MDEV-25989 Crash (or hang) on startup after restoring backup

If an ALTER TABLE that affected a table with FULLTEXT INDEX was
in progress while a backup was made, InnoDB would crash or hang
during the first startup after restoring the backup,
while trying to drop the #sql-alter- table for the DDL operation.

drop_garbage_tables_after_restore(): Invoke dict_sys.unlock()
before invoking the FTS functions. Also, invoke purge_sys.stop_FTS()
in debug builds to silence debug assertions. (Purge is not yet
running at this point.)

mysql-test/suite/mariabackup/alter_copy_excluded.result

 # xtrabackup backup
-CREATE TABLE t1(i int) ENGINE=InnoDB;
-INSERT into t1 values(1);
+CREATE TABLE t1(i int, t text, fulltext index(t)) ENGINE=InnoDB;
+INSERT into t1 values(1,'foo');
 connect  con2, localhost, root,,;
 connection con2;
 SET debug_sync='copy_data_between_tables_before_reset_backup_lock SIGNAL go  WAIT_FOR after_backup_stage_block_commit' ;
-SET debug_sync='now WAIT_FOR after_backup_stage_start';ALTER TABLE test.t1 FORCE, algorithm=COPY;|
+SET debug_sync='now WAIT_FOR after_backup_stage_start';ALTER TABLE test.t1 DROP t, algorithm=COPY;|
 connection default;
 connection con2;
 SET debug_sync='RESET';
@@ -15,11 +15,10 @@ connection default;
 # remove datadir
 # xtrabackup move back
 # restart
-SELECT COUNT(*) AS expect_0 FROM INFORMATION_SCHEMA.innodb_sys_tablespaces WHERE name like '%/#sql%';
-expect_0
-0
 SELECT * FROM t1;
-i
-1
+i	t
+1	foo
 DROP TABLE t1;
+SELECT * FROM INFORMATION_SCHEMA.innodb_sys_tablespaces WHERE name like '%/#sql%' or name like 'test/%';
+SPACE	NAME	FLAG	ROW_FORMAT	PAGE_SIZE	FILENAME	FS_BLOCK_SIZE	FILE_SIZE	ALLOCATED_SIZE
 # restart

mysql-test/suite/mariabackup/alter_copy_excluded.test

@@ -7,14 +7,14 @@
 echo # xtrabackup backup;
 let $targetdir=$MYSQLTEST_VARDIR/tmp/backup;
 
-CREATE TABLE t1(i int) ENGINE=InnoDB;
-INSERT into t1 values(1);
+CREATE TABLE t1(i int, t text, fulltext index(t)) ENGINE=InnoDB;
+INSERT into t1 values(1,'foo');
 
 connect  con2, localhost, root,,;
 connection con2;
 SET debug_sync='copy_data_between_tables_before_reset_backup_lock SIGNAL go  WAIT_FOR after_backup_stage_block_commit' ;
 DELIMITER |;
-send  SET debug_sync='now WAIT_FOR after_backup_stage_start';ALTER TABLE test.t1 FORCE, algorithm=COPY;|
+send  SET debug_sync='now WAIT_FOR after_backup_stage_start';ALTER TABLE test.t1 DROP t, algorithm=COPY;|
 DELIMITER ;|
 connection default;
 
@@ -53,9 +53,9 @@ exec $XTRABACKUP  --prepare --target-dir=$targetdir;
 --enable_result_log
 
 # Check there are no temp tablespaces in sys_tablespaces, after backup
-SELECT COUNT(*) AS expect_0 FROM INFORMATION_SCHEMA.innodb_sys_tablespaces WHERE name like '%/#sql%';
 SELECT * FROM t1;
 DROP TABLE t1;
+SELECT * FROM INFORMATION_SCHEMA.innodb_sys_tablespaces WHERE name like '%/#sql%' or name like 'test/%';
 
 # Restart once again to clear first_start_after_backup flag
 # This is to catch potential warnings, since "missing file" for #sql is suppressed

storage/innobase/handler/ha_innodb.cc

@@ -1969,6 +1969,9 @@ static void drop_garbage_tables_after_restore()
   mtr_t mtr;
   trx_t *trx= trx_create();
 
+  ut_ad(!purge_sys.enabled());
+  ut_d(purge_sys.stop_FTS());
+
   mtr.start();
   btr_pcur_open_at_index_side(true, dict_sys.sys_tables->indexes.start,
                               BTR_SEARCH_LEAF, &pcur, true, 0, &mtr);
@@ -2028,8 +2031,10 @@ static void drop_garbage_tables_after_restore()
       if (err == DB_SUCCESS &&
           (table->flags2 & (DICT_TF2_FTS_HAS_DOC_ID | DICT_TF2_FTS)))
       {
+        dict_sys.unlock();
         fts_optimize_remove_table(table);
         err= fts_lock_tables(trx, *table);
+        dict_sys.lock(SRW_LOCK_CALL);
       }
       table->release();
 
@@ -2058,6 +2063,7 @@ static void drop_garbage_tables_after_restore()
   btr_pcur_close(&pcur);
   mtr.commit();
   trx->free();
+  ut_d(purge_sys.resume_FTS());
 }
 
 static void innodb_ddl_recovery_done(handlerton*)