
       DBUG_RETURN(res);
   }
-  res= commit_one_phase_2(thd, all, trans, is_real_trans);
+  res= commit_one_phase_2(thd, all, trans, is_real_trans, rw_trans);
   DBUG_RETURN(res);
 }
 
 
 static int
-commit_one_phase_2(THD *thd, bool all, THD_TRANS *trans, bool is_real_trans)
+commit_one_phase_2(THD *thd, bool all, THD_TRANS *trans, bool is_real_trans,
+                   bool rw_trans)
 {
   int error= 0;
   uint count= 0;
   Ha_trx_info *ha_info= trans->ha_list, *ha_info_next;
+  MDL_request mdl_request;
   DBUG_ENTER("commit_one_phase_2");
   if (is_real_trans)
     DEBUG_SYNC(thd, "commit_one_phase_2");
+
+  if (rw_trans)
+  {
+    /*
+      Acquire a metadata lock which will ensure that COMMIT is blocked
+      by an active FLUSH TABLES WITH READ LOCK (and vice versa:
+      COMMIT in progress blocks FTWRL).
+
+      We allow the owner of FTWRL to COMMIT; we assume that it knows
+      what it does.
+    */
+    mdl_request.init(MDL_key::BACKUP, "", "", MDL_BACKUP_COMMIT, MDL_EXPLICIT);
+
+    if (!WSREP(thd) &&
+      thd->mdl_context.acquire_lock(&mdl_request,
+                                    thd->variables.lock_wait_timeout))
+    {
+      my_error(ER_ERROR_DURING_COMMIT, MYF(0), 1);
+      ha_rollback_trans(thd, all);
+      DBUG_RETURN(1);
+    }
+    DEBUG_SYNC(thd, "ha_commit_trans_after_acquire_commit_lock");
+  }
+
+#if defined(WITH_ARIA_STORAGE_ENGINE) && MYSQL_VERSION_ID < 100500
+  ha_maria::implicit_commit(thd, TRUE);
+#endif
+
   if (ha_info)
   {
     for (; ha_info; ha_info= ha_info_next)
@@ -1795,6 +1807,17 @@ commit_one_phase_2(THD *thd, bool all, THD_TRANS *trans, bool is_real_trans)
 #endif
     }
   }
+  if (mdl_request.ticket)
+  {
+    /*
+      We do not always immediately release transactional locks
+      after ha_commit_trans() (see uses of ha_enable_transaction()),
+      thus we release the commit blocker lock as soon as it's
+      not needed.
+    */
+    thd->mdl_context.release_lock(mdl_request.ticket);
+  }
+
   /* Free resources and perform other cleanup even for 'empty' transactions. */
   if (is_real_trans)
   {