Ensure stakes are always sorted before generating a leader_schedule

[sagar-solana]

Problem

Stakes need to be sorted, otherwise leader schedule becomes non-deterministic based on when stakes were first added to accounts.

Summary of Changes

Sort stakes before generating a leader schedule.

[codecov]

Codecov Report

Merging #3016 into master will increase coverage by <.1%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #3016     +/-   ##
========================================
+ Coverage    76.9%   76.9%   +<.1%     
========================================
  Files         130     130             
  Lines       19888   19917     +29     
========================================
+ Hits        15294   15319     +25     
- Misses       4594    4598      +4

[garious]

Great to see this fixed, but hurts to see an attempt to bring back code from earlier this week without its unit-tests. The original code was commented as well...

fn sort_stakes(stakes: &mut Vec<(Pubkey, u64)>) {
    // Sort first by stake. If stakes are the same, sort by pubkey to ensure a
    // deterministic result.
    // Note: Use unstable sort, because we dedup right after to remove the equal elements.
    stakes.sort_unstable_by(|(pubkey0, stake0), (pubkey1, stake1)| {
        if stake0 == stake1 {
            pubkey0.cmp(&pubkey1)
        } else {
            stake0.cmp(&stake1)
        }
    });

    // Now that it's sorted, we can do an O(n) dedup.
    stakes.dedup();
}

    #[test]
    fn test_sort_stakes_basic() {
        let pubkey0 = Keypair::new().pubkey();
        let pubkey1 = Keypair::new().pubkey();
        let mut stakes = vec![(pubkey0, 2), (pubkey1, 1)];
        sort_stakes(&mut stakes);
        assert_eq!(stakes, vec![(pubkey1, 1), (pubkey0, 2)]);
    }

    #[test]
    fn test_sort_stakes_with_dup() {
        let pubkey0 = Keypair::new().pubkey();
        let pubkey1 = Keypair::new().pubkey();
        let mut stakes = vec![(pubkey0, 1), (pubkey1, 2), (pubkey0, 1)];
        sort_stakes(&mut stakes);
        assert_eq!(stakes, vec![(pubkey0, 1), (pubkey1, 2)]);
    }

    #[test]
    fn test_sort_stakes_with_equal_stakes() {
        let pubkey0 = Pubkey::default();
        let pubkey1 = Keypair::new().pubkey();
        let mut stakes = vec![(pubkey0, 1), (pubkey1, 1)];
        sort_stakes(&mut stakes);
        assert_eq!(stakes, vec![(pubkey0, 1), (pubkey1, 1)]);
    }
}

[sagar-solana]

@garious, sorry about that, it was a hotfix pr for something Rob needed :(

[sagar-solana]

Everything in this PR was already included in #2992