MDEV-15554 InnoDB page_cleaner shutdown sometimes hangs

buf_flush_page_cleaner_coordinator(): Signal the worker threads
to exit while waiting for them to exit. Apparently, signals are
sometimes lost, causing shutdown to occasionally hang when
multiple page cleaners (and buffer pool instances) are used,
that is, when innodb_buffer_pool_size is at least 1 GiB.

buf_flush_page_cleaner_close(): Merge with the only caller.

storage/innobase/buf/buf0flu.cc

@@ -2718,25 +2718,6 @@ buf_flush_page_cleaner_init(void)
 	page_cleaner.is_running = true;
 }
 
-/**
-Close page_cleaner. */
-static
-void
-buf_flush_page_cleaner_close(void)
-{
-	ut_ad(!page_cleaner.is_running);
-
-	/* waiting for all worker threads exit */
-	while (page_cleaner.n_workers) {
-		os_thread_sleep(10000);
-	}
-
-	mutex_destroy(&page_cleaner.mutex);
-
-	os_event_destroy(page_cleaner.is_finished);
-	os_event_destroy(page_cleaner.is_requested);
-}
-
 /**
 Requests for all slots to flush all buffer pool instances.
 @param min_n	wished minimum mumber of blocks flushed
@@ -3438,9 +3419,17 @@ DECLARE_THREAD(buf_flush_page_cleaner_coordinator)(void*)
 	and no more access to page_cleaner structure by them.
 	Wakes worker threads up just to make them exit. */
 	page_cleaner.is_running = false;
+
+	/* waiting for all worker threads exit */
+	while (page_cleaner.n_workers) {
 		os_event_set(page_cleaner.is_requested);
+		os_thread_sleep(10000);
+	}
+
+	mutex_destroy(&page_cleaner.mutex);
 
-	buf_flush_page_cleaner_close();
+	os_event_destroy(page_cleaner.is_finished);
+	os_event_destroy(page_cleaner.is_requested);
 
 	buf_page_cleaner_is_active = false;