MDEV-18579 Assertion !ctx->online || num_fts_index == 0

instant_alter_column_possible(): Add the other MDEV-17459 work-around
condition. The existence of fulltext indexes only prevents instant
DROP COLUMN or changing the order of columns. Other forms of instant
ALTER TABLE are no problem.

Before commit 4e7ee166 that merged
the MDEV-18295 fix from 10.3, the work-around of MDEV-17459 in
instant_alter_column_possible() was categorically refusing any
ALGORITHM=INSTANT if any FULLTEXT INDEX was present. After that commit,
a related condition was only present in prepare_inplace_alter_table_dict()
but not in the other callers of instant_alter_column_possible().

mysql-test/suite/innodb/r/instant_alter.result

@@ -734,6 +734,9 @@ INSERT INTO t1 VALUES (1,1);
 ALTER TABLE t1 ADD COLUMN f INT AFTER a;
 ALTER TABLE t1 DROP b, DROP f;
 DROP TABLE t1;
+CREATE TABLE t1(t TEXT NOT NULL, FULLTEXT(t)) ENGINE=InnoDB ROW_FORMAT=REDUNDANT;
+ALTER TABLE t1 MODIFY COLUMN t TEXT;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -1414,6 +1417,9 @@ INSERT INTO t1 VALUES (1,1);
 ALTER TABLE t1 ADD COLUMN f INT AFTER a;
 ALTER TABLE t1 DROP b, DROP f;
 DROP TABLE t1;
+CREATE TABLE t1(t TEXT NOT NULL, FULLTEXT(t)) ENGINE=InnoDB ROW_FORMAT=COMPACT;
+ALTER TABLE t1 MODIFY COLUMN t TEXT;
+DROP TABLE t1;
 CREATE TABLE t1
 (id INT PRIMARY KEY, c2 INT UNIQUE,
 c3 POINT NOT NULL DEFAULT ST_GeomFromText('POINT(3 4)'),
@@ -2094,10 +2100,13 @@ INSERT INTO t1 VALUES (1,1);
 ALTER TABLE t1 ADD COLUMN f INT AFTER a;
 ALTER TABLE t1 DROP b, DROP f;
 DROP TABLE t1;
+CREATE TABLE t1(t TEXT NOT NULL, FULLTEXT(t)) ENGINE=InnoDB ROW_FORMAT=DYNAMIC;
+ALTER TABLE t1 MODIFY COLUMN t TEXT;
+DROP TABLE t1;
 disconnect analyze;
 SELECT variable_value-@old_instant instants
 FROM information_schema.global_status
 WHERE variable_name = 'innodb_instant_alter_column';
 instants
-170
+171
 SET GLOBAL innodb_purge_rseg_truncate_frequency= @saved_frequency;

mysql-test/suite/innodb/t/instant_alter.test

@@ -633,6 +633,11 @@ ALTER TABLE t1 ADD COLUMN f INT AFTER a;
 ALTER TABLE t1 DROP b, DROP f;
 DROP TABLE t1;
 
+# MDEV-18579 Assertion !ctx->online || num_fts_index == 0
+eval CREATE TABLE t1(t TEXT NOT NULL, FULLTEXT(t)) $engine;
+ALTER TABLE t1 MODIFY COLUMN t TEXT;
+DROP TABLE t1;
+
 dec $format;
 }
 disconnect analyze;

storage/innobase/handler/handler0alter.cc

@@ -1472,7 +1472,8 @@ instant_alter_column_possible(
 	    & (ALTER_STORED_COLUMN_ORDER | ALTER_DROP_STORED_COLUMN
 	       | ALTER_ADD_STORED_BASE_COLUMN)) {
 #if 1 // MDEV-17459: adjust fts_fetch_doc_from_rec() and friends; remove this
-		if (ib_table.fts) return false;
+		if (ib_table.fts || innobase_fulltext_exist(altered_table))
+			return false;
 #endif
 #if 1 // MDEV-17468: fix bugs with indexed virtual columns & remove this
 		for (const dict_index_t* index = ib_table.indexes.start;
@@ -6408,11 +6409,7 @@ prepare_inplace_alter_table_dict(
 		    || !ctx->new_table->persistent_autoinc);
 
 	if (ctx->need_rebuild() && instant_alter_column_possible(
-		    *user_table, ha_alter_info, old_table, altered_table)
-#if 1 // MDEV-17459: adjust fts_fetch_doc_from_rec() and friends; remove this
-		&& !innobase_fulltext_exist(altered_table)
-#endif
-	    ) {
+		    *user_table, ha_alter_info, old_table, altered_table)) {
 		for (uint a = 0; a < ctx->num_to_add_index; a++) {
 			ctx->add_index[a]->table = ctx->new_table;
 			ctx->add_index[a] = dict_index_add_to_cache(