Return actual iterator on assign_if_equal

Summary: There is no sense to return the iterator that points to the outdated node that just was overwritten by `assign_if_equal` call.

Reviewed By: yfeldblum

Differential Revision: D27678357

fbshipit-source-id: f07ce21df6556819f7eb103a623d3f7a46e72072

folly/concurrency/detail/ConcurrentHashMap-detail.h

@@ -670,6 +670,7 @@ class alignas(64) BucketTable {
             next->acquire_link(); // defined in hazptr_obj_base_linked
           }
           prev->store(cur, std::memory_order_release);
+          it.setNode(cur, buckets, bcount, idx);
           g.unlock();
           // Release not under lock.
           node->release();

folly/concurrency/test/ConcurrentHashMapTest.cpp

@@ -934,7 +934,9 @@ TYPED_TEST_P(ConcurrentHashMapTest, HeterogeneousInsert) {
 
   map.insert("foo", "baz");
   EXPECT_TRUE(map.assign(foo, "hello2"));
-  EXPECT_TRUE(map.assign_if_equal("foo", "hello2", "hello"));
+  auto mbIt = map.assign_if_equal("foo", "hello2", "hello");
+  EXPECT_TRUE(mbIt);
+  EXPECT_EQ(mbIt.value()->second, "hello");
   EXPECT_EQ(map[foo], "hello");
   auto it = map.find(foo);
   map.erase(it);