MDEV-6085 ALTER TABLE looses the connection string

sql/sql_table.cc

@@ -7239,6 +7239,9 @@ mysql_prepare_alter_table(THD *thd, TABLE *table,
   if (!(used_fields & HA_CREATE_USED_TRANSACTIONAL))
     create_info->transactional= table->s->transactional;
 
+  if (!(used_fields & HA_CREATE_USED_CONNECTION))
+    create_info->connect_string= table->s->connect_string;
+
   restore_record(table, s->default_values);     // Empty record for DEFAULT
 
   if ((create_info->fields_option_struct= (ha_field_option_struct**)

storage/connect/mysql-test/connect/r/mysql.result

@@ -273,3 +273,26 @@ a	b	c	d	e
 255	65535	4294967295	18446744073709551615	Hello
 DROP TABLE t2;
 DROP TABLE t1;
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (10),(20),(30);
+CREATE TABLE t2 ENGINE=CONNECT TABLE_TYPE=MYSQL CONNECTION='mysql://root@localhost:PORT/test/t1';
+SELECT * FROM t2;
+a
+10
+20
+30
+ALTER TABLE t2 MODIFY a TINYINT;
+Warnings:
+Warning	1105	This is an outward table, table data were not modified.
+SHOW CREATE TABLE t2;
+Table	Create Table
+t2	CREATE TABLE `t2` (
+  `a` tinyint(4) DEFAULT NULL
+) ENGINE=CONNECT DEFAULT CHARSET=latin1 CONNECTION='mysql://root@localhost:PORT/test/t1' `TABLE_TYPE`='MYSQL'
+SELECT * FROM t2;
+a
+10
+20
+30
+DROP TABLE t2;
+DROP TABLE t1;

storage/connect/mysql-test/connect/t/mysql.test

@@ -454,3 +454,19 @@ SELECT * FROM t2;
 
 DROP TABLE t2;
 DROP TABLE t1;
+
+#
+# MDEV-6085 ALTER TABLE looses the connection string
+#
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (10),(20),(30);
+--replace_result $PORT PORT
+--eval CREATE TABLE t2 ENGINE=CONNECT TABLE_TYPE=MYSQL CONNECTION='mysql://root@localhost:$PORT/test/t1'
+SELECT * FROM t2;
+ALTER TABLE t2 MODIFY a TINYINT;
+--replace_result $PORT PORT
+SHOW CREATE TABLE t2;
+SELECT * FROM t2;
+DROP TABLE t2;
+DROP TABLE t1;
+