[reland] Allow Future::then to return pre-extracted DataPtrs

lw · lw · commit 056bf4e754e0 · 2021-05-31T05:26:07.000-07:00
Reland of #58424 In CUDA mode, Future must inspect its value and extract DataPtrs. However some types are not supported, for example the C++/JIT custom classes, which include Message, which is widely used in RPC. Hence for these scenarios we allow the user to perform the custom DataPtr extraction on their own, and pass the pre-extracted DataPtrs. Note that `markCompleted` already allowed users to pass in pre-extracted DataPtrs, hence this PR simply extends this possibility to the `then` method too. Differential Revision: [D28623890](https://our.internmc.facebook.com/intern/diff/D28623890/) [ghstack-poisoned]
diff --git a/aten/src/ATen/core/ivalue_inl.h b/aten/src/ATen/core/ivalue_inl.h
@@ -518,20 +518,36 @@ struct C10_EXPORT ivalue::Future final : c10::intrusive_ptr_target {
    */
   template <typename T>
   c10::intrusive_ptr<Future> then(T callback, TypePtr type) {
+    using IValueWithDataPtrs = std::
+        tuple<IValue, std::vector<std::reference_wrapper<const at::DataPtr>>>;
 #if __cpp_lib_is_invocable >= 201703
     static_assert(
-        std::is_invocable_r<IValue, T, Future&>::value,
-        "The callback must have signature IValue(Future&)");
+        guts::disjunction<
+            std::is_invocable_r<IValue, T, Future&>,
+            std::is_invocable_r<IValueWithDataPtrs, T, Future&>>::value,
+        "The callback must have signature IValue(Future&) or "
+        "std::tuple<IValue, std::vector<std::reference_wrapper<const DataPtr>>>(Future&)");
 #endif
     auto childFut = createInstance(std::move(type));
-    addCallback(
-        [childFut, cb = std::move(callback)](Future& parentFut) mutable {
-          try {
-            childFut->markCompleted(cb(parentFut));
-          } catch (std::exception&) {
-            childFut->setError(std::current_exception());
-          }
-        });
+    addCallback([childFut,
+                 cb = std::move(callback)](Future& parentFut) mutable {
+      try {
+        guts::if_constexpr<std::is_convertible<
+            typename std::result_of<T && (Future&)>::type,
+            IValueWithDataPtrs>::value>(
+            [&](auto identity) {
+              IValue value;
+              std::vector<std::reference_wrapper<const at::DataPtr>> dataPtrs;
+              std::tie(value, dataPtrs) = identity(cb)(parentFut);
+              childFut->markCompleted(std::move(value), std::move(dataPtrs));
+            },
+            [&](auto identity) {
+              childFut->markCompleted(identity(cb)(parentFut));
+            });
+      } catch (std::exception&) {
+        childFut->setError(std::current_exception());
+      }
+    });
     return childFut;
   }
 
