MDEV-30528 Assertion in dtype_get_at_most_n_mbchars

1. Exclude merging history rows into fts index.

The check !history_fts && (index->type & DICT_FTS) was just incorrect
attempt to avoid history in fts index.

2. Don't check for duplicates for history rows.

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

@@ -1393,3 +1393,26 @@ INSERT INTO t1 VALUES(repeat("this is the test case", 500));
 ALTER TABLE t1 KEY_BLOCK_SIZE=4;
 ALTER TABLE t1 KEY_BLOCK_SIZE=0;
 DROP TABLE t1;
+#
+# MDEV-30528 Assertion in dtype_get_at_most_n_mbchars
+#
+create table t (f text) with system versioning character set utf8 engine=innodb;
+insert into t (f) values
+('mysql from tutorial dbms stands for database ...')  ,
+('when to use mysql well after that you went through a ...'),
+('where will optimizing mysql in what tutorial we will show ...'),
+('1001 mysql tricks 1. never run mysqld as root. 2. ...'),
+('mysql vs. yoursql in the following database comparison ...'),
+('mysql security when configured properly, mysql ...');
+delete from t where f like 'mysql%';
+alter table t add fulltext (f);
+select * from t where match(f) against ("use");
+f
+when to use mysql well after that you went through a ...
+select * from t where match(f) against ("run");
+f
+1001 mysql tricks 1. never run mysqld as root. 2. ...
+select * from t where match(f) against ("tutorial");
+f
+where will optimizing mysql in what tutorial we will show ...
+drop table t;

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

@@ -1341,3 +1341,21 @@ ALTER TABLE t1 KEY_BLOCK_SIZE=4;
 ALTER TABLE t1 KEY_BLOCK_SIZE=0;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-30528 Assertion in dtype_get_at_most_n_mbchars
+--echo #
+create table t (f text) with system versioning character set utf8 engine=innodb;
+insert into t (f) values
+        ('mysql from tutorial dbms stands for database ...')  ,
+        ('when to use mysql well after that you went through a ...'),
+        ('where will optimizing mysql in what tutorial we will show ...'),
+        ('1001 mysql tricks 1. never run mysqld as root. 2. ...'),
+        ('mysql vs. yoursql in the following database comparison ...'),
+        ('mysql security when configured properly, mysql ...');
+delete from t where f like 'mysql%';
+alter table t add fulltext (f);
+select * from t where match(f) against ("use");
+select * from t where match(f) against ("run");
+select * from t where match(f) against ("tutorial");
+# cleanup
+drop table t;

storage/innobase/row/row0merge.cc

@@ -502,7 +502,8 @@ row_merge_buf_add(
 	VCOL_STORAGE		vcol_storage;
 	DBUG_ENTER("row_merge_buf_add");
 
-	if (buf->n_tuples >= buf->max_tuples) {
+	if (buf->n_tuples >= buf->max_tuples
+	    || (history_fts && (buf->index->type & DICT_FTS))) {
 error:
 		n_row_added = 0;
 		goto end;
@@ -595,7 +596,8 @@ row_merge_buf_add(
 
 
 			/* Tokenize and process data for FTS */
-			if (!history_fts && (index->type & DICT_FTS)) {
+			if (index->type & DICT_FTS) {
+				ut_ad(!history_fts);
 				fts_doc_item_t*	doc_item;
 				byte*		value;
 				void*		ptr;
@@ -1876,6 +1878,7 @@ row_merge_read_clustered_index(
 	mach_write_to_8(new_sys_trx_start, trx->id);
 	mach_write_to_8(new_sys_trx_end, TRX_ID_MAX);
 	uint64_t	n_rows = 0;
+	bool history_row = false;
 
 	/* Scan the clustered index. */
 	for (;;) {
@@ -1892,7 +1895,7 @@ row_merge_read_clustered_index(
 		dtuple_t*	row;
 		row_ext_t*	ext;
 		page_cur_t*	cur	= btr_pcur_get_page_cur(&pcur);
-		bool history_row, history_fts = false;
+		bool history_fts = false;
 
 		page_cur_move_to_next(cur);
 
@@ -2527,7 +2530,8 @@ row_merge_read_clustered_index(
 						ut_ad(i == 0);
 						break;
 					}
-				} else if (dict_index_is_unique(buf->index)) {
+				} else if (!history_row
+					   && dict_index_is_unique(buf->index)) {
 					row_merge_dup_t	dup = {
 						buf->index, table, col_map, 0};