Fix ConcurrentHashMap iterator copy

Summary:
UndefinedBehaviorSanitizer: null-pointer-use buck-out/dev-asan-ubsan/gen/folly/concurrency/concurrent_hash_map#header-mode-symlink-tree-with-header-map,headers/folly/concurrency/ConcurrentHashMap.h:440:30 in
Finished test run: https://our.intern.facebook.com/intern/testinfra/testrun/882372015
Summary (total time 1.49s):
  PASS: 0
  FAIL: 0
  SKIP: 0
  FATAL: 1
    folly/concurrency/test:concurrent_hash_map_test - ConcurrentHashMap.CopyIterator
  TIMEOUT: 0
  OMIT: 0
```

Reviewed By: djwatson

Differential Revision: D7003359

fbshipit-source-id: d6764caf2dfb1f19cf5b38a9e89d7c0583c75412

folly/concurrency/ConcurrentHashMap.h

@@ -410,12 +410,14 @@ class ConcurrentHashMap {
     }
 
     ConstIterator& operator=(const ConstIterator& o) {
+      parent_ = o.parent_;
       it_ = o.it_;
       segment_ = o.segment_;
       return *this;
     }
 
     ConstIterator(const ConstIterator& o) {
+      parent_ = o.parent_;
       it_ = o.it_;
       segment_ = o.segment_;
     }

folly/concurrency/test/ConcurrentHashMapTest.cpp

@@ -257,6 +257,16 @@ TEST(ConcurrentHashMap, EraseTest) {
   foomap.erase(f1);
 }
 
+TEST(ConcurrentHashMap, CopyIterator) {
+  ConcurrentHashMap<int, int> map;
+  map.insert(0, 0);
+  auto const cbegin = map.cbegin();
+  for (auto cit = cbegin; cit != map.cend(); ++cit) {
+    std::pair<int const, int> const ckv{0, 0};
+    EXPECT_EQ(*cit, ckv);
+  }
+}
+
 TEST(ConcurrentHashMap, EraseInIterateTest) {
   ConcurrentHashMap<uint64_t, uint64_t> foomap(3);
   for (uint64_t k = 0; k < 10; ++k) {