src/core/lib/promise/observable.h

// 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_CORE_LIB_PROMISE_OBSERVABLE_H
#define GRPC_CORE_LIB_PROMISE_OBSERVABLE_H

#include <grpc/support/port_platform.h>

#include <stdint.h>

#include <limits>
#include <memory>
#include <type_traits>

#include "absl/base/thread_annotations.h"
#include "absl/types/optional.h"
#include "absl/types/variant.h"

#include "src/core/lib/gprpp/sync.h"
#include "src/core/lib/promise/activity.h"
#include "src/core/lib/promise/detail/promise_like.h"
#include "src/core/lib/promise/poll.h"
#include "src/core/lib/promise/wait_set.h"

namespace grpc_core {

namespace promise_detail {

using ObservableVersion = uint64_t;
static constexpr ObservableVersion kTombstoneVersion =
    std::numeric_limits<ObservableVersion>::max();

}  // namespace promise_detail

class WatchCommitter {
 public:
  void Commit() { version_seen_ = promise_detail::kTombstoneVersion; }

 protected:
  promise_detail::ObservableVersion version_seen_ = 0;
};

namespace promise_detail {

// Shared state between Observable and Observer.
template <typename T>
class ObservableState {
 public:
  explicit ObservableState(absl::optional<T> value)
      : value_(std::move(value)) {}

  // Publish that we're closed.
  void Close() {
    mu_.Lock();
    version_ = kTombstoneVersion;
    value_.reset();
    auto wakeup = waiters_.TakeWakeupSet();
    mu_.Unlock();
    wakeup.Wakeup();
  }

  // Synchronously publish a new value, and wake any waiters.
  void Push(T value) {
    mu_.Lock();
    version_++;
    value_ = std::move(value);
    auto wakeup = waiters_.TakeWakeupSet();
    mu_.Unlock();
    wakeup.Wakeup();
  }

  Poll<absl::optional<T>> PollGet(ObservableVersion* version_seen) {
    MutexLock lock(&mu_);
    if (!Started()) return Pending();
    *version_seen = version_;
    return value_;
  }

  Poll<absl::optional<T>> PollNext(ObservableVersion* version_seen) {
    MutexLock lock(&mu_);
    if (!NextValueReady(version_seen)) return Pending();
    return value_;
  }

  Poll<absl::optional<T>> PollWatch(ObservableVersion* version_seen) {
    if (*version_seen == kTombstoneVersion) return Pending();

    MutexLock lock(&mu_);
    if (!NextValueReady(version_seen)) return Pending();
    // Watch needs to be woken up if the value changes even if it's ready now.
    waiters_.AddPending(Activity::current()->MakeNonOwningWaker());
    return value_;
  }

 private:
  // Returns true if an initial value is set.
  // If one is not set, add ourselves as pending to waiters_, and return false.
  bool Started() ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
    if (!value_.has_value()) {
      if (version_ != kTombstoneVersion) {
        // We allow initial no-value, which does not indicate closure.
        waiters_.AddPending(Activity::current()->MakeNonOwningWaker());
        return false;
      }
    }
    return true;
  }

  // If no value is ready, add ourselves as pending to waiters_ and return
  // false.
  // If the next value is ready, update the last version seen and return true.
  bool NextValueReady(ObservableVersion* version_seen)
      ABSL_EXCLUSIVE_LOCKS_REQUIRED(mu_) {
    if (!Started()) return false;
    if (version_ == *version_seen) {
      waiters_.AddPending(Activity::current()->MakeNonOwningWaker());
      return false;
    }
    *version_seen = version_;
    return true;
  }

  Mutex mu_;
  WaitSet waiters_ ABSL_GUARDED_BY(mu_);
  ObservableVersion version_ ABSL_GUARDED_BY(mu_) = 1;
  absl::optional<T> value_ ABSL_GUARDED_BY(mu_);
};

// Promise implementation for Observer::Get.
template <typename T>
class ObservableGet {
 public:
  ObservableGet(ObservableVersion* version_seen, ObservableState<T>* state)
      : version_seen_(version_seen), state_(state) {}

  Poll<absl::optional<T>> operator()() {
    return state_->PollGet(version_seen_);
  }

 private:
  ObservableVersion* version_seen_;
  ObservableState<T>* state_;
};

// Promise implementation for Observer::Next.
template <typename T>
class ObservableNext {
 public:
  ObservableNext(ObservableVersion* version_seen, ObservableState<T>* state)
      : version_seen_(version_seen), state_(state) {}

