Add ImmutableRequestData

Summary: A simple folly::RequestData implementation.

Reviewed By: praihan

Differential Revision: D31738010

fbshipit-source-id: 3d7c3394fd3daf2dbb2942fe6cdac25207e04e32

folly/io/async/Request.h

@@ -20,6 +20,7 @@
 #include <memory>
 #include <mutex>
 #include <string>
+#include <type_traits>
 #include <utility>
 
 #include <folly/SingletonThreadLocal.h>
@@ -129,6 +130,29 @@ class RequestData {
   std::atomic<int> keepAliveCounter_{0};
 };
 
+/**
+ * ImmutableRequestData is a folly::RequestData that holds an immutable value.
+ * It is thread-safe (a requirement of RequestData) because it is immutable.
+ */
+template <typename T>
+class ImmutableRequestData : public folly::RequestData {
+ public:
+  template <
+      typename... Args,
+      typename = typename std::enable_if<
+          std::is_constructible<T, Args...>::value>::type>
+  explicit ImmutableRequestData(Args&&... args) noexcept(
+      std::is_nothrow_constructible<T, Args...>::value)
+      : val_(std::forward<Args>(args)...) {}
+
+  const T& value() const { return val_; }
+
+  bool hasCallback() override { return false; }
+
+ private:
+  const T val_;
+};
+
 using RequestDataItem = std::pair<RequestToken, std::unique_ptr<RequestData>>;
 
 // If you do not call create() to create a unique request context,

folly/io/async/test/RequestContextTest.cpp

@@ -600,3 +600,20 @@ TEST(RequestContextTryGetTest, TryGetTest) {
   });
   thread.join();
 }
+
+TEST(ImmutableRequestTest, simple) {
+  ImmutableRequestData<int> ird(4);
+  EXPECT_EQ(ird.value(), 4);
+}
+
+TEST(ImmutableRequestTest, type_traits) {
+  using IRDI = ImmutableRequestData<int>;
+
+  auto c1 = std::is_constructible<IRDI, int>::value;
+  EXPECT_TRUE(c1);
+  auto n1 = std::is_nothrow_constructible<IRDI, int>::value;
+  EXPECT_TRUE(n1);
+
+  auto c2 = std::is_constructible<IRDI, int, int>::value;
+  EXPECT_FALSE(c2);
+}