fence.h

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#ifndef CVMFS_FENCE_H_
#define CVMFS_FENCE_H_

#include "gtest/gtest_prod.h"
#include "util/atomic.h"
#include "util/posix.h"
#include "util/single_copy.h"

#ifdef CVMFS_NAMESPACE_GUARD
namespace CVMFS_NAMESPACE_GUARD {
#endif

class Fence : public SingleCopy {
  FRIEND_TEST(T_Fence, Basics);

 public:
  Fence() {
    atomic_init64(&counter_);
    atomic_init32(&blocking_);
  }

  void Enter() {
    while (atomic_read32(&blocking_)) {
      SafeSleepMs(kBusyWaitBackoffMs);
    }
    atomic_inc64(&counter_);
  }

  void Leave() { atomic_dec64(&counter_); }

  void Close() { atomic_cas32(&blocking_, 0, 1); }

  void Drain() {
    Close();
    while (atomic_read64(&counter_) > 0) {
      SafeSleepMs(kBusyWaitBackoffMs);
    }
  }

  void Open() { atomic_cas32(&blocking_, 1, 0); }

 private:
  static const unsigned kBusyWaitBackoffMs = 100;

  atomic_int64 counter_;

  atomic_int32 blocking_;
};


class FenceGuard {
 public:
  explicit FenceGuard(Fence *fence) : fence_(fence) { fence_->Enter(); }
  ~FenceGuard() { fence_->Leave(); }

 private:
  Fence *fence_;
};

#ifdef CVMFS_NAMESPACE_GUARD
}  // namespace CVMFS_NAMESPACE_GUARD
#endif

#endif  // CVMFS_FENCE_H_