Cleanup: Replace log_sys.n_pending_checkpoint_writes with a Boolean

Only one checkpoint may be in progress at a time.
The counter log_sys.n_pending_checkpoint_writes
was being protected by log_sys.mutex.
Let us replace it with the Boolean log_sys.checkpoint_pending.

storage/innobase/buf/buf0flu.cc

@@ -1820,7 +1820,7 @@ static bool log_checkpoint_low(lsn_t oldest_lsn, lsn_t end_lsn)
 
   ut_ad(log_sys.get_flushed_lsn() >= flush_lsn);
 
-  if (log_sys.n_pending_checkpoint_writes)
+  if (log_sys.checkpoint_pending)
   {
     /* A checkpoint write is running */
     mysql_mutex_unlock(&log_sys.mutex);

storage/innobase/include/log0log.h

@@ -595,11 +595,10 @@ struct log_t{
 					/*!< next checkpoint number */
   /** latest completed checkpoint (protected by log_sys.mutex) */
   Atomic_relaxed<lsn_t> last_checkpoint_lsn;
+  /** next checkpoint LSN (protected by log_sys.mutex) */
   lsn_t next_checkpoint_lsn;
-					/*!< next checkpoint lsn */
-	ulint		n_pending_checkpoint_writes;
-					/*!< number of currently pending
-					checkpoint writes */
+  /** whether a checkpoint is pending */
+  Atomic_relaxed<bool> checkpoint_pending;
 
   /** buffer for checkpoint header */
   byte *checkpoint_buf;

storage/innobase/log/log0log.cc

@@ -211,7 +211,7 @@ void log_t::create()
   max_checkpoint_age= 0;
   next_checkpoint_no= 0;
   next_checkpoint_lsn= 0;
-  n_pending_checkpoint_writes= 0;
+  checkpoint_pending= false;
 
   log_block_init(buf, LOG_START_LSN);
   log_block_set_first_rec_group(buf, LOG_BLOCK_HDR_SIZE);
@@ -939,7 +939,8 @@ ATTRIBUTE_COLD void log_write_checkpoint_info(lsn_t end_lsn)
 	ut_ad(LOG_CHECKPOINT_1 < srv_page_size);
 	ut_ad(LOG_CHECKPOINT_2 < srv_page_size);
 
-	++log_sys.n_pending_checkpoint_writes;
+	ut_ad(!log_sys.checkpoint_pending);
+	log_sys.checkpoint_pending = true;
 
 	mysql_mutex_unlock(&log_sys.mutex);
 
@@ -954,8 +955,8 @@ ATTRIBUTE_COLD void log_write_checkpoint_info(lsn_t end_lsn)
 
 	mysql_mutex_lock(&log_sys.mutex);
 
-	--log_sys.n_pending_checkpoint_writes;
-	ut_ad(log_sys.n_pending_checkpoint_writes == 0);
+	ut_ad(log_sys.checkpoint_pending);
+	log_sys.checkpoint_pending = false;
 
 	log_sys.next_checkpoint_no++;
 
@@ -1149,8 +1150,8 @@ ATTRIBUTE_COLD void logs_empty_and_mark_files_at_shutdown()
 
 	if (log_sys.is_initialised()) {
 		mysql_mutex_lock(&log_sys.mutex);
-		const ulint	n_write	= log_sys.n_pending_checkpoint_writes;
-		const ulint	n_flush	= log_sys.pending_flushes;
+		const size_t n_write{log_sys.checkpoint_pending};
+		const size_t n_flush{log_sys.get_pending_flushes()};
 		mysql_mutex_unlock(&log_sys.mutex);
 
 		if (n_write || n_flush) {
@@ -1291,7 +1292,7 @@ log_print(
 		ULINTPF " pending chkp writes\n"
 		ULINTPF " log i/o's done, %.2f log i/o's/second\n",
 		log_sys.pending_flushes.load(),
-		log_sys.n_pending_checkpoint_writes,
+		ulint{log_sys.checkpoint_pending},
 		log_sys.n_log_ios,
 		static_cast<double>(
 			log_sys.n_log_ios - log_sys.n_log_ios_old)

storage/innobase/srv/srv0mon.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 2010, 2016, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2021, MariaDB Corporation.
+Copyright (c) 2013, 2022, 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
@@ -1909,10 +1909,7 @@ srv_mon_process_existing_counter(
 		break;
 
 	case MONITOR_PENDING_CHECKPOINT_WRITE:
-		mysql_mutex_lock(&log_sys.mutex);
-		value = static_cast<mon_type_t>(
-		    log_sys.n_pending_checkpoint_writes);
-		mysql_mutex_unlock(&log_sys.mutex);
+		value = log_sys.checkpoint_pending;
 		break;
 
 	case MONITOR_LOG_IO: