let TLObserver be moveable

Summary: It is currently copyable but not moveable, so all apparent moves are actually copies. This can be surprisingly expensive when unexpected.

Differential Revision: D57691075

fbshipit-source-id: 3f5332bb1f2767f3c01208899bffc88ac3ba2cc8

third-party/folly/src/folly/observer/Observer-inl.h

@@ -162,22 +162,28 @@ T AtomicObserver<T>::get() const {
 
 template <typename T>
 TLObserver<T>::TLObserver(Observer<T> observer)
-    : observer_(std::move(observer)),
-      snapshot_([&] { return Snapshot<T>(observer_.getSnapshot()); }) {}
+    : observer_(std::move(observer)) {}
 
 template <typename T>
 TLObserver<T>::TLObserver(const TLObserver<T>& other)
     : TLObserver(other.observer_) {}
 
+template <typename T>
+TLObserver<T>::TLObserver(TLObserver<T>&& other) noexcept = default;
+
 template <typename T>
 const Snapshot<T>& TLObserver<T>::getSnapshotRef() const {
-  auto& snapshot = *snapshot_;
-  if (observer_.needRefresh(snapshot) ||
+  Snapshot<T>* snapshot = snapshot_.get();
+  if (FOLLY_UNLIKELY(!snapshot)) {
+    snapshot = new Snapshot<T>(observer_.getSnapshot());
+    snapshot_.reset(snapshot);
+  }
+  if (observer_.needRefresh(*snapshot) ||
       observer_detail::ObserverManager::inManagerThread()) {
-    snapshot = observer_.getSnapshot();
+    *snapshot = observer_.getSnapshot();
   }
 
-  return snapshot;
+  return *snapshot;
 }
 
 template <typename T>

third-party/folly/src/folly/observer/Observer.h

@@ -334,6 +334,7 @@ class TLObserver {
  public:
   explicit TLObserver(Observer<T> observer);
   TLObserver(const TLObserver<T>& other);
+  TLObserver(TLObserver<T>&& other) noexcept;
 
   const Snapshot<T>& getSnapshotRef() const;
   const Snapshot<T>& operator*() const { return getSnapshotRef(); }
@@ -342,7 +343,7 @@ class TLObserver {
 
  private:
   Observer<T> observer_;
-  ThreadLocal<Snapshot<T>> snapshot_;
+  mutable ThreadLocalPtr<Snapshot<T>> snapshot_;
 };
 
 template <typename T>

third-party/folly/src/folly/observer/test/ObserverTest.cpp

@@ -271,6 +271,10 @@ TEST(Observer, TLObserver) {
   EXPECT_EQ(42, ***k);
   k = std::make_unique<folly::observer::TLObserver<int>>(createTLObserver(41));
   EXPECT_EQ(41, ***k);
+  k = std::make_unique<folly::observer::TLObserver<int>>( // copy-ctor
+      static_cast<folly::observer::TLObserver<int> const&>(
+          createTLObserver(40)));
+  EXPECT_EQ(40, ***k);
 }
 
 TEST(ReadMostlyTLObserver, ReadMostlyTLObserver) {