src/kudu/util/promise.h

// Copyright 2014 Cloudera, 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.
#ifndef KUDU_UTIL_PROMISE_H
#define KUDU_UTIL_PROMISE_H

#include "kudu/gutil/macros.h"
#include "kudu/util/countdown_latch.h"

namespace kudu {

// A promise boxes a value which is to be provided at some time in the future.
// A single producer calls Set(...), and any number of consumers can call Get()
// to retrieve the produced value.
//
// In Guava terms, this is a SettableFuture<T>.
template<typename T>
class Promise {
 public:
  Promise() : latch_(1) {}
  ~Promise() {}

  // Reset the promise to be used again.
  // For this to be safe, there must be some kind of external synchronization
  // ensuring that no threads are still accessing the value from the previous
  // incarnation of the promise.
  void Reset() {
    latch_.Reset(1);
    val_ = T();
  }

  // Block until a value is available, and return a reference to it.
  const T& Get() const {
    latch_.Wait();
    return val_;
  }

  // Wait for the promised value to become available with the given timeout.
  //
  // Returns NULL if the timeout elapses before a value is available.
  // Otherwise returns a pointer to the value. This pointer's lifetime is
  // tied to the lifetime of the Promise object.
  const T* WaitFor(const MonoDelta& delta) const {
    if (latch_.WaitFor(delta)) {
      return &val_;
    } else {
      return NULL;
    }
  }

  // Set the value of this promise.
  // This may be called at most once.
  void Set(const T& val) {
    DCHECK_EQ(latch_.count(), 1) << "Already set!";
    val_ = val;
    latch_.CountDown();
  }

 private:
  CountDownLatch latch_;
  T val_;
  DISALLOW_COPY_AND_ASSIGN(Promise);
};

} // namespace kudu
#endif /* KUDU_UTIL_PROMISE_H */