Calculate memory usage correctly for unordered_maps that use PoolAllocator

Extracts the resource from a PoolAllocator and uses it for
calculation of the node's memory usage.

src/test/pool_tests.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <memusage.h>
 #include <support/allocators/pool.h>
 #include <test/util/poolresourcetester.h>
 #include <test/util/random.h>
@@ -153,4 +154,37 @@ BOOST_AUTO_TEST_CASE(random_allocations)
     PoolResourceTester::CheckAllDataAccountedFor(resource);
 }
 
+BOOST_AUTO_TEST_CASE(memusage_test)
+{
+    auto std_map = std::unordered_map<int, int>{};
+
+    using Map = std::unordered_map<int,
+                                   int,
+                                   std::hash<int>,
+                                   std::equal_to<int>,
+                                   PoolAllocator<std::pair<const int, int>,
+                                                 sizeof(std::pair<const int, int>) + sizeof(void*) * 4,
+                                                 alignof(void*)>>;
+    auto resource = Map::allocator_type::ResourceType(1024);
+
+    PoolResourceTester::CheckAllDataAccountedFor(resource);
+
+    {
+        auto resource_map = Map{0, std::hash<int>{}, std::equal_to<int>{}, &resource};
+
+        // can't have the same resource usage
+        BOOST_TEST(memusage::DynamicUsage(std_map) != memusage::DynamicUsage(resource_map));
+
+        for (size_t i = 0; i < 10000; ++i) {
+            std_map[i];
+            resource_map[i];
+        }
+
+        // Eventually the resource_map should have a much lower memory usage because it has less malloc overhead
+        BOOST_TEST(memusage::DynamicUsage(resource_map) <= memusage::DynamicUsage(std_map) * 90 / 100);
+    }
+
+    PoolResourceTester::CheckAllDataAccountedFor(resource);
+}
+
 BOOST_AUTO_TEST_SUITE_END()