From 752e5cc25274556a8dbab7a0da772d54bf7c89bb Mon Sep 17 00:00:00 2001
From: Giulio Eulisse <giulio.eulisse@cern.ch>
Date: Sun, 14 Jun 2020 18:33:59 +0200
Subject: [PATCH] DPL: improve ScopedExit

---
 Framework/Core/src/DataProcessingDevice.cxx |   4 +-
 Framework/Core/src/ScopedExit.h             | 143 +++++++++++++++++---
 2 files changed, 129 insertions(+), 18 deletions(-)

diff --git a/Framework/Core/src/DataProcessingDevice.cxx b/Framework/Core/src/DataProcessingDevice.cxx
index a835ea02ea50e..b2e1799a88ad5 100644
--- a/Framework/Core/src/DataProcessingDevice.cxx
+++ b/Framework/Core/src/DataProcessingDevice.cxx
@@ -424,7 +424,7 @@ bool DataProcessingDevice::handleData(FairMQParts& parts, InputChannelInfo& info
   // fine grain control what is exposed at each state.
   auto& monitoringService = mServiceRegistry.get<Monitoring>();
   StateMonitoring<DataProcessingStatus>::moveTo(DataProcessingStatus::IN_DPL_OVERHEAD);
-  ScopedExit metricFlusher([&monitoringService] {
+  auto metricFlusher = make_scope_guard([]() noexcept {
     StateMonitoring<DataProcessingStatus>::moveTo(DataProcessingStatus::IN_DPL_OVERHEAD);
   });
 
@@ -554,7 +554,7 @@ bool DataProcessingDevice::tryDispatchComputation(std::vector<DataRelayer::Recor
   // FIXME: I should use a different id for this state.
   auto& monitoringService = mServiceRegistry.get<Monitoring>();
   StateMonitoring<DataProcessingStatus>::moveTo(DataProcessingStatus::IN_DPL_OVERHEAD);
-  ScopedExit metricFlusher([&monitoringService] {
+  auto metricFlusher = make_scope_guard([]() noexcept -> void {
     StateMonitoring<DataProcessingStatus>::moveTo(DataProcessingStatus::IN_DPL_OVERHEAD);
   });
 
diff --git a/Framework/Core/src/ScopedExit.h b/Framework/Core/src/ScopedExit.h
index feb40fee2556d..cf5533591fcd9 100644
--- a/Framework/Core/src/ScopedExit.h
+++ b/Framework/Core/src/ScopedExit.h
@@ -7,30 +7,141 @@
 // In applying this license CERN does not waive the privileges and immunities
 // granted to it by virtue of its status as an Intergovernmental Organization
 // or submit itself to any jurisdiction.
-#include <functional>
+#include <type_traits>
+#include <utility>
 
-namespace o2
+namespace o2::framework
 {
-namespace framework
+namespace detail
 {
+// Original from https://github.com/ricab/scope_guard
+// which is licensed to public domain
+// Type trait determining whether a type is callable with no arguments
+template <typename T, typename = void>
+struct is_noarg_callable_t
+  : public std::false_type {
+}; // in general, false
 
-/// Helper class which invokes @a callback when going out of scope.
-class ScopedExit
+template <typename T>
+struct is_noarg_callable_t<T, decltype(std::declval<T&&>()())>
+  : public std::true_type {
+}; // only true when call expression valid
+
+// Type trait determining whether a no-argument callable returns void
+template <typename T>
+struct returns_void_t
+  : public std::is_same<void, decltype(std::declval<T&&>()())> {
+};
+
+/* Type trait determining whether a no-arg callable is nothrow invocable if
+  required. This is where SG_REQUIRE_NOEXCEPT logic is encapsulated. */
+template <typename T>
+struct is_nothrow_invocable_if_required_t
+  : public std::is_nothrow_invocable<T> /* Note: _r variants not enough to
+                                        confirm void return: any return can be
+                                        discarded so all returns are
+                                        compatible with void */
+{
+};
+
+template <typename A, typename B, typename... C>
+struct and_t : public and_t<A, and_t<B, C...>> {
+};
+
+template <typename A, typename B>
+struct and_t<A, B> : public std::conditional<A::value, B, A>::type {
+};
+
+template <typename T>
+struct is_proper_sg_callback_t
+  : public and_t<is_noarg_callable_t<T>,
+                 returns_void_t<T>,
+                 is_nothrow_invocable_if_required_t<T>,
+                 std::is_nothrow_destructible<T>> {
+};
+
+template <typename Callback,
+          typename = typename std::enable_if<
+            is_proper_sg_callback_t<Callback>::value>::type>
+class scope_guard;
+
+template <typename Callback>
+detail::scope_guard<Callback> make_scope_guard(Callback&& callback) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value);
+
+template <typename Callback>
+class scope_guard<Callback> final
 {
  public:
-  ScopedExit(std::function<void(void)> callback)
-    : mCallback{callback}
-  {
-  }
+  typedef Callback callback_type;
 
-  ~ScopedExit()
-  {
-    mCallback();
-  }
+  scope_guard(scope_guard&& other) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value);
+
+  ~scope_guard() noexcept; // highlight noexcept dtor
+
+  void dismiss() noexcept;
+
+ public:
+  scope_guard() = delete;
+  scope_guard(const scope_guard&) = delete;
+  scope_guard& operator=(const scope_guard&) = delete;
+  scope_guard& operator=(scope_guard&&) = delete;
+
+ private:
+  explicit scope_guard(Callback&& callback) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value); /*
+                                                    meant for friends only */
+
+  friend scope_guard<Callback> make_scope_guard<Callback>(Callback&&) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value); /*
+    only make_scope_guard can create scope_guards from scratch (i.e. non-move)
+    */
 
  private:
-  std::function<void(void)> mCallback;
+  Callback mCallback;
+  bool mActive;
 };
 
-} // namespace framework
-} // namespace o2
+} // namespace detail
+
+using detail::make_scope_guard; // see comment on declaration above
+
+template <typename Callback>
+detail::scope_guard<Callback>::scope_guard(Callback&& callback) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
+  : mCallback(std::forward<Callback>(callback)) /* use () instead of {} because
+    of DR 1467 (https://is.gd/WHmWuo), which still impacts older compilers
+    (e.g. GCC 4.x and clang <=3.6, see https://godbolt.org/g/TE9tPJ and
+    https://is.gd/Tsmh8G) */
+    ,
+    mActive{true}
+{
+}
+
+template <typename Callback>
+detail::scope_guard<Callback>::~scope_guard() noexcept
+{
+  if (mActive) {
+    mCallback();
+  }
+}
+
+template <typename Callback>
+detail::scope_guard<Callback>::scope_guard(scope_guard&& other) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
+  : mCallback(std::forward<Callback>(other.mCallback)) // idem
+    ,
+    mActive{std::move(other.mActive)}
+{
+  other.mActive = false;
+}
+
+template <typename Callback>
+inline void detail::scope_guard<Callback>::dismiss() noexcept
+{
+  mActive = false;
+}
+
+template <typename Callback>
+inline auto detail::make_scope_guard(Callback&& callback) noexcept(std::is_nothrow_constructible<Callback, Callback&&>::value)
+  -> detail::scope_guard<Callback>
+{
+  return detail::scope_guard<Callback>{std::forward<Callback>(callback)};
+}
+
+} // namespace o2::framework