MDEV-28912 NON-UNIQUE FTS_DOC_ID index mistaken as FTS_DOC_ID_INDEX

- InnoDB mistakenly identifies the non-unique FTS_DOC_ID index as
FTS_DOC_ID_INDEX while loading the table. dict_load_indexes()
should check whether the index is unique before assigning
fts_doc_id_index

mysql-test/suite/innodb_fts/r/fulltext2.result

@@ -271,3 +271,10 @@ fts_doc_id	first_name	last_name	score
 6	Ned	Flanders	0
 7	Nelson	Muntz	0
 DROP TABLE t1;
+CREATE TABLE t1(a INT, b TEXT, FTS_DOC_ID BIGINT UNSIGNED NOT NULL,
+KEY FTS_DOC_ID_INDEX(FTS_DOC_ID))ENGINE=InnoDB;
+ALTER TABLE t1 ADD COLUMN c INT as (a) VIRTUAL;
+ALTER TABLE t1 ADD d INT NULL;
+ALTER TABLE t1 ADD FULLTEXT(b);
+ERROR HY000: Index 'FTS_DOC_ID_INDEX' is of wrong type for an InnoDB FULLTEXT index
+DROP TABLE t1;

mysql-test/suite/innodb_fts/t/fulltext2.test

@@ -257,3 +257,14 @@ INSERT INTO t1 (id, first_name, last_name) VALUES
 analyze table t1;
 SELECT fts_doc_id, first_name, last_name, MATCH(first_name) AGAINST('Homer' IN BOOLEAN MODE) AS score FROM t1;
 DROP TABLE t1;
+
+#
+# MDEV-28912  NON-UNIQUE FTS_DOC_ID mistaken as FTS_DOC_ID_INDEX
+#
+CREATE TABLE t1(a INT, b TEXT, FTS_DOC_ID BIGINT UNSIGNED NOT NULL,
+		KEY FTS_DOC_ID_INDEX(FTS_DOC_ID))ENGINE=InnoDB;
+ALTER TABLE t1 ADD COLUMN c INT as (a) VIRTUAL;
+ALTER TABLE t1 ADD d INT NULL;
+--error ER_INNODB_FT_WRONG_DOCID_INDEX
+ALTER TABLE t1 ADD FULLTEXT(b);
+DROP TABLE t1;

storage/innobase/dict/dict0load.cc

@@ -2590,8 +2590,11 @@ dict_load_indexes(
 	ut_ad(table->fts_doc_id_index == NULL);
 
 	if (table->fts != NULL) {
-		table->fts_doc_id_index = dict_table_get_index_on_name(
+		dict_index_t *idx = dict_table_get_index_on_name(
 			table, FTS_DOC_ID_INDEX_NAME);
+		if (idx && dict_index_is_unique(idx)) {
+			table->fts_doc_id_index = idx;
+		}
 	}
 
 	/* If the table contains FTS indexes, populate table->fts->indexes */