MDEV-21189: Dropping partition with 'wsrep_OSU_method=RSU' and 'SESSION...

MDEV-21189: Dropping partition with 'wsrep_OSU_method=RSU' and 'SESSION sql_log_bin = 0' cases the galera node to hang

Found two bugs

(1) have_committing_connections was missing mutex unlock on one
exit case. As this function is called on a loop it caused mutex
lock when we already owned the mutex. This could cause hang.

(2) wsrep_RSU_begin did set up error code when partition to
be dropped could not be MDL-locked because of concurrent
operations but wrong error code was propagated to upper layer
causing error to be ignored. This could have also caused
the hang.

mysql-test/suite/galera/galera_4nodes.cnf

@@ -16,6 +16,7 @@ wsrep-sync-wait=15
 #galera_port=@OPT.port
 #ist_port=@OPT.port
 #sst_port=@OPT.port
+wsrep-on=1
 wsrep-cluster-address=gcomm://
 wsrep_provider_options='base_port=@mysqld.1.#galera_port;gcache.size=10M'
 wsrep_node_incoming_address=127.0.0.1:@mysqld.1.port
@@ -25,6 +26,7 @@ wsrep_sst_receive_address='127.0.0.1:@mysqld.1.#sst_port'
 #galera_port=@OPT.port
 #ist_port=@OPT.port
 #sst_port=@OPT.port
+wsrep-on=1
 wsrep_cluster_address='gcomm://127.0.0.1:@mysqld.1.#galera_port'
 wsrep_provider_options='base_port=@mysqld.2.#galera_port;gcache.size=10M'
 wsrep_node_incoming_address=127.0.0.1:@mysqld.2.port
@@ -35,6 +37,7 @@ wsrep_sst_receive_address='127.0.0.1:@mysqld.2.#sst_port'
 #galera_port=@OPT.port
 #ist_port=@OPT.port
 #sst_port=@OPT.port
+wsrep-on=1
 wsrep_cluster_address='gcomm://127.0.0.1:@mysqld.1.#galera_port'
 wsrep_provider_options='base_port=@mysqld.3.#galera_port;gcache.size=10M'
 wsrep_node_incoming_address=127.0.0.1:@mysqld.3.port
@@ -45,6 +48,7 @@ wsrep_sst_receive_address='127.0.0.1:@mysqld.3.#sst_port'
 #galera_port=@OPT.port
 #ist_port=@OPT.port
 #sst_port=@OPT.port
+wsrep-on=1
 wsrep_cluster_address='gcomm://127.0.0.1:@mysqld.1.#galera_port'
 wsrep_provider_options='base_port=@mysqld.4.#galera_port;gcache.size=10M'
 wsrep_node_incoming_address=127.0.0.1:@mysqld.4.port

mysql-test/suite/galera/t/galera_partition.cnf

+!include ../galera_4nodes.cnf
+
+[mysqld.1]
+wsrep_provider_options='base_port=@mysqld.1.#galera_port;gcache.size=10M;gmcast.segment=1'
+wsrep_slave_threads=10
+
+[mysqld.2]
+wsrep_provider_options='base_port=@mysqld.2.#galera_port;gcache.size=10M;gmcast.segment=1'
+wsrep_slave_threads=10
+
+[mysqld.3]
+wsrep_provider_options='base_port=@mysqld.3.#galera_port;gcache.size=10M;gmcast.segment=2'
+wsrep_slave_threads=10
+
+[mysqld.4]
+wsrep_provider_options='base_port=@mysqld.4.#galera_port;gcache.size=10M;gmcast.segment=3'
+wsrep_slave_threads=10
+

sql/wsrep_mysqld.cc

@@ -1784,7 +1784,7 @@ static int wsrep_RSU_begin(THD *thd, char *db_, char *table_)
     }
 
     my_error(ER_LOCK_DEADLOCK, MYF(0));
-    return(1);
+    return(-1);
   }
 
   wsrep_seqno_t seqno = wsrep->pause(wsrep);
@@ -2311,6 +2311,7 @@ static my_bool have_committing_connections()
 
     if (is_committing_connection(tmp))
     {
+      mysql_mutex_unlock(&LOCK_thread_count);
       return TRUE;
     }
   }