MDEV-14154: Document some time_t fields better

srv_slot_t::suspend_time, os_aio_slot_t::reservation_time,
sync_cell_t::reservation_time: Explain what could happen
if the system time has is being adjusted.

fts_sync_t::start_time: Document that the field is mostly unused.

storage/innobase/include/fts0types.h

 /*****************************************************************************
 
 Copyright (c) 2007, 2016, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, 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
@@ -122,7 +123,8 @@ struct fts_sync_t {
 	doc_id_t	max_doc_id;	/*!< The doc id at which the cache was
 					noted as being full, we use this to
 					set the upper_limit field */
-	ib_time_t	start_time;	/*!< SYNC start time */
+	time_t		start_time;	/*!< SYNC start time; only used if
+					fts_enable_diag_print */
 	bool		in_progress;	/*!< flag whether sync is in progress.*/
 	bool		unlock_cache;	/*!< flag whether unlock cache when
 					write fts node */

storage/innobase/sync/sync0arr.cc

@@ -122,8 +122,10 @@ struct sync_cell_t {
 					has not been signalled in the
 					period between the reset and
 					wait call. */
-	time_t		reservation_time;/*!< time when the thread reserved
-					the wait cell */
+	/** time(NULL) when the wait cell was reserved.
+	FIXME: sync_array_print_long_waits_low() may display bogus
+	warnings when the system time is adjusted to the past! */
+	time_t		reservation_time;
 };
 
 /* NOTE: It is allowed for a thread to wait

storage/xtradb/include/fts0types.h

 /*****************************************************************************
 
 Copyright (c) 2007, 2016, Oracle and/or its affiliates. All Rights Reserved.
+Copyright (c) 2019, 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
@@ -122,7 +123,8 @@ struct fts_sync_t {
 	doc_id_t	max_doc_id;	/*!< The doc id at which the cache was
 					noted as being full, we use this to
 					set the upper_limit field */
-	ib_time_t	start_time;	/*!< SYNC start time */
+	time_t		start_time;	/*!< SYNC start time; only used if
+					fts_enable_diag_print */
 	bool		in_progress;	/*!< flag whether sync is in progress.*/
 	bool		unlock_cache;	/*!< flag whether unlock cache when
 					write fts node */

storage/xtradb/include/srv0srv.h

@@ -3,7 +3,7 @@
 Copyright (c) 1995, 2017, Oracle and/or its affiliates. All rights reserved.
 Copyright (c) 2008, 2009, Google Inc.
 Copyright (c) 2009, Percona Inc.
-Copyright (c) 2013, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2019, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -1329,10 +1329,11 @@ struct srv_slot_t{
 	ibool		suspended;		/*!< TRUE if the thread is
 						waiting for the event of this
 						slot */
-	ib_time_t	suspend_time;		/*!< time when the thread was
-						suspended. Initialized by
-						lock_wait_table_reserve_slot()
-						for lock wait */
+	/** time(NULL) when the thread was suspended.
+	FIXME: Use my_interval_timer() or similar, to avoid bogus
+	timeouts in lock_wait_check_and_cancel() or lock_wait_suspend_thread()
+	when the system time is adjusted to the past! */
+	time_t		suspend_time;
 	ulong		wait_timeout;		/*!< wait time that if exceeded
 						the thread will be timed out.
 						Initialized by

storage/xtradb/os/os0file.cc

@@ -214,7 +214,10 @@ struct os_aio_slot_t{
 	ulint		pos;		/*!< index of the slot in the aio
 					array */
 	ibool		reserved;	/*!< TRUE if this slot is reserved */
-	time_t		reservation_time;/*!< time when reserved */
+	/** time(NULL) when reserved.
+	FIXME: os_aio_simulated_handle() may malfunction if
+	the system time is adjusted! */
+	time_t		reservation_time;
 	ulint		len;		/*!< length of the block to read or
 					write */
 	byte*		buf;		/*!< buffer used in i/o */

storage/xtradb/sync/sync0arr.cc

@@ -123,8 +123,10 @@ struct sync_cell_t {
 					has not been signalled in the
 					period between the reset and
 					wait call. */
-	time_t		reservation_time;/*!< time when the thread reserved
-					the wait cell */
+	/** time(NULL) when the wait cell was reserved.
+	FIXME: sync_array_print_long_waits_low() may display bogus
+	warnings when the system time is adjusted to the past! */
+	time_t		reservation_time;
 };
 
 /* NOTE: It is allowed for a thread to wait