MDEV-18096 The server would crash when has configs rpl_semi_sync_master_enabled = OFF rpl_semi_sync_master_wait_no_slave = OFF

The patch fixes a fired assert in the semisync master module. The assert
caught attempt to switch semisync off (per rpl_semi_sync_master_wait_no_slave = OFF)
when it was not even initialized (per rpl_semi_sync_master_enabled = OFF).
The switching-off execution branch is relocated under one that executes
enable_master() first.

A minor cleaup is done to remove the int return from two functions that
did not return anything but an error which could not happen in the functions.

Author: andrelkin

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

@@ -0,0 +1,8 @@
+include/master-slave.inc
+[connection master]
+connection master;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 SET a=1;
+DROP TABLE t1;
+connection slave;
+include/rpl_end.inc

mysql-test/suite/rpl/t/rpl_semi_sync_wait_no_slave-master.opt

@@ -0,0 +1 @@
+--rpl_semi_sync_master_enabled=0 --rpl_semi_sync_master_wait_no_slave=0

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

@@ -0,0 +1,14 @@
+# The test verifies master crash of MDEV-18096 when the server starts with
+# rpl_semi_sync_master_enabled = OFF rpl_semi_sync_master_wait_no_slave = OFF
+
+--source include/master-slave.inc
+--source include/have_binlog_format_mixed.inc
+
+--connection master
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 SET a=1;
+DROP TABLE t1;
+
+--sync_slave_with_master
+
+--source include/rpl_end.inc

sql/semisync_master.cc

@@ -228,7 +228,7 @@ bool Active_tranx::is_tranx_end_pos(const char *log_file_name,
   DBUG_RETURN(entry != NULL);
 }
 
-int Active_tranx::clear_active_tranx_nodes(const char *log_file_name,
+void Active_tranx::clear_active_tranx_nodes(const char *log_file_name,
                                            my_off_t log_file_pos)
 {
   Tranx_node *new_front;
@@ -307,7 +307,7 @@ int Active_tranx::clear_active_tranx_nodes(const char *log_file_name,
                             m_trx_front->log_name, (ulong)m_trx_front->log_pos));
   }
 
-  DBUG_RETURN(0);
+  DBUG_VOID_RETURN;
 }
 
 
@@ -371,20 +371,21 @@ int Repl_semi_sync_master::init_object()
   {
     result = enable_master();
     if (!result)
+    {
       result= ack_receiver.start(); /* Start the ACK thread. */
+      /*
+        If rpl_semi_sync_master_wait_no_slave is disabled, let's temporarily
+        switch off semisync to avoid hang if there's none active slave.
+      */
+      if (!rpl_semi_sync_master_wait_no_slave)
+        switch_off();
+    }
   }
   else
   {
     result = disable_master();
   }
 
-  /*
-    If rpl_semi_sync_master_wait_no_slave is disabled, let's temporarily
-    switch off semisync to avoid hang if there's none active slave.
-  */
-  if (!rpl_semi_sync_master_wait_no_slave)
-    switch_off();
-
   return result;
 }
 
@@ -961,25 +962,23 @@ int Repl_semi_sync_master::commit_trx(const char* trx_wait_binlog_name,
  * the current sending event catches up with last wait position.  If it
  * does match, semi-sync will be switched on again.
  */
-int Repl_semi_sync_master::switch_off()
+void Repl_semi_sync_master::switch_off()
 {
-  int result;
-
   DBUG_ENTER("Repl_semi_sync_master::switch_off");
 
   m_state = false;
 
   /* Clear the active transaction list. */
   assert(m_active_tranxs != NULL);
-  result = m_active_tranxs->clear_active_tranx_nodes(NULL, 0);
+  m_active_tranxs->clear_active_tranx_nodes(NULL, 0);
 
   rpl_semi_sync_master_off_times++;
   m_wait_file_name_inited   = false;
   m_reply_file_name_inited  = false;
   sql_print_information("Semi-sync replication switched OFF.");
   cond_broadcast();                            /* wake up all waiting threads */
 
-  DBUG_RETURN(result);
+  DBUG_VOID_RETURN;
 }
 
 int Repl_semi_sync_master::try_switch_on(int server_id,

sql/semisync_master.h

@@ -343,11 +343,8 @@ class Active_tranx
    * position.
    * If log_file_name is NULL, everything will be cleared: the sorted
    * list and the hash table will be reset to empty.
-   * 
-   * Return:
-   *  0: success;  non-zero: error
    */
-  int clear_active_tranx_nodes(const char *log_file_name,
+  void clear_active_tranx_nodes(const char *log_file_name,
                                my_off_t    log_file_pos);
 
   /* Given a position, check to see whether the position is an active
@@ -449,7 +446,7 @@ class Repl_semi_sync_master
   }
 
   /* Switch semi-sync off because of timeout in transaction waiting. */
-  int switch_off();
+  void switch_off();
 
   /* Switch semi-sync on when slaves catch up. */
   int try_switch_on(int server_id,