pool: make sure PoolAllocator uses the correct alignment

This changes the PoolAllocator to default the alignment to the given type. This makes the code simpler, and most importantly
fixes a bug on ARM 32bit that caused OOM: The class CTxOut has a member CAmount which is an int64_t and on ARM 32bit int64_t
are 8 byte aligned which is larger than the pointer alignment of 4 bytes. So for CCoinsMap to be able to use the pool, we
need to use the alignment of the member instead of just alignof(void*).

Github-Pull: bitcoin#28913
Rebased-From: ce881bf

src/bench/pool.cpp

@@ -37,8 +37,7 @@ static void PoolAllocator_StdUnorderedMapWithPoolResource(benchmark::Bench& benc
                                    std::hash<uint64_t>,
                                    std::equal_to<uint64_t>,
                                    PoolAllocator<std::pair<const uint64_t, uint64_t>,
-                                                 sizeof(std::pair<const uint64_t, uint64_t>) + 4 * sizeof(void*),
-                                                 alignof(void*)>>;
+                                                 sizeof(std::pair<const uint64_t, uint64_t>) + 4 * sizeof(void*)>>;
 
     // make sure the resource supports large enough pools to hold the node. We do this by adding the size of a few pointers to it.
     auto pool_resource = Map::allocator_type::ResourceType();

src/coins.h

@@ -145,8 +145,7 @@ using CCoinsMap = std::unordered_map<COutPoint,
                                      SaltedOutpointHasher,
                                      std::equal_to<COutPoint>,
                                      PoolAllocator<std::pair<const COutPoint, CCoinsCacheEntry>,
-                                                   sizeof(std::pair<const COutPoint, CCoinsCacheEntry>) + sizeof(void*) * 4,
-                                                   alignof(void*)>>;
+                                                   sizeof(std::pair<const COutPoint, CCoinsCacheEntry>) + sizeof(void*) * 4>>;
 
 using CCoinsMapMemoryResource = CCoinsMap::allocator_type::ResourceType;
 

src/support/allocators/pool.h

@@ -272,7 +272,7 @@ class PoolResource final
 /**
  * Forwards all allocations/deallocations to the PoolResource.
  */
-template <class T, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES>
+template <class T, std::size_t MAX_BLOCK_SIZE_BYTES, std::size_t ALIGN_BYTES = alignof(T)>
 class PoolAllocator
 {
     PoolResource<MAX_BLOCK_SIZE_BYTES, ALIGN_BYTES>* m_resource;

src/test/pool_tests.cpp

@@ -163,8 +163,7 @@ BOOST_AUTO_TEST_CASE(memusage_test)
                                    std::hash<int>,
                                    std::equal_to<int>,
                                    PoolAllocator<std::pair<const int, int>,
-                                                 sizeof(std::pair<const int, int>) + sizeof(void*) * 4,
-                                                 alignof(void*)>>;
+                                                 sizeof(std::pair<const int, int>) + sizeof(void*) * 4>>;
     auto resource = Map::allocator_type::ResourceType(1024);
 
     PoolResourceTester::CheckAllDataAccountedFor(resource);