MDEV-32017 Auto-increment no longer works for explicit FTS_DOC_ID

- InnoDB should avoid the sync commit operation when
there is nothing in fulltext cache. This is caused by
commit 1248fe72 (MDEV-27582)

mysql-test/suite/innodb_fts/r/innodb-fts-ddl.result

@@ -318,3 +318,16 @@ DROP TABLE t1;
 disconnect con1;
 SET GLOBAL innodb_optimize_fulltext_only=OFF;
 SET GLOBAL innodb_ft_aux_table = default;
+#
+#  MDEV-32017  Auto-increment no longer works for
+#		 explicit FTS_DOC_ID
+#
+CREATE TABLE t (
+FTS_DOC_ID BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+f1 char(255), f2 char(255), f3 char(255), fulltext key (f3)
+) ENGINE=InnoDB;
+INSERT INTO t (f1,f2,f3) VALUES ('foo','bar','baz');
+ALTER TABLE t ADD FULLTEXT INDEX ft1(f1);
+ALTER TABLE t ADD FULLTEXT INDEX ft2(f2);
+INSERT INTO t (f1,f2,f3) VALUES ('bar','baz','qux');
+DROP TABLE t;

mysql-test/suite/innodb_fts/t/innodb-fts-ddl.test

@@ -397,3 +397,17 @@ DROP TABLE t1;
 disconnect con1;
 SET GLOBAL innodb_optimize_fulltext_only=OFF;
 SET GLOBAL innodb_ft_aux_table = default;
+
+--echo #
+--echo #  MDEV-32017  Auto-increment no longer works for
+--echo #		 explicit FTS_DOC_ID
+--echo #
+CREATE TABLE t (
+  FTS_DOC_ID BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
+  f1 char(255), f2 char(255), f3 char(255), fulltext key (f3)
+) ENGINE=InnoDB;
+INSERT INTO t (f1,f2,f3) VALUES ('foo','bar','baz');
+ALTER TABLE t ADD FULLTEXT INDEX ft1(f1);
+ALTER TABLE t ADD FULLTEXT INDEX ft2(f2);
+INSERT INTO t (f1,f2,f3) VALUES ('bar','baz','qux');
+DROP TABLE t;

storage/innobase/fts/fts0fts.cc

@@ -4304,6 +4304,11 @@ fts_sync(
 	size_t		fts_cache_size= 0;
 	rw_lock_x_lock(&cache->lock);
 
+	if (cache->total_size == 0) {
+		rw_lock_x_unlock(&cache->lock);
+		return DB_SUCCESS;
+	}
+
 	/* Check if cache is being synced.
 	Note: we release cache lock in fts_sync_write_words() to
 	avoid long wait for the lock by other threads. */