MDEV-8212 alter table - failing to ADD PRIMARY KEY IF NOT EXISTS when existing...

MDEV-8212 alter table - failing to ADD PRIMARY KEY IF NOT EXISTS when existing index of same as column name.
        The default name for the primary key is rather 'PRIMARY' instead of the indexed column name.

mysql-test/r/alter_table.result

@@ -1488,6 +1488,17 @@ t2	CREATE TABLE `t2` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t2;
 DROP TABLE t1;
+CREATE TABLE t1 (
+`transaction_id` int(11) NOT NULL DEFAULT '0', 
+KEY `transaction_id` (`transaction_id`));
+ALTER TABLE t1 DROP KEY IF EXISTS transaction_id, ADD PRIMARY KEY IF NOT EXISTS (transaction_id);
+SHOW CREATE TABLE t1;
+Table	Create Table
+t1	CREATE TABLE `t1` (
+  `transaction_id` int(11) NOT NULL DEFAULT '0',
+  PRIMARY KEY (`transaction_id`)
+) ENGINE=MyISAM DEFAULT CHARSET=latin1
+DROP TABLE t1;
 # Bug#11748057 (formerly known as 34972): ALTER TABLE statement doesn't
 #                                         identify correct column name.
 #

mysql-test/t/alter_table.test

@@ -1326,6 +1326,14 @@ SHOW CREATE TABLE t2;
 DROP TABLE t2;
 DROP TABLE t1;
 
+CREATE TABLE t1 (
+    `transaction_id` int(11) NOT NULL DEFAULT '0', 
+    KEY `transaction_id` (`transaction_id`));
+ALTER TABLE t1 DROP KEY IF EXISTS transaction_id, ADD PRIMARY KEY IF NOT EXISTS (transaction_id);
+SHOW CREATE TABLE t1;
+
+DROP TABLE t1;
+
 --echo # Bug#11748057 (formerly known as 34972): ALTER TABLE statement doesn't
 --echo #                                         identify correct column name.
 --echo #

sql/sql_table.cc

@@ -5859,12 +5859,17 @@ drop_create_field:
       /* let us check the name of the first key part. */
       if ((keyname= key->name.str) == NULL)
       {
-        List_iterator<Key_part_spec> part_it(key->columns);
-        Key_part_spec *kp;
-        if ((kp= part_it++))
-          keyname= kp->field_name.str;
-        if (keyname == NULL)
-          continue;
+        if (key->type == Key::PRIMARY)
+          keyname= primary_key_name;
+        else
+        {
+          List_iterator<Key_part_spec> part_it(key->columns);
+          Key_part_spec *kp;
+          if ((kp= part_it++))
+            keyname= kp->field_name.str;
+          if (keyname == NULL)
+            continue;
+        }
       }
       if (key->type != Key::FOREIGN_KEY)
       {