MDEV-23026 purge fails with assert !rw_lock_own_flagged(lock, RW_LOCK…

…_FLAG_X | RW_LOCK_FLAG_S)

InnoDB purge thread locks the root page of clustered index
while accessing the undo log records and later same thread
tries to open the table, initialize statistics and tries
to lock the clustered index root page while doing virtual
column computation.

Solution:
=========
InnoDB should prevent statistics initialization when the
table is being opened by purge thread

storage/innobase/handler/ha_innodb.cc

@@ -6261,8 +6261,10 @@ ha_innobase::open(const char* name, int, uint)
 
 	innobase_copy_frm_flags_from_table_share(ib_table, table->s);
 
-	/* No point to init any statistics if tablespace is still encrypted. */
-	if (ib_table->is_readable()) {
+	/* No point to init any statistics if tablespace is still encrypted
+	or if table is being opened by background thread */
+	if (THDVAR(thd, background_thread)) {
+	} else if (ib_table->is_readable()) {
 		dict_stats_init(ib_table);
 	} else {
 		ib_table->stat_initialized = 1;