Summary

Our random-number generator function __wt_random(), even when it is initialized from different seeds, eventually converges to the same state.

Analysis

The convergence happen as soon as z or w become 0:

    if (z == 0 || w == 0) {
        __wt_random_init(&rnd);
        w = M_W(rnd);
        z = M_Z(rnd);
    }

Function __wt_random_init() uses fixed values, so as soon as this branch is hit, the generation of random numbers across different runs (even with the same seed) converges.

Near-instant convergence

To make the matter worse, this converges instantly in test push_pull! The test initializes the seed like this:

    /* Seed the random number generator */
    v = (uint32_t)getpid() + num_records + (2 * counter);
    __wt_random_init_custom_seed(&rnd, v);

This generates a number that is less than 2^32, which means that value z of the test state starts by being initialized to 0. This triggers re-initialization to the fixed values as explained above on the first use of the function.