Use a platform-independent PRNG in the mongo shell for Random.rand()

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));

}