MDEV-27039 Trying to lock mutex ... when the mutex was already locked

The reason of the double lock was an extraneous ha_flush_logs().
Unlike the upstream it is unnecessary in Mariadb that exploits a binlog
checkpoint mechanism for not letting PURGE or RESET-MASTER to trouble
transaction recovery. That is in case should a trx
be prepared but its binlog file gone, the trx then is committed on disk too.
Those facts have been always verified by existing tests of

  binlog.binlog_{checkpoint,xa_recover}.test.

A regression test for the bug is included though.

sql/log.cc

@@ -7029,7 +7029,6 @@ void MYSQL_BIN_LOG::purge()
     DBUG_EXECUTE_IF("expire_logs_always", { purge_time = my_time(0); });
     if (purge_time >= 0)
     {
-      ha_flush_logs();
       purge_logs_before_date(purge_time);
     }
     DEBUG_SYNC(current_thd, "after_purge_logs_before_date");

storage/spider/mysql-test/spider/r/spider_fixes.result

@@ -596,6 +596,13 @@ connection child2_1;
 DROP DATABASE IF EXISTS auto_test_remote;
 connection child2_2;
 DROP DATABASE IF EXISTS auto_test_remote2;
+connection master_1;
+SET @@global.expire_logs_days=11;
+connect  master_purge, localhost, root, , , $MASTER_1_MYPORT, $MASTER_1_MYSOCK;
+SET @@global.binlog_checksum=NONE;
+SET @@global.binlog_checksum=$binlog_checksum;
+SET @@global.expire_logs_days=$expire_logs_days;
+disconnect master_purge;
 for slave1_1
 for master_1
 for child2

storage/spider/mysql-test/spider/t/spider_fixes.test

@@ -1410,6 +1410,21 @@ if ($USE_CHILD_GROUP2)
   --connection child2_2
   DROP DATABASE IF EXISTS auto_test_remote2;
 }
+
+
+# MDEV-27039 LOCK_global_system_variables attempted to re-acquire
+# The test proves no assert anymore.
+--connection master_1
+--let $binlog_checksum=`SELECT @@global.binlog_checksum`
+--let $expire_logs_days=`SELECT @@global.expire_logs_days`
+SET @@global.expire_logs_days=11;
+
+--connect (master_purge, localhost, root, , , $MASTER_1_MYPORT, $MASTER_1_MYSOCK)
+SET @@global.binlog_checksum=NONE;
+--evalp SET @@global.binlog_checksum=$binlog_checksum
+--evalp SET @@global.expire_logs_days=$expire_logs_days
+--disconnect master_purge
+
 --disable_query_log
 --disable_result_log
 --source slave_test_deinit.inc