Clarify the access to trx_sys.rseg_history_len

trx_sys_t::rseg_history_len: Make private, and clarify the
documentation.

trx_sys_t::history_size(): Read rseg_history_len.

trx_sys_t::history_insert(), trx_sys_t::history_remove(),
trx_sys_t::history_add(): Update rseg_history_len.

storage/innobase/btr/btr0cur.cc

@@ -1166,7 +1166,7 @@ btr_cur_search_to_nth_level_func(
 		Free blocks and read IO bandwidth should be prior
 		for them, when the history list is glowing huge. */
 		if (lock_intention == BTR_INTENTION_DELETE
-		    && trx_sys.rseg_history_len > BTR_CUR_FINE_HISTORY_LENGTH
+		    && trx_sys.history_size() > BTR_CUR_FINE_HISTORY_LENGTH
 			&& buf_get_n_pending_read_ios()) {
 			mtr_x_lock(dict_index_get_lock(index), mtr);
 		} else if (dict_index_is_spatial(index)
@@ -2305,7 +2305,7 @@ btr_cur_open_at_index_side_func(
 		Free blocks and read IO bandwidth should be prior
 		for them, when the history list is glowing huge. */
 		if (lock_intention == BTR_INTENTION_DELETE
-		    && trx_sys.rseg_history_len > BTR_CUR_FINE_HISTORY_LENGTH
+		    && trx_sys.history_size() > BTR_CUR_FINE_HISTORY_LENGTH
 		    && buf_get_n_pending_read_ios()) {
 			mtr_x_lock(dict_index_get_lock(index), mtr);
 		} else {
@@ -2651,7 +2651,7 @@ btr_cur_open_at_rnd_pos_func(
 		Free blocks and read IO bandwidth should be prior
 		for them, when the history list is glowing huge. */
 		if (lock_intention == BTR_INTENTION_DELETE
-		    && trx_sys.rseg_history_len > BTR_CUR_FINE_HISTORY_LENGTH
+		    && trx_sys.history_size() > BTR_CUR_FINE_HISTORY_LENGTH
 		    && buf_get_n_pending_read_ios()) {
 			mtr_x_lock(dict_index_get_lock(index), mtr);
 		} else {

storage/innobase/include/trx0sys.h

@@ -817,6 +817,11 @@ class trx_sys_t
   MY_ALIGNED(CACHE_LINE_SIZE) trx_id_t m_rw_trx_hash_version;
 
 
+  /**
+    TRX_RSEG_HISTORY list length (number of committed transactions to purge)
+  */
+  MY_ALIGNED(CACHE_LINE_SIZE) int32 rseg_history_len;
+
   /** Active views. */
   MY_ALIGNED(CACHE_LINE_SIZE) UT_LIST_BASE_NODE_T(ReadView) m_views;
 
@@ -850,11 +855,6 @@ class trx_sys_t
 					single-threaded mode; not protected
 					by any mutex, because it is read-only
 					during multi-threaded operation */
-	ulint		rseg_history_len;
-					/*!< Length of the TRX_RSEG_HISTORY
-					list (update undo logs for committed
-					transactions), protected by
-					rseg->mutex */
 
 
   /**
@@ -1122,6 +1122,20 @@ class trx_sys_t
     return count;
   }
 
+  /** @return number of committed transactions waiting for purge */
+  ulint history_size() const
+  {
+    return uint32(my_atomic_load32(&rseg_history_len));
+  }
+  /** Add to the TRX_RSEG_HISTORY length (on database startup). */
+  void history_add(int32 len)
+  {
+    my_atomic_add32(&rseg_history_len, len);
+  }
+  /** Register a committed transaction. */
+  void history_insert() { history_add(1); }
+  /** Note that a committed transaction was purged. */
+  void history_remove() { history_add(-1); }
 
 private:
   static my_bool get_min_trx_id_callback(rw_trx_hash_element_t *element,

storage/innobase/lock/lock0lock.cc

@@ -5294,7 +5294,7 @@ lock_print_info_summary(
 	fprintf(file, "\n");
 
 	fprintf(file,
-		"History list length " ULINTPF "\n", trx_sys.rseg_history_len);
+		"History list length " ULINTPF "\n", trx_sys.history_size());
 
 #ifdef PRINT_NUM_OF_LOCK_STRUCTS
 	fprintf(file,

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, 2017, MariaDB Corporation.
+Copyright (c) 2013, 2018, 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
@@ -1952,7 +1952,7 @@ srv_mon_process_existing_counter(
 		break;
 
 	case MONITOR_RSEG_HISTORY_LEN:
-		value = trx_sys.rseg_history_len;
+		value = trx_sys.history_size();
 		break;
 
 	case MONITOR_RSEG_CUR_SIZE:

storage/innobase/srv/srv0srv.cc

@@ -1975,7 +1975,7 @@ srv_wake_purge_thread_if_not_active()
 
 	if (purge_sys->state == PURGE_STATE_RUN
 	    && !my_atomic_loadlint(&srv_sys.n_threads_active[SRV_PURGE])
-	    && my_atomic_loadlint(&trx_sys.rseg_history_len)) {
+	    && trx_sys.history_size()) {
 
 		srv_release_threads(SRV_PURGE, 1);
 	}
@@ -2614,7 +2614,7 @@ srv_do_purge(ulint* n_total_purged)
 	}
 
 	do {
-		if (trx_sys.rseg_history_len > rseg_history_len
+		if (trx_sys.history_size() > rseg_history_len
 		    || (srv_max_purge_lag > 0
 			&& rseg_history_len > srv_max_purge_lag)) {
 
