// Invariant: the system is in a safe state.
ResourceMgr::Request(int resourceID, int threadID) {
    lock.Acquire();
    assert(isSafe());
    while (!wouldBeSafe(resourceID, threadID)) {
        cv.Wait(&lock);
    }
    alloc[resourceID][threadID]++;
    avail[resourceID]--;
    assert(isSafe());
    lock.Release();
}