add Executor::KeepAlive::copy() method

Summary: We do not want implicit copies of `KeepAlive`, because they may be accidental and they impose an overhead. However, making a copy by calling `folly::getKeepAliveToken(keep_alive.get())` does not look very nice. Also, in case that `keep_alive` is a dummy, it makes an unnecessary call to `Executor::keepAliveAcquire`, which will return false but cannot be inlined usually because it's a virtual function. This diff adds a `copy() const` method which returns a copy of the `KeepAlive` object. It only calls `Executor::keepAliveAcquire` if the original object is not a dummy.

Reviewed By: yfeldblum, andriigrynenko

Differential Revision: D8203839

fbshipit-source-id: bb595d3836405bd84e55c4214ac19127b1e0e42a

folly/Executor.h

@@ -110,6 +110,10 @@ class Executor {
       return get();
     }
 
+    KeepAlive copy() const {
+      return getKeepAliveToken(get());
+    }
+
    private:
     static constexpr intptr_t kDummyFlag = 1;
     static constexpr intptr_t kExecutorMask = ~kDummyFlag;

folly/test/ExecutorTest.cpp

@@ -91,4 +91,23 @@ TEST(ExecutorTest, KeepAliveConvert) {
   EXPECT_EQ(0, exec.refCount);
 }
 
+TEST(ExecutorTest, KeepAliveCopy) {
+  KeepAliveTestExecutor exec;
+
+  {
+    Executor::KeepAlive<KeepAliveTestExecutor>&& ka = getKeepAliveToken(exec);
+    EXPECT_TRUE(ka);
+    EXPECT_EQ(std::addressof(exec), ka.get());
+    EXPECT_EQ(1, exec.refCount);
+
+    Executor::KeepAlive<KeepAliveTestExecutor>&& ka2 = ka.copy();
+    EXPECT_TRUE(ka);
+    EXPECT_TRUE(ka2);
+    EXPECT_EQ(std::addressof(exec), ka2.get());
+    EXPECT_EQ(2, exec.refCount);
+  }
+
+  EXPECT_EQ(0, exec.refCount);
+}
+
 } // namespace folly