From 8f2e8075d660a354a3aabf065998f42802f9cc11 Mon Sep 17 00:00:00 2001
From: Gabriel Corona <gabriel.corona@loria.fr>
Date: Mon, 20 Jun 2016 13:59:40 +0200
Subject: [PATCH] Make FutureContinuation reusable in simgrid::xbt::Task

---
 include/simgrid/kernel/future.hpp | 61 ++++---------------------
 include/xbt/functional.hpp        | 75 +++++++++++++++++++++++++++++++
 2 files changed, 84 insertions(+), 52 deletions(-)

diff --git a/include/simgrid/kernel/future.hpp b/include/simgrid/kernel/future.hpp
index c58f46b164..a11c8ec40c 100644
--- a/include/simgrid/kernel/future.hpp
+++ b/include/simgrid/kernel/future.hpp
@@ -10,6 +10,7 @@
 #include <boost/optional.hpp>
 
 #include <xbt/base.h>
+#include <xbt/functional.hpp>
 
 #include <functional>
 #include <future>
@@ -27,8 +28,6 @@ template<class T> class Promise;
 // Those are implementation details:
 enum class FutureStatus;
 template<class T> class FutureState;
-class FutureContinuation;
-template<class T, class F> class FutureContinuationImpl;
 
 enum class FutureStatus {
   not_ready,
@@ -36,47 +35,6 @@ enum class FutureStatus {
   done,
 };
 
-/** A continuation attached to a future to be executed when it is ready */
-XBT_PUBLIC_CLASS FutureContinuation {
-public:
-  FutureContinuation() {}
-
-  // No copy:
-  FutureContinuation(FutureContinuation&) = delete;
-  FutureContinuation& operator=(FutureContinuation&) = delete;
-
-  virtual ~FutureContinuation() {}
-  virtual void operator()() = 0;
-};
-
-/** Default implementation of `FutureContinuation`
- *
- *  @param T   value type of the future
- *  @param F   type of the wrapped code/callback/continuation
- */
-template<class T, class F>
-class FutureContinuationImpl : public FutureContinuation {
-public:
-  FutureContinuationImpl(std::shared_ptr<FutureState<T>> ptr, F callback)
-    : ptr_(std::move(ptr)), callback_(std::move(callback)) {}
-  ~FutureContinuationImpl() override {}
-  void operator()() override
-  {
-    try {
-      callback_(Future<T>(ptr_));
-    }
-    // Those exceptions are lost.
-    // If we want to implement callback chaining, we'll have to catch them and
-    // foward them to the next future.
-    catch (...) {
-      // We could log this.
-    }
-  }
-private:
-  std::shared_ptr<FutureState<T>> ptr_;
-  F callback_;
-};
-
 /** Bases stuff for all @ref simgrid::kernel::FutureState<T> */
 class FutureStateBase {
 public:
@@ -93,7 +51,7 @@ public:
     this->set_ready();
   }
 
-  void set_continuation(std::unique_ptr<FutureContinuation> continuation)
+  void set_continuation(simgrid::xbt::Task<void()> continuation)
   {
     xbt_assert(!continuation_);
     switch (status_) {
@@ -105,7 +63,7 @@ public:
     case FutureStatus::ready:
       // The future is ready, execute the continuation directly.
       // We might execute it from the event loop instead:
-      (*continuation)();
+      continuation();
       break;
     case FutureStatus::not_ready:
       // The future is not ready so we mast keep the continuation for
@@ -138,7 +96,7 @@ protected:
       // We need to do this becase the current implementation of the
       // continuation has a shared_ptr to the FutureState.
       auto continuation = std::move(continuation_);
-      (*continuation)();
+      continuation();
     }
   }
 
@@ -160,7 +118,7 @@ protected:
 private:
   FutureStatus status_ = FutureStatus::not_ready;
   std::exception_ptr exception_;
-  std::unique_ptr<FutureContinuation> continuation_;
+  simgrid::xbt::Task<void()> continuation_;
 };
 
 /** Shared state for future and promises
@@ -345,11 +303,10 @@ public:
   {
     if (state_ == nullptr)
       throw std::future_error(std::future_errc::no_state);
-    std::unique_ptr<FutureContinuation> ptr =
-      std::unique_ptr<FutureContinuation>(
-        new FutureContinuationImpl<T,F>(state_, std::move(continuation)));
-    state_->set_continuation(std::move(ptr));
-    state_ = nullptr;
+    // Give shared-ownership to the continuation:
+    auto state = std::move(state_);
+    state->set_continuation(simgrid::xbt::makeTask(
+      std::move(continuation), state));
   }
 
   /** Get the value from the future
diff --git a/include/xbt/functional.hpp b/include/xbt/functional.hpp
index 537ba8f9dc..1f8abfbb20 100644
--- a/include/xbt/functional.hpp
+++ b/include/xbt/functional.hpp
@@ -13,6 +13,7 @@
 #include <functional>
 #include <future>
 #include <utility>
+#include <tuple>
 
 #include <xbt/sysdep.h>
 #include <xbt/utility.hpp>
@@ -153,6 +154,80 @@ constexpr auto apply(F&& f, Tuple&& t)
     >());
 }
 
+template<class T> class Task;
+
+/** Type-erased run-once task
+ *
+ *  * Like std::function but callable only once.
+ *    However, it works with move-only types.
+ *
+ *  * Like std::packaged_task<> but without the shared state.
+ */
+template<class R, class... Args>
+class Task<R(Args...)> {
+private:
+  // Type-erasure for the code:
+  class Base {
+  public:
+    virtual ~Base() {}
+    virtual R operator()(Args...) = 0;
+  };
+  template<class F>
+  class Impl : public Base {
+  public:
+    Impl(F&& code) : code_(std::move(code)) {}
+    Impl(F const& code) : code_(code) {}
+    ~Impl() override {}
+    R operator()(Args... args)
+    {
+      return code_(std::forward<Args>(args)...);
+    }
+  private:
+    F code_;
+  };
+  std::unique_ptr<Base> code_;
+public:
+  Task() {}
+  Task(std::nullptr_t) {}
+
+  template<class F>
+  Task(F&& code) :
+    code_(new Impl<F>(std::forward<F>(code))) {}
+
+  operator bool() const { return code_ != nullptr; }
+  bool operator!() const { return code_ == nullptr; }
+
+  template<class... OtherArgs>
+  R operator()(OtherArgs&&... args)
+  {
+    std::unique_ptr<Base> code = std::move(code_);
+    return (*code)(std::forward<OtherArgs>(args)...);
+  }
+};
+
+template<class F, class... Args>
+auto makeTask(F code, Args... args)
+-> Task< decltype(code(std::move(args)...))() >
+{
+  typedef decltype(code(std::move(args)...)) result_type;
+
+  class Impl {
+  private:
+    F code_;
+    std::tuple<Args...> args_;
+  public:
+    Impl(F code, std::tuple<Args...> args) :
+      code_(std::move(code)),
+      args_(std::move(args)) {}
+    result_type operator()()
+    {
+      return simgrid::xbt::apply(std::move(code_), std::move(args_));
+    }
+  };
+
+  return Impl(std::move(code), std::make_tuple(std::move(args)...));
+}
+
 }
 }
 
-- 
2.20.1