Don't `wait` without a condition

Reason

A wait without a condition can miss a wakeup or wake up simply to find that there is no work to do.

Example, bad

std::condition_variable cv;
std::mutex mx;

void thread1()
{
    while (true) {
        // do some work ...
        std::unique_lock<std::mutex> lock(mx);
        cv.notify_one();    // wake other thread
    }
}

void thread2()
{
    while (true) {
        std::unique_lock<std::mutex> lock(mx);
        cv.wait(lock);    // might block forever
        // do work ...
    }
}

Here, if some other thread consumes thread1's notification, thread2 can wait forever.

Example

template<typename T>
class Sync_queue {
public:
    void put(const T& val);
    void put(T&& val);
    void get(T& val);
private:
    mutex mtx;
    condition_variable cond;    // this controls access
    list<T> q;
};

template<typename T>
void Sync_queue<T>::put(const T& val)
{
    lock_guard<mutex> lck(mtx);
    q.push_back(val);
    cond.notify_one();
}

template<typename T>
void Sync_queue<T>::get(T& val)
{
    unique_lock<mutex> lck(mtx);
    cond.wait(lck, [this] { return !q.empty(); });    // prevent spurious wakeup
    val = q.front();
    q.pop_front();
}

Now if the queue is empty when a thread executing get() wakes up (e.g., because another thread has gotten to get() before it), it will immediately go back to sleep, waiting.

Enforcement

Flag all waits without conditions.