Sealable: fixed semantic; atomic type for the flag variable

Summary:
* Fixed semantic: all kinds of derivative instances lose `sealed` flag (which is expected);
 * Using atomic<bool> for `sealed_` ivar.

Reviewed By: fkgozali

Differential Revision: D7230674

fbshipit-source-id: abe786610c20a45a0fabb9068120e24adeeeac7f

ReactCommon/fabric/core/primitives/Sealable.cpp

@@ -12,6 +12,36 @@
 namespace facebook {
 namespace react {
 
+/*
+ * Note:
+ * We must explictly implement all *the rule of five* methods because:
+ *   1. Using `std::atomic` behind `sealed_` implicitly deletes default
+ *      constructors;
+ *   2. We have to establish behaviour where any new cloned or moved instances
+ *      of the object lose `sealed` flag.
+ *
+ * See more about the rule of three/five/zero:
+ * http://en.cppreference.com/w/cpp/language/rule_of_three
+ */
+
+Sealable::Sealable(): sealed_(false) {}
+
+Sealable::Sealable(const Sealable& other): sealed_(false) {};
+
+Sealable::Sealable(Sealable&& other) noexcept: sealed_(false) {};
+
+Sealable::~Sealable() noexcept {};
+
+Sealable& Sealable::operator= (const Sealable& other) {
+  ensureUnsealed();
+  return *this;
+}
+
+Sealable& Sealable::operator= (Sealable&& other) noexcept {
+  ensureUnsealed();
+  return *this;
+};
+
 void Sealable::seal() const {
   sealed_ = true;
 }

ReactCommon/fabric/core/primitives/Sealable.h

@@ -7,21 +7,27 @@
 
 #pragma once
 
+#import <atomic>
+
 namespace facebook {
 namespace react {
 
 /*
- * Represents something which can be *sealed* (imperatively marked as immutable).
+ * Represents an object which can be *sealed* (imperatively marked as immutable).
+ *
+ * The `sealed` flag is tight to a particular instance of the class and resets
+ * to `false` for all newly created (by copy-constructor, assignment operator
+ * and so on) derivative objects.
  *
  * Why do we need this? In Fabric, some objects are semi-immutable
- * even if they explicitly marked as `const`. It means that in some special
- * cases those objects can be const-cast-away and then mutated. That comes from
+ * even if they are explicitly marked as `const`. It means that in some special
+ * cases those objects can be const-casted-away and then mutated. That comes from
  * the fact that we share some object's life-cycle responsibilities with React
  * and the immutability is guaranteed by some logic splitted between native and
- * JavaScript worlds (which makes impossible to fully use immutability
+ * JavaScript worlds (which makes it impossible to fully use immutability
  * enforcement at a language level).
  * To detect possible errors as early as possible we additionally mark objects
- * as *sealed* after some stage and then enforce this at run-time.
+ * as *sealed* after some stages and then enforce this at run-time.
  *
  * How to use:
  *   1. Inherit your class from `Sealable`.
@@ -31,6 +37,13 @@ namespace react {
  */
 class Sealable {
 public:
+  Sealable();
+  Sealable(const Sealable& other);
+  Sealable(Sealable&& other) noexcept;
+  ~Sealable() noexcept;
+  Sealable& operator=(const Sealable& other);
+  Sealable& operator=(Sealable&& other) noexcept;
+
   /*
    * Seals the object. This operation is irreversible;
    * the object cannot be "unsealed" after being sealing.
@@ -50,7 +63,7 @@ protected:
   void ensureUnsealed() const;
 
 private:
-  mutable bool sealed_ {false};
+  mutable std::atomic<bool> sealed_ {false};
 };
 
 } // namespace react