fix Arena bytes-used accounting across merge

Summary: Fix `Arena` bytes-used accounting across calls to member `merge`.

Reviewed By: luciang

Differential Revision: D25656949

fbshipit-source-id: b230b0cdbf51b146fd566cbf688f49817bdf874d

folly/memory/Arena-inl.h

@@ -77,5 +77,7 @@ void Arena<Alloc>::merge(Arena<Alloc>&& other) {
   other.ptr_ = other.end_ = nullptr;
   totalAllocatedSize_ += other.totalAllocatedSize_;
   other.totalAllocatedSize_ = 0;
+  bytesUsed_ += other.bytesUsed_;
+  other.bytesUsed_ = 0;
 }
 } // namespace folly

folly/memory/test/ArenaTest.cpp

@@ -15,15 +15,17 @@
  */
 
 #include <folly/memory/Arena.h>
-#include <folly/Memory.h>
-#include <folly/portability/GFlags.h>
-#include <folly/portability/GTest.h>
 
 #include <set>
 #include <vector>
 
 #include <glog/logging.h>
 
+#include <folly/Memory.h>
+#include <folly/memory/Malloc.h>
+#include <folly/portability/GFlags.h>
+#include <folly/portability/GTest.h>
+
 using namespace folly;
 
 static_assert(AllocatorHasTrivialDeallocate<SysArena>::value, "");
@@ -233,6 +235,29 @@ TEST(Arena, ClearAfterLarge) {
   EXPECT_EQ(0, arena.bytesUsed());
 }
 
+TEST(Arena, Merge) {
+  constexpr size_t blockSize = 1024;
+  size_t blockAllocSize = goodMallocSize(blockSize + SysArena::kBlockOverhead);
+
+  SysArena arena1(blockSize);
+  SysArena arena2(blockSize);
+
+  arena1.allocate(16);
+  arena2.allocate(32);
+
+  EXPECT_EQ(blockAllocSize + sizeof(SysArena), arena1.totalSize());
+  EXPECT_EQ(blockAllocSize + sizeof(SysArena), arena2.totalSize());
+  EXPECT_EQ(16, arena1.bytesUsed());
+  EXPECT_EQ(32, arena2.bytesUsed());
+
+  arena1.merge(std::move(arena2));
+
+  EXPECT_EQ(blockAllocSize * 2 + sizeof(SysArena), arena1.totalSize());
+  EXPECT_EQ(blockAllocSize * 0 + sizeof(SysArena), arena2.totalSize());
+  EXPECT_EQ(48, arena1.bytesUsed());
+  EXPECT_EQ(0, arena2.bytesUsed());
+}
+
 int main(int argc, char* argv[]) {
   testing::InitGoogleTest(&argc, argv);
   gflags::ParseCommandLineFlags(&argc, &argv, true);