MDEV-18281 COM_RESET_CONNECTION changes the connection encoding

Store original charset during client authentication, and restore it for
COM_RESET_CONNECTION

client/mysqltest.cc

@@ -5938,6 +5938,7 @@ void do_connect(struct st_command *command)
   int read_timeout= 0;
   int write_timeout= 0;
   int connect_timeout= 0;
+  char *csname=0;
   struct st_connection* con_slot;
 
   static DYNAMIC_STRING ds_connection_name;
@@ -6049,6 +6050,11 @@ void do_connect(struct st_command *command)
     {
       connect_timeout= atoi(con_options + sizeof("connect_timeout=")-1);
     }
+    else if (strncasecmp(con_options, "CHARSET=",
+      sizeof("CHARSET=") - 1) == 0)
+    {
+      csname= strdup(con_options + sizeof("CHARSET=") - 1);
+    }
     else
       die("Illegal option to connect: %.*s", 
           (int) (end - con_options), con_options);
@@ -6086,7 +6092,7 @@ void do_connect(struct st_command *command)
     mysql_options(con_slot->mysql, MYSQL_OPT_COMPRESS, NullS);
   mysql_options(con_slot->mysql, MYSQL_OPT_LOCAL_INFILE, 0);
   mysql_options(con_slot->mysql, MYSQL_SET_CHARSET_NAME,
-                charset_info->csname);
+                csname?csname: charset_info->csname);
   if (opt_charsets_dir)
     mysql_options(con_slot->mysql, MYSQL_SET_CHARSET_DIR,
                   opt_charsets_dir);
@@ -6197,6 +6203,7 @@ void do_connect(struct st_command *command)
 #ifdef HAVE_SMEM
   dynstr_free(&ds_shm);
 #endif
+  free(csname);
   DBUG_VOID_RETURN;
 }
 

mysql-test/r/reset_connection.result

@@ -5,3 +5,23 @@ Com_select	10
 SHOW local STATUS LIKE 'com_select';
 Variable_name	Value
 Com_select	0
+# Test if charset changes after reset (utf8)
+connect utf8_conn,localhost,root,,,,,CHARSET=utf8;
+connection utf8_conn;
+SELECT IF(@@character_set_client='utf8','OK', 'FAIL') AS RESULT;
+RESULT
+OK
+SELECT IF(@@character_set_client='utf8','OK', 'FAIL') AS RESULT;
+RESULT
+OK
+disconnect utf8_conn;
+# Test if charset changes after reset (latin1)
+connect latin1_conn,localhost,root,,,,,CHARSET=latin1;
+connection latin1_conn;
+SELECT IF(@@character_set_client='latin1','OK', 'FAIL') AS RESULT;
+RESULT
+OK
+SELECT IF(@@character_set_client='latin1','OK', 'FAIL') AS RESULT;
+RESULT
+OK
+disconnect latin1_conn;

mysql-test/t/reset_connection.test

@@ -23,3 +23,18 @@ SHOW local STATUS LIKE 'com_select';
 
 SHOW local STATUS LIKE 'com_select';
 
+--echo # Test if charset changes after reset (utf8)
+connect(utf8_conn,localhost,root,,,,,CHARSET=utf8);
+connection utf8_conn;
+SELECT IF(@@character_set_client='utf8','OK', 'FAIL') AS RESULT;
+--reset_connection
+SELECT IF(@@character_set_client='utf8','OK', 'FAIL') AS RESULT;
+disconnect utf8_conn;
+
+--echo # Test if charset changes after reset (latin1)
+connect(latin1_conn,localhost,root,,,,,CHARSET=latin1);
+connection latin1_conn;
+SELECT IF(@@character_set_client='latin1','OK', 'FAIL') AS RESULT;
+--reset_connection
+SELECT IF(@@character_set_client='latin1','OK', 'FAIL') AS RESULT;
+disconnect latin1_conn;
\ No newline at end of file

sql/sql_class.cc

@@ -840,6 +840,7 @@ THD::THD(my_thread_id id, bool is_wsrep_applier)
   prepare_derived_at_open= FALSE;
   create_tmp_table_for_derived= FALSE;
   save_prep_leaf_list= FALSE;
+  org_charset= 0;
   /* Restore THR_THD */
   set_current_thd(old_THR_THD);
   inc_thread_count();

sql/sql_class.h

@@ -2312,6 +2312,9 @@ class THD :public Statement,
   uint dbug_sentry; // watch out for memory corruption
 #endif
   struct st_my_thread_var *mysys_var;
+
+  /* Original charset number from the first client packet, or COM_CHANGE_USER*/
+  CHARSET_INFO *org_charset;
 private:
   /*
     Type of current query: COM_STMT_PREPARE, COM_QUERY, etc. Set from

sql/sql_connect.cc

@@ -793,6 +793,7 @@ bool thd_init_client_charset(THD *thd, uint cs_number)
                cs->csname);
       return true;
     }
+    thd->org_charset= cs;
     thd->update_charset(cs,cs,cs);
   }
   return false;

sql/sql_parse.cc

@@ -1689,6 +1689,9 @@ bool dispatch_command(enum enum_server_command command, THD *thd,
     thd->status_var.com_other++;
     thd->change_user();
     thd->clear_error();                         // if errors from rollback
+    /* Restore original charset from client authentication packet.*/
+    if(thd->org_charset)
+      thd->update_charset(thd->org_charset,thd->org_charset,thd->org_charset);
     my_ok(thd, 0, 0, 0);
     break;
   }