From 1383754f90945eeb8409bcce024be01557f0ebfb Mon Sep 17 00:00:00 2001
From: Benjamin Kietzman <bengilgit@gmail.com>
Date: Tue, 6 Oct 2020 10:27:22 -0400
Subject: [PATCH 1/2] ARROW-10196: [C++] Add Future::DeferNotOk

---
 cpp/src/arrow/util/future.h      | 27 +++++++++++----------------
 cpp/src/arrow/util/thread_pool.h |  4 +---
 2 files changed, 12 insertions(+), 19 deletions(-)

diff --git a/cpp/src/arrow/util/future.h b/cpp/src/arrow/util/future.h
index 19fd632c065..44f66549a41 100644
--- a/cpp/src/arrow/util/future.h
+++ b/cpp/src/arrow/util/future.h
@@ -154,9 +154,8 @@ class FutureStorage : public FutureStorageBase {
 
   Status status() const { return result_.status(); }
 
-  template <typename U>
-  void MarkFinished(U&& value) {
-    result_ = std::forward<U>(value);
+  void MarkFinished(Result<T> result) {
+    result_ = std::move(result);
     if (ARROW_PREDICT_TRUE(result_.ok())) {
       impl_->MarkFinished();
     } else {
@@ -250,7 +249,7 @@ class Future {
   // The default constructor creates an invalid Future.  Use Future::Make()
   // for a valid Future.  This constructor is mostly for the convenience
   // of being able to presize a vector of Futures.
-  Future() : impl_(NULLPTR) {}
+  Future() = default;
 
   // Consumer API
 
@@ -358,6 +357,13 @@ class Future {
     return fut;
   }
 
+  static Future DeferNotOk(Result<Future> maybe_future) {
+    if (ARROW_PREDICT_FALSE(!maybe_future.ok())) {
+      return MakeFinished(std::move(maybe_future).status());
+    }
+    return std::move(maybe_future).MoveValueUnsafe();
+  }
+
  protected:
   void CheckValid() const {
 #ifndef NDEBUG
@@ -368,7 +374,7 @@ class Future {
   }
 
   std::shared_ptr<FutureStorage<T>> storage_;
-  FutureImpl* impl_;
+  FutureImpl* impl_ = NULLPTR;
 
   friend class FutureWaiter;
 };
@@ -419,15 +425,4 @@ inline std::vector<int> WaitForAny(const std::vector<Future<T>*>& futures,
   return waiter->MoveFinishedFutures();
 }
 
-#define ARROW_ASSIGN_OR_RETURN_FUTURE_IMPL(result_name, lhs, T, rexpr) \
-  auto result_name = (rexpr);                                          \
-  if (ARROW_PREDICT_FALSE(!(result_name).ok())) {                      \
-    return Future<T>::MakeFinished(std::move(result_name).status());   \
-  }                                                                    \
-  lhs = std::move(result_name).MoveValueUnsafe();
-
-#define ARROW_ASSIGN_OR_RETURN_FUTURE(lhs, T, rexpr) \
-  ARROW_ASSIGN_OR_RETURN_FUTURE_IMPL(                \
-      ARROW_ASSIGN_OR_RAISE_NAME(_error_or_value, __COUNTER__), lhs, T, rexpr);
-
 }  // namespace arrow
diff --git a/cpp/src/arrow/util/thread_pool.h b/cpp/src/arrow/util/thread_pool.h
index fa6583e7371..45dc1ca7145 100644
--- a/cpp/src/arrow/util/thread_pool.h
+++ b/cpp/src/arrow/util/thread_pool.h
@@ -143,10 +143,8 @@ class ARROW_EXPORT Executor {
       typename RT = typename detail::ExecutorResultTraits<FunctionRetType>,
       typename ValueType = typename RT::ValueType>
   Future<ValueType> SubmitAsFuture(Function&& func, Args&&... args) {
-    ARROW_ASSIGN_OR_RETURN_FUTURE(
-        auto future, ValueType,
+    return Future<ValueType>::DeferNotOk(
         Submit(std::forward<Function>(func), std::forward<Args>(args)...));
-    return future;
   }
 
   // Return the level of parallelism (the number of tasks that may be executed

From 1f54d6152c5880836856017c9033debd6ae6eb43 Mon Sep 17 00:00:00 2001
From: Benjamin Kietzman <bengilgit@gmail.com>
Date: Wed, 7 Oct 2020 10:29:44 -0400
Subject: [PATCH 2/2] add doccomment

---
 cpp/src/arrow/util/future.h | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/cpp/src/arrow/util/future.h b/cpp/src/arrow/util/future.h
index 44f66549a41..575f5cb3c41 100644
--- a/cpp/src/arrow/util/future.h
+++ b/cpp/src/arrow/util/future.h
@@ -310,6 +310,15 @@ class Future {
     return impl_->Wait(seconds);
   }
 
+  /// If a Result<Future> holds an error instead of a Future, construct a finished Future
+  /// holding that error.
+  static Future DeferNotOk(Result<Future> maybe_future) {
+    if (ARROW_PREDICT_FALSE(!maybe_future.ok())) {
+      return MakeFinished(std::move(maybe_future).status());
+    }
+    return std::move(maybe_future).MoveValueUnsafe();
+  }
+
   // Producer API
 
   /// \brief Producer API: execute function and mark Future finished
@@ -357,13 +366,6 @@ class Future {
     return fut;
   }
 
-  static Future DeferNotOk(Result<Future> maybe_future) {
-    if (ARROW_PREDICT_FALSE(!maybe_future.ok())) {
-      return MakeFinished(std::move(maybe_future).status());
-    }
-    return std::move(maybe_future).MoveValueUnsafe();
-  }
-
  protected:
   void CheckValid() const {
 #ifndef NDEBUG