MDEV-359: Server crash when SET GLOBAL rpl_semi_sync_master_enabled = OFF

The semisync code does a fast-but-unsafe check for enabled or not without lock,
followed by a slow-but-safe check under lock. However, if the slow check failed,
the code still referenced not valid data (in an assert() expression), causing a
crash.

Fixed by not running the incorrect assert when semisync is disabled.

mysql-test/suite/rpl/r/rpl_mdev359.result

+include/master-slave.inc
+[connection master]
+CREATE TABLE t1 (a INT) ENGINE=MyISAM;
+INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
+SET GLOBAL rpl_semi_sync_master_enabled = ON;
+SET DEBUG_SYNC = "rpl_semisync_master_commit_trx_before_lock SIGNAL m1_ready WAIT_FOR m1_cont";
+INSERT INTO t1 SELECT * FROM t1;
+SET DEBUG_SYNC= "now WAIT_FOR m1_ready";
+SET GLOBAL rpl_semi_sync_master_enabled = OFF;
+SET DEBUG_SYNC= "now SIGNAL m1_cont";
+DROP TABLE t1;
+UNINSTALL PLUGIN rpl_semi_sync_master;
+include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_mdev359.test

+--source include/have_semisync_plugin.inc
+--source include/not_embedded.inc
+--source include/have_binlog_format_mixed_or_statement.inc
+--source include/master-slave.inc
+--source include/have_debug_sync.inc
+
+# MDEV-359: There was a server crash when the code first checks if semisync
+# is enabled without lock, then if so takes the lock and tests again.
+# If semisync was disabled in-between the first and the second test, an
+# assert was incorrectly made that referenced a NULL pointer.
+#
+# This tests uses debug_sync to pause one thread at the critical point in
+# the code, disable the semisync, and then continue the paused thread.
+
+CREATE TABLE t1 (a INT) ENGINE=MyISAM;
+INSTALL PLUGIN rpl_semi_sync_master SONAME 'semisync_master.so';
+SET GLOBAL rpl_semi_sync_master_enabled = ON;
+--connection master1
+SET DEBUG_SYNC = "rpl_semisync_master_commit_trx_before_lock SIGNAL m1_ready WAIT_FOR m1_cont";
+--send
+INSERT INTO t1 SELECT * FROM t1;
+--connection master
+SET DEBUG_SYNC= "now WAIT_FOR m1_ready";
+SET GLOBAL rpl_semi_sync_master_enabled = OFF;
+SET DEBUG_SYNC= "now SIGNAL m1_cont";
+--connection master1
+--reap
+
+connection master;
+DROP TABLE t1;
+
+disable_warnings;
+UNINSTALL PLUGIN rpl_semi_sync_master;
+enable_warnings;
+
+--source include/rpl_end.inc

plugin/semisync/semisync_master.cc

@@ -608,6 +608,7 @@ int ReplSemiSyncMaster::commitTrx(const char* trx_wait_binlog_name,
 
     set_timespec(start_ts, 0);
 
+    DEBUG_SYNC(current_thd, "rpl_semisync_master_commit_trx_before_lock");
     /* Acquire the mutex. */
     lock();
 
@@ -738,7 +739,6 @@ int ReplSemiSyncMaster::commitTrx(const char* trx_wait_binlog_name,
       }
     }
 
-  l_end:
     /*
       At this point, the binlog file and position of this transaction
       must have been removed from ActiveTranx.
@@ -747,6 +747,7 @@ int ReplSemiSyncMaster::commitTrx(const char* trx_wait_binlog_name,
            !active_tranxs_->is_tranx_end_pos(trx_wait_binlog_name,
                                              trx_wait_binlog_pos));
     
+  l_end:
     /* Update the status counter. */
     if (is_on())
       rpl_semi_sync_master_yes_transactions++;