MDEV-16678 Prefer MDL to dict_sys.latch for innodb background tasks

Use std::queue backed by std::deque instead of list because it does less
allocations which still having O(1) push and pop operations.

Also store trx_purge_rec_t directly, because its only 16 bytes and allocating
it is to wasteful. This should be faster.

purge_node_t::purge_node_t: container is already empty after creation

purge_node_t::end(): replace clearing container with assertion that it's clear

storage/innobase/include/row0purge.h

@@ -34,7 +34,7 @@ Created 3/14/1997 Heikki Tuuri
 #include "row0types.h"
 #include "row0mysql.h"
 #include "mysqld.h"
-#include <list>
+#include <queue>
 
 class MDL_ticket;
 /** Determines if it is possible to remove a secondary index entry.
@@ -150,7 +150,7 @@ struct purge_node_t{
 	int			mdl_hold_recs;
 
 	/** Undo recs to purge */
-	std::list<trx_purge_rec_t*>	undo_recs;
+	std::queue<trx_purge_rec_t>	undo_recs;
 
 	/** Constructor */
 	explicit purge_node_t(que_thr_t* parent) :
@@ -166,7 +166,6 @@ struct purge_node_t{
 		purge_thd(NULL),
 		mdl_hold_recs(0)
 	{
-		undo_recs.clear();
 	}
 
 #ifdef UNIV_DEBUG
@@ -258,7 +257,7 @@ struct purge_node_t{
   {
     DBUG_ASSERT(common.type == QUE_NODE_PURGE);
     close_table();
-    undo_recs.clear();
+    ut_ad(undo_recs.empty());
     ut_d(in_progress= false);
     purge_thd= nullptr;
     mem_heap_empty(heap);

storage/innobase/row/row0purge.cc

@@ -1155,12 +1155,11 @@ row_purge_step(
 	node->start();
 
 	if (!node->undo_recs.empty()) {
-		trx_purge_rec_t*	purge_rec =
-			node->undo_recs.front();
-		node->undo_recs.pop_front();
-		node->roll_ptr = purge_rec->roll_ptr;
+		trx_purge_rec_t purge_rec = node->undo_recs.front();
+		node->undo_recs.pop();
+		node->roll_ptr = purge_rec.roll_ptr;
 
-		row_purge(node, purge_rec->undo_rec, thr);
+		row_purge(node, purge_rec.undo_rec, thr);
 
 		if (node->undo_recs.empty()) {
 			row_purge_end(thr);

storage/innobase/trx/trx0purge.cc

@@ -1141,7 +1141,7 @@ trx_purge_attach_undo_recs(ulint n_purge_threads)
 
 	while (UNIV_LIKELY(srv_undo_sources) || !srv_fast_shutdown) {
 		purge_node_t*		node;
-		trx_purge_rec_t*	purge_rec;
+		trx_purge_rec_t		purge_rec;
 
 		ut_a(!thr->is_active);
 
@@ -1149,9 +1149,6 @@ trx_purge_attach_undo_recs(ulint n_purge_threads)
 		node = (purge_node_t*) thr->child;
 		ut_a(que_node_get_type(node) == QUE_NODE_PURGE);
 
-		purge_rec = static_cast<trx_purge_rec_t*>(
-			mem_heap_zalloc(purge_sys.heap, sizeof(*purge_rec)));
-
 		/* Track the max {trx_id, undo_no} for truncating the
 		UNDO logs once we have purged the records. */
 
@@ -1160,18 +1157,18 @@ trx_purge_attach_undo_recs(ulint n_purge_threads)
 		}
 
 		/* Fetch the next record, and advance the purge_sys.tail. */
-		purge_rec->undo_rec = trx_purge_fetch_next_rec(
-			&purge_rec->roll_ptr, &n_pages_handled,
+		purge_rec.undo_rec = trx_purge_fetch_next_rec(
+			&purge_rec.roll_ptr, &n_pages_handled,
 			purge_sys.heap);
 
-		if (purge_rec->undo_rec == NULL) {
+		if (purge_rec.undo_rec == NULL) {
 			break;
-		} else if (purge_rec->undo_rec == &trx_purge_dummy_rec) {
+		} else if (purge_rec.undo_rec == &trx_purge_dummy_rec) {
 			continue;
 		}
 
 		table_id_t table_id = trx_undo_rec_get_table_id(
-			purge_rec->undo_rec);
+			purge_rec.undo_rec);
 
 		auto it = table_id_map.find(table_id);
 
@@ -1189,7 +1186,7 @@ trx_purge_attach_undo_recs(ulint n_purge_threads)
 			table_id_map.insert({table_id, node});
 		}
 
-		node->undo_recs.push_back(purge_rec);
+		node->undo_recs.push(purge_rec);
 
 		if (n_pages_handled >= batch_size) {
 			break;