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// Copyright 2021 gRPC authors.
//
// 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.
#ifndef GRPC_SRC_CORE_LIB_PROMISE_LOOP_H
#define GRPC_SRC_CORE_LIB_PROMISE_LOOP_H
#include <grpc/support/port_platform.h>
#include <type_traits>
#include "y_absl/status/status.h"
#include "y_absl/status/statusor.h"
#include "y_absl/types/variant.h"
#include "src/core/lib/gprpp/construct_destruct.h"
#include "src/core/lib/promise/detail/promise_factory.h"
#include "src/core/lib/promise/poll.h"
namespace grpc_core {
// Special type - signals to loop to take another iteration, instead of
// finishing
struct Continue {};
// Result of polling a loop promise - either Continue looping, or return a value
// T
template <typename T>
using LoopCtl = y_absl::variant<Continue, T>;
namespace promise_detail {
template <typename T>
struct LoopTraits;
template <typename T>
struct LoopTraits<LoopCtl<T>> {
using Result = T;
static LoopCtl<T> ToLoopCtl(LoopCtl<T> value) { return value; }
};
template <typename T>
struct LoopTraits<y_absl::StatusOr<LoopCtl<T>>> {
using Result = y_absl::StatusOr<T>;
static LoopCtl<Result> ToLoopCtl(y_absl::StatusOr<LoopCtl<T>> value) {
if (!value.ok()) return value.status();
const auto& inner = *value;
if (y_absl::holds_alternative<Continue>(inner)) return Continue{};
return y_absl::get<T>(inner);
}
};
template <>
struct LoopTraits<y_absl::StatusOr<LoopCtl<y_absl::Status>>> {
using Result = y_absl::Status;
static LoopCtl<Result> ToLoopCtl(
y_absl::StatusOr<LoopCtl<y_absl::Status>> value) {
if (!value.ok()) return value.status();
const auto& inner = *value;
if (y_absl::holds_alternative<Continue>(inner)) return Continue{};
return y_absl::get<y_absl::Status>(inner);
}
};
template <typename F>
class Loop {
private:
using Factory = promise_detail::RepeatedPromiseFactory<void, F>;
using PromiseType = decltype(std::declval<Factory>().Make());
using PromiseResult = typename PromiseType::Result;
public:
using Result = typename LoopTraits<PromiseResult>::Result;
explicit Loop(F f) : factory_(std::move(f)) {}
~Loop() {
if (started_) Destruct(&promise_);
}
Loop(Loop&& loop) noexcept : factory_(std::move(loop.factory_)) {}
Loop(const Loop& loop) = delete;
Loop& operator=(const Loop& loop) = delete;
Poll<Result> operator()() {
if (!started_) {
started_ = true;
Construct(&promise_, factory_.Make());
}
while (true) {
// Poll the inner promise.
auto promise_result = promise_();
// If it returns a value:
if (auto* p = promise_result.value_if_ready()) {
// - then if it's Continue, destroy the promise and recreate a new one
// from our factory.
auto lc = LoopTraits<PromiseResult>::ToLoopCtl(*p);
if (y_absl::holds_alternative<Continue>(lc)) {
Destruct(&promise_);
Construct(&promise_, factory_.Make());
continue;
}
// - otherwise there's our result... return it out.
return y_absl::get<Result>(lc);
} else {
// Otherwise the inner promise was pending, so we are pending.
return Pending();
}
}
}
private:
GPR_NO_UNIQUE_ADDRESS Factory factory_;
GPR_NO_UNIQUE_ADDRESS union {
GPR_NO_UNIQUE_ADDRESS PromiseType promise_;
};
bool started_ = false;
};
} // namespace promise_detail
// Looping combinator.
// Expects F returns LoopCtl<T> - if it's Continue, then run the loop again -
// otherwise yield the returned value as the result of the loop.
template <typename F>
promise_detail::Loop<F> Loop(F f) {
return promise_detail::Loop<F>(std::move(f));
}
} // namespace grpc_core
#endif // GRPC_SRC_CORE_LIB_PROMISE_LOOP_H
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