BUG#43406 CHANGE MASTER TO should log previous state in error log

      
When issuing a 'CHANGE MASTER TO' statement, key elements of the previous
state, namely the host, port, the master_log_file and the master_log_pos
are dumped into the error log.

sql/sql_repl.cc

@@ -1134,6 +1134,10 @@ bool change_master(THD* thd, Master_info* mi)
   int thread_mask;
   const char* errmsg= 0;
   bool need_relay_log_purge= 1;
+  char saved_host[HOSTNAME_LENGTH + 1];
+  uint saved_port;
+  char saved_log_name[FN_REFLEN];
+  my_off_t saved_log_pos;
   DBUG_ENTER("change_master");
 
   lock_slave_threads(mi);
@@ -1162,6 +1166,17 @@ bool change_master(THD* thd, Master_info* mi)
     could possibly modify the data structures from running
   */
 
+  /*
+    Before processing the command, save the previous state.
+  */
+  char *pos;
+  pos= strmake(saved_host, mi->host, HOSTNAME_LENGTH);
+  pos= '\0';
+  saved_port= mi->port;
+  pos= strmake(saved_log_name, mi->master_log_name, FN_REFLEN - 1);
+  pos= '\0';
+  saved_log_pos= mi->master_log_pos;
+
   /*
     If the user specified host or port without binlog or position,
     reset binlog's name to FIRST and position to 4.
@@ -1325,6 +1340,15 @@ bool change_master(THD* thd, Master_info* mi)
   /* Clear the errors, for a clean start */
   mi->rli.clear_error();
   mi->rli.clear_until_condition();
+
+  sql_print_information("'CHANGE MASTER TO executed'. "
+    "Previous state master_host='%s', master_port='%u', master_log_file='%s', "
+    "master_log_pos='%ld'. "
+    "New state master_host='%s', master_port='%u', master_log_file='%s', "
+    "master_log_pos='%ld'.", saved_host, saved_port, saved_log_name,
+    (ulong) saved_log_pos, mi->host, mi->port, mi->master_log_name,
+    (ulong) mi->master_log_pos);
+
   /*
     If we don't write new coordinates to disk now, then old will remain in
     relay-log.info until START SLAVE is issued; but if mysqld is shutdown