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.

1 files changed, 2 insertions, 1 deletions

diff --git a/plugin/semisync/semisync_master.cc b/plugin/semisync/semisync_master.cc
index 9e857f83d39..cb60d8e67d3 100644
--- a/plugin/semisync/semisync_master.cc
+++ b/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++;