Increase refcount size

Summary:
The refcounting logic is correct, however, decreasing the next node's
refcount only happens in ~NodeT, after going through hazptr bulkReclaim.

The current default for bulkReclaim is 1k, which exceeds the value of
refcount uint8_t.  Increase size to uint16_t so that we don't wrap refcount
before bulkReclaim calls the node destructors.

Reviewed By: magedm

Differential Revision: D6900530

fbshipit-source-id: 9b2530dc6cd0609b4795b909fd46aeffbb1ae821

folly/concurrency/detail/ConcurrentHashMap-detail.h

@@ -156,7 +156,7 @@ class NodeT : public folly::hazptr::hazptr_obj_base<
 
  private:
   ValueHolder<KeyType, ValueType, Allocator> item_;
-  Atom<uint8_t> refcount_{1};
+  Atom<uint16_t> refcount_{1};
 };
 
 } // namespace concurrenthashmap

folly/concurrency/test/ConcurrentHashMapTest.cpp

@@ -547,3 +547,25 @@ TEST(ConcurrentHashMap, assignStressTest) {
     join;
   }
 }
+
+TEST(ConcurrentHashMap, RefcountTest) {
+  struct badhash {
+    size_t operator()(uint64_t) const {
+      return 0;
+    }
+  };
+  ConcurrentHashMap<
+      uint64_t,
+      uint64_t,
+      badhash,
+      std::equal_to<uint64_t>,
+      std::allocator<uint8_t>,
+      0>
+      foomap(3);
+  foomap.insert(0, 0);
+  foomap.insert(1, 1);
+  foomap.insert(2, 2);
+  for (int32_t i = 0; i < 300; ++i) {
+    foomap.insert_or_assign(1, i);
+  }
+}