@@ -2643,7 +2643,7 @@ srv_do_purge(ulint* n_total_purged)
 		ut_a(n_use_threads <= n_threads);
 
 		/* Take a snapshot of the history list before purge. */
-		if ((rseg_history_len = trx_sys.rseg_history_len) == 0) {
+		if (!(rseg_history_len = trx_sys.history_size())) {
 			break;
 		}
 
@@ -2698,7 +2698,7 @@ srv_purge_coordinator_suspend(
 		/* We don't wait right away on the the non-timed wait because
 		we want to signal the thread that wants to suspend purge. */
 		const bool wait = stop
-			|| rseg_history_len <= trx_sys.rseg_history_len;
+			|| rseg_history_len <= trx_sys.history_size();
 		const bool timeout = srv_resume_thread(
 			slot, sig_count, wait,
 			stop ? 0 : SRV_PURGE_MAX_TIMEOUT);
@@ -2715,8 +2715,8 @@ srv_purge_coordinator_suspend(
 			purge_sys->running = true;
 
 			if (timeout
-			    && rseg_history_len == trx_sys.rseg_history_len
-			    && trx_sys.rseg_history_len < 5000) {
+			    && rseg_history_len < 5000
+			    && rseg_history_len == trx_sys.history_size()) {
 				/* No new records were added since the
 				wait started. Simply wait for new
 				records. The magic number 5000 is an
@@ -2777,7 +2777,7 @@ DECLARE_THREAD(srv_purge_coordinator_thread)(
 
 	slot = srv_reserve_slot(SRV_PURGE);
 
-	ulint	rseg_history_len = trx_sys.rseg_history_len;
+	ulint	rseg_history_len = trx_sys.history_size();
 
 	do {
 		/* If there are no records to purge or the last

storage/innobase/trx/trx0purge.cc

@@ -324,8 +324,6 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 	flst_add_first(rseg_header + TRX_RSEG_HISTORY,
 		       undo_header + TRX_UNDO_HISTORY_NODE, mtr);
 
-	my_atomic_addlint(&trx_sys.rseg_history_len, 1);
-
 	mlog_write_ull(undo_header + TRX_UNDO_TRX_NO, trx->no, mtr);
 	/* This is needed for upgrading old undo log pages from
 	before MariaDB 10.3.1. */
@@ -342,6 +340,8 @@ trx_purge_add_undo_to_history(const trx_t* trx, trx_undo_t*& undo, mtr_t* mtr)
 		rseg->needs_purge = true;
 	}
 
+	trx_sys.history_insert();
+
 	if (undo->state == TRX_UNDO_CACHED) {
 		UT_LIST_ADD_FIRST(rseg->undo_cached, undo);
 		MONITOR_INC(MONITOR_NUM_UNDO_SLOT_CACHED);
@@ -366,7 +366,7 @@ trx_purge_remove_log_hdr(
 {
 	flst_remove(rseg_hdr + TRX_RSEG_HISTORY,
 		    log_hdr + TRX_UNDO_HISTORY_NODE, mtr);
-	my_atomic_addlint(&trx_sys.rseg_history_len, -1);
+	trx_sys.history_remove();
 }
 
 /** Free an undo log segment, and remove the header from the history list.
@@ -1539,7 +1539,7 @@ trx_purge_dml_delay(void)
 	if (srv_max_purge_lag > 0) {
 		float	ratio;
 
-		ratio = float(trx_sys.rseg_history_len) / srv_max_purge_lag;
+		ratio = float(trx_sys.history_size()) / srv_max_purge_lag;
 
 		if (ratio > 1.0) {
 			/* If the history list length exceeds the

storage/innobase/trx/trx0rseg.cc

@@ -213,7 +213,7 @@ trx_rseg_mem_restore(trx_rseg_t* rseg, trx_id_t& max_trx_id, mtr_t* mtr)
 		+ 1 + trx_undo_lists_init(rseg, max_trx_id, rseg_header);
 
 	if (ulint len = flst_get_len(rseg_header + TRX_RSEG_HISTORY)) {
-		my_atomic_addlint(&trx_sys.rseg_history_len, len);
+		trx_sys.history_add(int32(len));
 
 		fil_addr_t	node_addr = trx_purge_get_log_from_hist(
 			flst_get_last(rseg_header + TRX_RSEG_HISTORY, mtr));

storage/innobase/trx/trx0sys.cc

@@ -402,6 +402,7 @@ trx_sys_t::create()
 	mutex_create(LATCH_ID_TRX_SYS, &mutex);
 	UT_LIST_INIT(mysql_trx_list, &trx_t::mysql_trx_list);
 	UT_LIST_INIT(m_views, &ReadView::m_view_list);
+	my_atomic_store32(&rseg_history_len, 0);
 
 	rw_trx_hash.init();
 }
@@ -529,8 +530,6 @@ trx_sys_t::close()
 
 	ut_a(UT_LIST_GET_LEN(mysql_trx_list) == 0);
 	ut_ad(UT_LIST_GET_LEN(m_views) == 0);
-
-	/* We used placement new to create this mutex. Call the destructor. */
 	mutex_free(&mutex);
 	m_initialised = false;
 }