folly::Synchronized race condition examples

lock.cpp

#include <cstdint>
#include <cstdio>
#include <mutex>

#define GLOG_USE_GLOG_EXPORT
#include <folly/Synchronized.h>

class IntBumper {
public:
  IntBumper(uint64_t &x) : m_ptr(&x) {}
  void bump() { (*m_ptr)++; }
private:
  uint64_t *m_ptr;
};

static folly::Synchronized<uint64_t, std::mutex> X;

void background_loop() {
  for (int i = 0; i < 500'000; i++) {
    IntBumper bumper = {*X.lock()};
    bumper.bump();
  }
}

int main() {
  auto thread1 = std::thread(background_loop);
  auto thread2 = std::thread(background_loop);
  thread1.join();
  thread2.join();
  // BUG: This should print 1000000, but on my machine it prints something like 974918.
  X.withLock([](auto &x) { printf("%ld\n", x); });
}

withLock.cpp

#include <cstdint>
#include <cstdio>
#include <mutex>

#define GLOG_USE_GLOG_EXPORT
#include <folly/Synchronized.h>

class IntBumper {
public:
  IntBumper(uint64_t &x) : m_ptr(&x) {}
  void bump() { (*m_ptr)++; }
private:
  uint64_t *m_ptr;
};

static folly::Synchronized<uint64_t, std::mutex> X;

void background_loop() {
  for (int i = 0; i < 500'000; i++) {
    auto bumper = X.withLock([](auto &x) { return IntBumper{x}; });
    bumper.bump();
  }
}

int main() {
  auto thread1 = std::thread(background_loop);
  auto thread2 = std::thread(background_loop);
  thread1.join();
  thread2.join();
  // BUG: This should print 1000000, but on my machine it prints something like 974918.
  X.withLock([](auto &x) { printf("%ld\n", x); });
}