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

@@ -370,7 +370,6 @@ class Future {
         return true;
     }
 
-    template<typename ConcretePromise>
     struct PromiseTypeBase {
         Promise<T> _promise;
         std::optional<djinni::expected<T, std::exception_ptr>> _result{};
@@ -379,7 +378,9 @@ 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 {
+                static_assert(std::is_convertible_v<P*, PromiseTypeBase*>);
                 auto& promise_type = finished.promise();
                 if (*promise_type._result) {
                     if constexpr (std::is_void_v<T>) {
@@ -406,7 +407,7 @@ class Future {
         }
     };
 
-    struct PromiseType: PromiseTypeBase<PromiseType>{
+    struct PromiseType: PromiseTypeBase {
         template <typename V, typename = std::enable_if_t<std::is_convertible_v<V, T>>>
         void return_value(V&& value) {
             this->_result.emplace(std::forward<V>(value));
@@ -424,7 +425,7 @@ class Future {
 
 #if defined(DJINNI_FUTURE_HAS_COROUTINE_SUPPORT)
 template<>
-struct Future<void>::PromiseType : PromiseTypeBase<PromiseType> {
+struct Future<void>::PromiseType : PromiseTypeBase {
     void return_void() {
         _result.emplace();
     }

support-lib/cpp/SharedFuture.hpp

@@ -0,0 +1,162 @@
+/**
+ * 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"
+
+#if defined(DJINNI_FUTURE_HAS_COROUTINE_SUPPORT)
+
+#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 {
+        waitIgnoringExceptions().wait();
+    }
+
+    decltype(auto) get() const {
+        wait();
+        return await_resume();
+    }
+
+    template <typename Func>
+    using ResultT = std::remove_cv_t<std::remove_reference_t<std::invoke_result_t<Func, const SharedFuture<T>&>>>;
+
+    // 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>
+    Future<ResultT<Func>> then(Func transform) const {
+        auto cpy = SharedFuture(*this); // retain copy during coroutine suspension
+        co_await cpy.waitIgnoringExceptions();
+        co_return transform(cpy);
+    }
+
+    // Same as above but returns SharedFuture.
+    template<typename Func>
+    SharedFuture<ResultT<Func>> thenShared(Func transform) const {
+        return SharedFuture<ResultT<Func>>(then(std::move(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 (!*_sharedStates->storedValue) {
+            std::rethrow_exception(_sharedStates->storedValue->error());
+        }
+        if constexpr (!std::is_void_v<T>) {
+            return const_cast<const T &>(_sharedStates->storedValue->value());
+        }
+    }
+
+    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:
+    Future<void> waitIgnoringExceptions() const {
+        try {
+            co_await *this;
+        } catch (...) {
+            // Ignore exceptions.
+        }
+    }
+
+    struct SharedStates {
+        std::recursive_mutex mutex;
+        std::optional<djinni::expected<T, std::exception_ptr>> 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);
+            try {
+                if constexpr (std::is_void_v<T>) {
+                    futureResult.get();
+                    sharedStates->storedValue.emplace();
+                } else {
+                    sharedStates->storedValue = futureResult.get();
+                }
+            } catch (...) {
+                sharedStates->storedValue = make_unexpected(std::current_exception());
+            }
+            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
+
+#endif

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 +1,91 @@
+#import <Foundation/Foundation.h>
+#import <XCTest/XCTest.h>
+
+#include "../../../support-lib/cpp/SharedFuture.hpp"
+
+@interface DBSharedFutureTest : XCTestCase
+@end
+
+@implementation DBSharedFutureTest
+
+#ifdef DJINNI_FUTURE_HAS_COROUTINE_SUPPORT
+
+- (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.thenShared([](const auto& resolved) { return 2 * resolved.get(); });
+
+    intPromise.setValue(42);
+    XCTAssertEqual(transformedInt.get(), 84);
+
+    // Also verify multiple consumers and chaining.
+    auto transformedString = futureInt.thenShared([](const auto& resolved) { return std::to_string(resolved.get()); });
+    auto futurePlusOneTimesTwo = futureInt.then([](auto resolved) { return resolved.get() + 1; }).then([](auto resolved) {
+        return 2 * resolved.get();
+    });
+    auto futureStringLen = transformedString.then([](auto resolved) { return resolved.get().length(); });
+
+    XCTAssertEqual(transformedString.get(), std::string("42"));
+    XCTAssertEqual(futurePlusOneTimesTwo.get(), (42 + 1) * 2);
+    XCTAssertEqual(futureStringLen.get(), 2);
+
+    XCTAssertEqual(futureInt.get(), 42);
+
+    auto voidFuture = transformedString.thenShared([](auto) {});
+    voidFuture.wait();
+
+    auto intFuture2 = voidFuture.thenShared([](auto) { return 43; });
+    XCTAssertEqual(intFuture2.get(), 43);
+}
+
+- (void)testException
+{
+    // Also verify exception handling.
+    djinni::Promise<int> intPromise;
+    djinni::SharedFuture<int> futureInt(intPromise.getFuture());
+
+    intPromise.setException(std::runtime_error("mocked"));
+
+    XCTAssertThrows(futureInt.get());
+
+    auto thenResult = futureInt.then([](auto resolved) { return resolved.get(); });
+    XCTAssertThrows(thenResult.get());
+
+    auto withExceptionHandling = futureInt.thenShared([](const auto& resolved) {
+      try {
+        return resolved.get();
+      } catch (...) {
+        return -1;
+      }
+    });
+    withExceptionHandling.wait();
+    XCTAssertEqual(withExceptionHandling.get(), -1);
+}
+
+#endif
+
+@end