[#MDEV-27017] Assertion failure 'table->get_ref_count() == 0' on DDL that involves FULLTEXT INDEX

~~MDEV-25506~~ introduced a race condition that was inadequately fixed by ~~MDEV-25936~~.
Here is an example stack trace:

10.6
#2 0x000056397eb3ce0b in ut_dbg_assertion_failed (
expr=0x56397ff9f440 "table->get_ref_count() == 0",
file=0x56397ff9f700 "/data/Server/bb-10.6-markoO/storage/innobase/dict/dict0dict.cc", line=1824)
at /data/Server/bb-10.6-markoO/storage/innobase/ut/ut0dbg.cc:60
#3 0x000056397ec887aa in dict_sys_t::remove (this=0x563980edd580 <dict_sys>,
table=0x618000274908, lru=false, keep=false)
at /data/Server/bb-10.6-markoO/storage/innobase/dict/dict0dict.cc:1824
#4 0x000056397ed045b3 in trx_t::commit (this=0x4f4764645068,
deleted=std::vector of length 9, capacity 16 = {...})
at /data/Server/bb-10.6-markoO/storage/innobase/dict/drop.cc:249
#5 0x000056397e66b546 in ha_innobase::delete_table (this=0x62b000217be0,
name=0x1aa33317c300 "./test/#sql-backup-68681-24")
at /data/Server/bb-10.6-markoO/storage/innobase/handler/ha_innodb.cc:13626

The assertion would fail on an internal table whose name starts with FTS_. The reason is that the call purge_sys.stop_FTS() does not actually wait that purge operations for all the FTS tables (related to the current operation) have ceased.

[MDEV-27017] Assertion failure 'table->get_ref_count() == 0' on DDL that involves FULLTEXT INDEX Created: 2021-11-10 Updated: 2022-01-27 Resolved: 2022-01-05
Status:	Closed
Project:	MariaDB Server
Component/s:	Full-text Search, Storage Engine - InnoDB
Affects Version/s:	10.6.3, 10.6.4, 10.6.5, 10.7.1
Fix Version/s:	10.6.6, 10.7.2

[MDEV-27017] Assertion failure 'table->get_ref_count() == 0' on DDL that involves FULLTEXT INDEX Created: 2021-11-10 Updated: 2022-01-27 Resolved: 2022-01-05