MDEV-21500 Server hang when using simulated AIO

The write-heavy test innodb_zip.wl6501_scale_1 timed out on
10.2 60d7011c for me.
Out of os_aio_n_segments=6, 5 are waiting for an event in
os_aio_simulated_handler(). One thread is waiting for a
write to complete in buf_dblwr_add_to_batch(), but that
would never happen, because nothing is waking up the simulated AIO
handler threads.

This hang appears to have been introduced in MySQL 5.6.12
in mysql/mysql-server@26cfde776cdf5ce61bd5cc494dfc1df28c76977f.

storage/innobase/buf/buf0dblwr.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -963,6 +963,7 @@ buf_dblwr_flush_buffered_writes(void)
 		ib_int64_t	sig_count = os_event_reset(buf_dblwr->b_event);
 		mutex_exit(&buf_dblwr->mutex);
 
+		os_aio_simulated_wake_handler_threads();
 		os_event_wait_low(buf_dblwr->b_event, sig_count);
 		goto try_again;
 	}
@@ -1096,6 +1097,7 @@ buf_dblwr_add_to_batch(
 		checkpoint. */
 		ib_int64_t	sig_count = os_event_reset(buf_dblwr->b_event);
 		mutex_exit(&buf_dblwr->mutex);
+		os_aio_simulated_wake_handler_threads();
 
 		os_event_wait_low(buf_dblwr->b_event, sig_count);
 		goto try_again;

storage/xtradb/buf/buf0dblwr.cc

 /*****************************************************************************
 
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All Rights Reserved.
-Copyright (c) 2013, 2019, MariaDB Corporation.
+Copyright (c) 2013, 2020, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -963,6 +963,7 @@ buf_dblwr_flush_buffered_writes(void)
 		ib_int64_t	sig_count = os_event_reset(buf_dblwr->b_event);
 		mutex_exit(&buf_dblwr->mutex);
 
+		os_aio_simulated_wake_handler_threads();
 		os_event_wait_low(buf_dblwr->b_event, sig_count);
 		goto try_again;
 	}
@@ -1112,6 +1113,7 @@ buf_dblwr_add_to_batch(
 		checkpoint. */
 		ib_int64_t	sig_count = os_event_reset(buf_dblwr->b_event);
 		mutex_exit(&buf_dblwr->mutex);
+		os_aio_simulated_wake_handler_threads();
 
 		os_event_wait_low(buf_dblwr->b_event, sig_count);
 		goto try_again;