Add SharedFuture

.bazelrc

@@ -1 +1 @@
-build --cxxopt=-std=c++17 --host_cxxopt=-std=c++17 --incompatible_java_common_parameters=false --define=android_dexmerger_tool=d8_dexmerger --define=android_incremental_dexing_tool=d8_dexbuilder --nouse_workers_with_dexbuilder
+build --cxxopt=-std=c++17 --cxxopt=-fcoroutines-ts --host_cxxopt=-std=c++17 --host_cxxopt=-fcoroutines-ts --incompatible_java_common_parameters=false --define=android_dexmerger_tool=d8_dexmerger --define=android_incremental_dexing_tool=d8_dexbuilder --nouse_workers_with_dexbuilder

support-lib/cpp/Future.hpp

@@ -379,7 +379,8 @@ class Future {
             constexpr bool await_ready() const noexcept {
                 return false;
             }
-            bool await_suspend(detail::CoroutineHandle<ConcretePromise> finished) const noexcept {
+            template <typename P>
+            bool await_suspend(detail::CoroutineHandle<P> finished) const noexcept {
                 auto& promise_type = finished.promise();
                 if (*promise_type._result) {
                     if constexpr (std::is_void_v<T>) {

support-lib/cpp/SharedFuture.hpp

@@ -0,0 +1,138 @@
+/**
+ * Copyright 2021 Snap, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+ #pragma once
+
+#include "Future.hpp"
+
+#include <memory>
+#include <optional>
+#include <type_traits>
+#include <variant>
+#include <vector>
+
+namespace djinni {
+
+// SharedFuture is a wrapper around djinni::Future to allow multiple consumers (i.e. like std::shared_future)
+// The API is designed to be similar to djinni::Future.
+template<typename T>
+class SharedFuture {
+public:
+    // Create SharedFuture from Future. Runtime error if the future is already consumed.
+    explicit SharedFuture(Future<T>&& future);
+
+    // Transform into Future<T>.
+    Future<T> toFuture() const {
+        if (await_ready()) {
+            co_return await_resume(); // return stored value directly
+        }
+        co_return co_await SharedFuture(*this); // retain copy during coroutine suspension
+    }
+
+    void wait() const {
+        return [this]() -> Future<void> { co_await *this; }().wait();
+    }
+
+    decltype(auto) get() const {
+        wait();
+        return await_resume();
+    }
+
+    // Transform the result of this future into a new future. The behavior is same as Future::then except that
+    // it doesn't consume the future, and can be called multiple times.
+    template<typename Func>
+    SharedFuture<std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<Func, T>>>> then(Func transform) const {
+        co_return transform(co_await SharedFuture(*this)); // retain copy during coroutine suspension
+    }
+
+    // Overload for T = void or `transform` takes no arugment.
+    template<typename Func, typename = std::enable_if_t<!std::is_invocable_v<Func, T>>>
+    SharedFuture<std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<Func>>>> then(Func transform) const {
+        co_await SharedFuture(*this); // retain copy during coroutine suspension
+        co_return transform();
+    }
+
+    // -- coroutine support implementation only; not intended externally --
+
+    bool await_ready() const {
+        std::scoped_lock lock(_sharedStates->mutex);
+        return _sharedStates->storedValue.has_value();
+    }
+
+    decltype(auto) await_resume() const {
+        if constexpr (!std::is_void_v<T>) {
+            return *_sharedStates->storedValue;
+        }
+    }
+
+    bool await_suspend(detail::CoroutineHandle<> h) const;
+
+    struct Promise : public Future<T>::promise_type {
+        SharedFuture<T> get_return_object() noexcept {
+            return SharedFuture(Future<T>::promise_type::get_return_object());
+        }
+    };
+    using promise_type = Promise;
+
+private:
+    struct SharedStates {
+        std::recursive_mutex mutex;
+        std::optional<std::conditional_t<std::is_void_v<T>, std::monostate, T>> storedValue = std::nullopt;
+        std::vector<detail::CoroutineHandle<>> coroutineHandles;
+    };
+    // Use a shared_ptr to allow copying SharedFuture.
+    std::shared_ptr<SharedStates> _sharedStates = std::make_shared<SharedStates>();
+};
+
+// CTAD deduction guide to construct from Future directly.
+template<typename T>
+SharedFuture(Future<T>&&) -> SharedFuture<T>;
+
+// ------------------ Implementation ------------------
+
+template<typename T>
+SharedFuture<T>::SharedFuture(Future<T>&& future) {
+    // `future` will invoke all continuations when it is ready.
+    future.then([sharedStates = _sharedStates](auto futureResult) {
+        std::vector toCall = [&] {
+            std::scoped_lock lock(sharedStates->mutex);
+            if constexpr (std::is_void_v<T>) {
+                sharedStates->storedValue.emplace();
+            } else {
+                sharedStates->storedValue = futureResult.get();
+            }
+            return std::move(sharedStates->coroutineHandles);
+        }();
+        for (auto& handle : toCall) {
+            handle();
+        }
+    });
+}
+
+template<typename T>
+bool SharedFuture<T>::await_suspend(detail::CoroutineHandle<> h) const {
+    {
+        std::unique_lock lock(_sharedStates->mutex);
+        if (!_sharedStates->storedValue) {
+            _sharedStates->coroutineHandles.push_back(std::move(h));
+            return true;
+        }
+    }
+    h();
+    return true;
+}
+
+} // namespace djinni

