YugabyteDB (2.13.1.0-b60, 21121d69985fbf76aa6958d8f04a9bfa936293b5)

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//

//   http://www.apache.org/licenses/LICENSE-2.0

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// under the License.

//

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// Portions Copyright (c) YugaByte, Inc.

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// Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except

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//

// http://www.apache.org/licenses/LICENSE-2.0

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#ifndef YB_UTIL_PROMISE_H

#define YB_UTIL_PROMISE_H

#include "yb/gutil/macros.h"

#include "yb/util/countdown_latch.h"

namespace yb {

// 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!"0
;

    val_ = val;

    latch_.CountDown();

  }

 private:

  CountDownLatch latch_;

  T val_;

  DISALLOW_COPY_AND_ASSIGN(Promise);

};

} // namespace yb

#endif /* YB_UTIL_PROMISE_H */