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ARROW-12286: [C++] Create AsyncGenerator from Future<AsyncGenerator<T>> #9943

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westonpace wants to merge 1 commit into from
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ARROW-12286: [C++] Create AsyncGenerator from Future<AsyncGenerator<T>> #9943

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7 changes: 7 additions & 0 deletions cpp/src/arrow/util/async_generator.h
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Original file line number Diff line number Diff line change
Expand Up @@ -640,6 +640,7 @@ class SerialReadaheadGenerator {
std::shared_ptr<State> state_;
};

/// \see MakeFromFuture
template <typename T>
class FutureFirstGenerator {
public:
Expand Down Expand Up @@ -669,6 +670,12 @@ class FutureFirstGenerator {
std::shared_ptr<State> state_;
};

/// \brief Transforms a Future<AsyncGenerator<T>> into an AsyncGenerator<T>
/// that waits for the future to complete as part of the first item.
///
/// This generator is not async-reentrant (even if the generator yielded by future is)
///
/// This generator does not queue
template <typename T>
AsyncGenerator<T> MakeFromFuture(Future<AsyncGenerator<T>> future) {
return FutureFirstGenerator<T>(std::move(future));
Expand Down
35 changes: 29 additions & 6 deletions cpp/src/arrow/util/async_generator_test.cc
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Expand Up @@ -222,8 +222,7 @@ class GeneratorTestFixture : public ::testing::TestWithParam<bool> {
AsyncGenerator<TestInt> MakeSource(const std::vector<TestInt>& items) {
std::vector<TestInt> wrapped(items.begin(), items.end());
auto gen = AsyncVectorIt(std::move(wrapped));
bool slow = GetParam();
if (slow) {
if (IsSlow()) {
return SlowdownABit(std::move(gen));
}
return gen;
Expand All @@ -233,22 +232,22 @@ class GeneratorTestFixture : public ::testing::TestWithParam<bool> {
AsyncGenerator<TestInt> gen = [] {
return Future<TestInt>::MakeFinished(Status::Invalid("XYZ"));
};
bool slow = GetParam();
if (slow) {
if (IsSlow()) {
return SlowdownABit(std::move(gen));
}
return gen;
}

int GetNumItersForStress() {
bool slow = GetParam();
// Run fewer trials for the slow case since they take longer
if (slow) {
if (IsSlow()) {
return 10;
} else {
return 100;
}
}

bool IsSlow() { return GetParam(); }
};

template <typename T>
Expand Down Expand Up @@ -461,6 +460,30 @@ TEST(TestAsyncUtil, Concatenated) {
AssertAsyncGeneratorMatch(expected, concat);
}

class FromFutureFixture : public GeneratorTestFixture {};

TEST_P(FromFutureFixture, Basic) {
auto source = Future<std::vector<TestInt>>::MakeFinished(RangeVector(3));
if (IsSlow()) {
source = SleepABitAsync().Then(
[](...) -> Result<std::vector<TestInt>> { return RangeVector(3); });
}
auto slow = IsSlow();
auto to_gen = source.Then([slow](const std::vector<TestInt>& vec) {
auto vec_gen = MakeVectorGenerator(vec);
if (slow) {
return SlowdownABit(std::move(vec_gen));
}
return vec_gen;
});
auto gen = MakeFromFuture(std::move(to_gen));
auto collected = CollectAsyncGenerator(std::move(gen));
ASSERT_FINISHES_OK_AND_EQ(RangeVector(3), collected);
}

INSTANTIATE_TEST_SUITE_P(FromFutureTests, FromFutureFixture,
::testing::Values(false, true));

class MergedGeneratorTestFixture : public GeneratorTestFixture {};

TEST_P(MergedGeneratorTestFixture, Merged) {
Expand Down