test-suite/BUILD

@@ -55,6 +55,7 @@ objc_library(
     copts = [
         "-ObjC++",
         "-std=c++17",
+        "-fcoroutines-ts"
     ],
     srcs = glob([
         "generated-src/objc/**/*.mm",

test-suite/handwritten-src/objc/tests/DBSharedFutureTest.mm

@@ -0,0 +1,61 @@
+#import <Foundation/Foundation.h>
+#import <XCTest/XCTest.h>
+
+#include "../../../support-lib/cpp/SharedFuture.hpp"
+
+@interface DBSharedFutureTest : XCTestCase
+@end
+
+@implementation DBSharedFutureTest
+
+- (void)setUp
+{
+    [super setUp];
+}
+
+- (void)tearDown
+{
+    [super tearDown];
+}
+
+- (void)testCreateFuture
+{
+    djinni::SharedFuture<int> resolvedInt(djinni::Promise<int>::resolve(42));
+    XCTAssertEqual(resolvedInt.get(), 42);
+
+    djinni::Promise<NSString*> strPromise;
+    djinni::SharedFuture futureString(strPromise.getFuture());
+
+    strPromise.setValue(@"foo");
+    XCTAssertEqualObjects(futureString.get(), @"foo");
+}
+
+- (void)testThen
+{
+    djinni::Promise<int> intPromise;
+    djinni::SharedFuture<int> futureInt(intPromise.getFuture());
+
+    auto transformedInt = futureInt.then([](int i) { return 2 * i; });
+
+    intPromise.setValue(42);
+    XCTAssertEqual(transformedInt.get(), 84);
+
+    // Also verify multiple consumers and chaining.
+    auto transformedString = futureInt.then([](int i) { return std::to_string(i); });
+    auto futurePlusOneTimesTwo = futureInt.then([](int i) { return i + 1; }).then([](int i) { return 2 * i; });
+    auto futureStringLen = transformedString.then([](const std::string& s) { return s.length(); }).toFuture();
+
+    XCTAssertEqual(transformedString.get(), std::string("42"));
+    XCTAssertEqual(futurePlusOneTimesTwo.get(), (42 + 1) * 2);
+    XCTAssertEqual(futureStringLen.get(), 2);
+
+    XCTAssertEqual(futureInt.get(), 42);
+
+    auto voidFuture = transformedString.then([]() {});
+    voidFuture.wait();
+
+    auto intFuture2 = voidFuture.then([]() { return 43; });
+    XCTAssertEqual(intFuture2.get(), 43);
+}
+
+@end