The Math.random() function in JavaScript isn't seeded, so the mongo shell provides a Random.rand() function that can be seeded using the Random.srand() function. The C++ function that underlies Random.rand() uses the rand_r() function on POSIX systems and the rand() function on Windows. The RAND_MAX value used to scale the results to be a floating-point number between 0 and 1 is platform-specific. This means that even for the same seed, the pseudo-random number generator will produce different sequences on Windows, Linux, and OS X.

The Random.setRandomSeed() function seeds the pseudo-random number generator with a value generated by a SecureRandom instance. Some of our more complex JavaScript integration tests use a pseudo-random number generator to vary the test data and/or operations to make the test case more interesting each time. Having the pseudo-random number generator in the mongo shell be platform-independent would greatly aid in reproducing the exact test setup without requiring using the same platform as what the test failure occurred on.

#if !defined(_WIN32)
ThreadLocalValue<unsigned int> _randomSeed;
#endif

BSONObj JSSrand(const BSONObj& a, void* data) {
    unsigned int seed;
    // grab the least significant bits of either the supplied argument or
    // a random number from SecureRandom.
    if (a.nFields() == 1 && a.firstElement().isNumber())
        seed = static_cast<unsigned int>(a.firstElement().numberLong());
    else {
        std::unique_ptr<SecureRandom> rand(SecureRandom::create());
        seed = static_cast<unsigned int>(rand->nextInt64());
    }
#if !defined(_WIN32)
    _randomSeed.set(seed);
#else
    srand(seed);
#endif
    return BSON("" << static_cast<double>(seed));
}

BSONObj JSRand(const BSONObj& a, void* data) {
    uassert(12519, "rand accepts no arguments", a.nFields() == 0);
    unsigned r;
#if !defined(_WIN32)
    r = rand_r(&_randomSeed.getRef());
#else
    r = rand();
#endif
    return BSON("" << double(r) / (double(RAND_MAX) + 1));
}