fix Arena bytes-used accounting across clear

Summary: Fix `Arena` bytes-used accounting across calls to member `clear` after a sequence of all-large allocations.

Reviewed By: luciang

Differential Revision: D25656875

fbshipit-source-id: e79aca5f58b61bf1bd4790f8badd0dad6063a6ed

folly/memory/Arena.h

@@ -121,11 +121,11 @@ class Arena {
   void merge(Arena&& other);
 
   void clear() {
+    bytesUsed_ = 0;
     freeLargeBlocks(); // We don't reuse large blocks
     if (blocks_.empty()) {
       return;
     }
-    bytesUsed_ = 0;
     currentBlock_ = blocks_.begin();
     char* start = currentBlock_->start();
     ptr_ = start;

folly/memory/test/ArenaTest.cpp

@@ -222,6 +222,17 @@ TEST(Arena, Clear) {
   }
 }
 
+TEST(Arena, ClearAfterLarge) {
+  constexpr size_t blockSize = 1024;
+  constexpr size_t mult = 10;
+  SysArena arena(blockSize);
+  EXPECT_EQ(0, arena.bytesUsed());
+  arena.allocate(blockSize * mult);
+  EXPECT_EQ(blockSize * mult, arena.bytesUsed());
+  arena.clear();
+  EXPECT_EQ(0, arena.bytesUsed());
+}
+
 int main(int argc, char* argv[]) {
   testing::InitGoogleTest(&argc, argv);
   gflags::ParseCommandLineFlags(&argc, &argv, true);