folly/futures: replace MoveWrappers with generalised lambda capture

[folly.git] / folly / futures / detail / Core.h

diff --git a/folly/futures/detail/Core.h b/folly/futures/detail/Core.h
index ecacb280591590ab8a4044d17174ff5541ef1cc5..a41059009d48cf87d8b600e530141ad2a2bcfd98 100644 (file)
--- a/folly/futures/detail/Core.h
+++ b/folly/futures/detail/Core.h
@@ -21,13 +21,13 @@
 #include <stdexcept>
 #include <vector>
 
-#include <folly/Optional.h>
+#include <folly/Executor.h>
+#include <folly/Function.h>
 #include <folly/MicroSpinLock.h>
-
-#include <folly/futures/Try.h>
-#include <folly/futures/Promise.h>
+#include <folly/Optional.h>
 #include <folly/futures/Future.h>
-#include <folly/Executor.h>
+#include <folly/futures/Promise.h>
+#include <folly/futures/Try.h>
 #include <folly/futures/detail/FSM.h>
 
 #include <folly/io/async/Request.h>
@@ -99,6 +99,26 @@ class Core {
   Core(Core&&) noexcept = delete;
   Core& operator=(Core&&) = delete;
 
+  // Core is assumed to be convertible only if the type is convertible
+  // and the size is the same. This is a compromise for the complexity
+  // of having to make Core truly have a conversion constructor which
+  // would cause various other problems.
+  // If we made Core move constructible then we would need to update the
+  // Promise and Future with the location of the new Core. This is complex
+  // and may be inefficient.
+  // Core should only be modified so that for size(T) == size(U),
+  // sizeof(Core<T>) == size(Core<U>).
+  // This assumption is used as a proxy to make sure that
+  // the members of Core<T> and Core<U> line up so that we can use a
+  // reinterpret cast.
+  template <
+      class U,
+      typename = typename std::enable_if<std::is_convertible<U, T>::value &&
+                                         sizeof(U) == sizeof(T)>::type>
+  static Core<T>* convert(Core<U>* from) {
+    return reinterpret_cast<Core<T>*>(from);
+  }
+
   /// May call from any thread
   bool hasResult() const {
     switch (fsm_.getState()) {
@@ -127,34 +147,13 @@ class Core {
     }
   }
 
-  template <typename F>
-  class LambdaBufHelper {
-   public:
-    template <typename FF>
-    explicit LambdaBufHelper(FF&& func) : func_(std::forward<FF>(func)) {}
-    void operator()(Try<T>&& t) {
-      SCOPE_EXIT { this->~LambdaBufHelper(); };
-      func_(std::move(t));
-    }
-   private:
-    F func_;
-  };
-
   /// Call only from Future thread.
   template <typename F>
-  void setCallback(F func) {
+  void setCallback(F&& func) {
     bool transitionToArmed = false;
     auto setCallback_ = [&]{
       context_ = RequestContext::saveContext();
-
-      // Move the lambda into the Core if it fits
-      if (sizeof(LambdaBufHelper<F>) <= lambdaBufSize) {
-        auto funcLoc = reinterpret_cast<LambdaBufHelper<F>*>(&lambdaBuf_);
-        new (funcLoc) LambdaBufHelper<F>(std::forward<F>(func));
-        callback_ = std::ref(*funcLoc);
-      } else {
-        callback_ = std::move(func);
-      }
+      callback_ = std::forward<F>(func);
     };
 
     FSM_START(fsm_)
@@ -327,9 +326,10 @@ class Core {
       executorLock_.unlock();
     }
 
+    // keep Core alive until callback did its thing
+    ++attached_;
+
     if (x) {
-      // keep Core alive until executor did its thing
-      ++attached_;
       try {
         if (LIKELY(x->getNumPriorities() == 1)) {
           x->add([this]() mutable {
@@ -354,6 +354,7 @@ class Core {
         callback_(std::move(*result_));
       }
     } else {
+      SCOPE_EXIT { detachOne(); };
       RequestContext::setContext(context_);
       SCOPE_EXIT { callback_ = {}; };
       callback_(std::move(*result_));
@@ -369,13 +370,18 @@ class Core {
     }
   }
 
-  // lambdaBuf occupies exactly one cache line
-  static constexpr size_t lambdaBufSize = 8 * sizeof(void*);
-  typename std::aligned_storage<lambdaBufSize>::type lambdaBuf_;
+  // Core should only be modified so that for size(T) == size(U),
+  // sizeof(Core<T>) == size(Core<U>).
+  // See Core::convert for details.
+
+  folly::Function<
+      void(Try<T>&&),
+      folly::FunctionMoveCtor::MAY_THROW,
+      8 * sizeof(void*)>
+      callback_;
   // place result_ next to increase the likelihood that the value will be
   // contained entirely in one cache line
   folly::Optional<Try<T>> result_;
-  std::function<void(Try<T>&&)> callback_ {nullptr};
   FSM<State> fsm_;
   std::atomic<unsigned char> attached_;
   std::atomic<bool> active_ {true};