  Poll<absl::optional<T>> operator()() {
    return state_->PollNext(version_seen_);
  }

 private:
  ObservableVersion* version_seen_;
  ObservableState<T>* state_;
};

template <typename T, typename F>
class ObservableWatch final : private WatchCommitter {
 private:
  using Promise = PromiseLike<decltype(std::declval<F>()(
      std::declval<T>(), std::declval<WatchCommitter*>()))>;
  using Result = typename Promise::Result;

 public:
  explicit ObservableWatch(F factory, std::shared_ptr<ObservableState<T>> state)
      : state_(std::move(state)), factory_(std::move(factory)) {}
  ObservableWatch(const ObservableWatch&) = delete;
  ObservableWatch& operator=(const ObservableWatch&) = delete;
  ObservableWatch(ObservableWatch&& other) noexcept
      : state_(std::move(other.state_)),
        promise_(std::move(other.promise_)),
        factory_(std::move(other.factory_)) {}
  ObservableWatch& operator=(ObservableWatch&&) noexcept = default;

  Poll<Result> operator()() {
    auto r = state_->PollWatch(&version_seen_);
    if (auto* p = absl::get_if<kPollReadyIdx>(&r)) {
      if (p->has_value()) {
        promise_ = Promise(factory_(std::move(**p), this));
      } else {
        promise_ = {};
      }
    }
    if (promise_.has_value()) {
      return (*promise_)();
    } else {
      return Pending();
    }
  }

 private:
  std::shared_ptr<ObservableState<T>> state_;
  absl::optional<Promise> promise_;
  F factory_;
};

}  // namespace promise_detail

template <typename T>
class Observable;

// Observer watches an Observable for updates.
// It can see either the latest value or wait for a new value, but is not
// guaranteed to see every value pushed to the Observable.
template <typename T>
class Observer {
 public:
  Observer(const Observer&) = delete;
  Observer& operator=(const Observer&) = delete;
  Observer(Observer&& other) noexcept
      : version_seen_(other.version_seen_), state_(std::move(other.state_)) {}
  Observer& operator=(Observer&& other) noexcept {
    version_seen_ = other.version_seen_;
    state_ = std::move(other.state_);
    return *this;
  }

  // Return a promise that will produce an optional<T>.
  // If the Observable is still present, this will be a value T, but if the
  // Observable has been closed, this will be nullopt. Borrows data from the
  // Observer, so this value must stay valid until the promise is resolved. Only
  // one Next, Get call is allowed to be outstanding at a time.
  promise_detail::ObservableGet<T> Get() {
    return promise_detail::ObservableGet<T>{&version_seen_, &*state_};
  }

  // Return a promise that will produce the next unseen value as an optional<T>.
  // If the Observable is still present, this will be a value T, but if the
  // Observable has been closed, this will be nullopt. Borrows data from the
  // Observer, so this value must stay valid until the promise is resolved. Only
  // one Next, Get call is allowed to be outstanding at a time.
  promise_detail::ObservableNext<T> Next() {
    return promise_detail::ObservableNext<T>{&version_seen_, &*state_};
  }

 private:
  using State = promise_detail::ObservableState<T>;
  friend class Observable<T>;
  explicit Observer(std::shared_ptr<State> state) : state_(state) {}
  promise_detail::ObservableVersion version_seen_ = 0;
  std::shared_ptr<State> state_;
};

// Observable models a single writer multiple reader broadcast channel.
// Readers can observe the latest value, or await a new latest value, but they
// are not guaranteed to observe every value.
template <typename T>
class Observable {
 public:
  Observable() : state_(std::make_shared<State>(absl::nullopt)) {}
  explicit Observable(T value)
      : state_(std::make_shared<State>(std::move(value))) {}
  ~Observable() { state_->Close(); }
  Observable(const Observable&) = delete;
  Observable& operator=(const Observable&) = delete;

  // Push a new value into the observable.
  void Push(T value) { state_->Push(std::move(value)); }

  // Create a new Observer - which can pull the current state from this
  // Observable.
  Observer<T> MakeObserver() { return Observer<T>(state_); }

  // Create a new Watch - a promise that pushes state into the passed in promise
  // factory. The promise factory takes two parameters - the current value and a
  // commit token. If the commit token is used (the Commit function on it is
  // called), then no further Watch updates are provided.
  template <typename F>
  promise_detail::ObservableWatch<T, F> Watch(F f) {
    return promise_detail::ObservableWatch<T, F>(std::move(f), state_);
  }

 private:
  using State = promise_detail::ObservableState<T>;
  std::shared_ptr<State> state_;
};

}  // namespace grpc_core

#endif  // GRPC_CORE_LIB_PROMISE_OBSERVABLE_H