Fix alignment issue with new arena context system

UBSAN fails the modified test here without the changes in arena

PiperOrigin-RevId: 638901311

src/core/lib/resource_quota/arena.cc

@@ -36,8 +36,9 @@ void* ArenaStorage(size_t& initial_size) {
   static constexpr size_t base_size =
       GPR_ROUND_UP_TO_ALIGNMENT_SIZE(sizeof(Arena));
   initial_size = GPR_ROUND_UP_TO_ALIGNMENT_SIZE(initial_size);
-  initial_size = std::max(initial_size,
-                          arena_detail::BaseArenaContextTraits::ContextSize());
+  initial_size = std::max(
+      initial_size, GPR_ROUND_UP_TO_ALIGNMENT_SIZE(
+                        arena_detail::BaseArenaContextTraits::ContextSize()));
   size_t alloc_size = base_size + initial_size;
   static constexpr size_t alignment =
       (GPR_CACHELINE_SIZE > GPR_MAX_ALIGNMENT &&
@@ -76,7 +77,8 @@ RefCountedPtr<Arena> Arena::Create(size_t initial_size,
 
 Arena::Arena(size_t initial_size, RefCountedPtr<ArenaFactory> arena_factory)
     : initial_zone_size_(initial_size),
-      total_used_(arena_detail::BaseArenaContextTraits::ContextSize()),
+      total_used_(GPR_ROUND_UP_TO_ALIGNMENT_SIZE(
+          arena_detail::BaseArenaContextTraits::ContextSize())),
       arena_factory_(std::move(arena_factory)) {
   for (size_t i = 0; i < arena_detail::BaseArenaContextTraits::NumContexts();
        ++i) {

test/core/resource_quota/arena_test.cc

@@ -284,12 +284,16 @@ struct ArenaContextType<Foo> {
   static void Destroy(Foo* p) { p->~Foo(); }
 };
 
+struct alignas(16) VeryAligned {};
+
 TEST(ArenaTest, FooContext) {
   auto arena = SimpleArenaAllocator()->MakeArena();
   EXPECT_EQ(arena->GetContext<Foo>(), nullptr);
   arena->SetContext(arena->New<Foo>(42));
   ASSERT_NE(arena->GetContext<Foo>(), nullptr);
   EXPECT_EQ(*arena->GetContext<Foo>()->p, 42);
+  arena->New<VeryAligned>();
+  arena->New<VeryAligned>();
 }
 
 class MockArenaFactory : public ArenaFactory {