Staking locked balances can be more than the actual free balances

[liuchengxu]

ChainX/xpallets/mining/staking/src/lib.rs

Line 351 in 8d78cde

ensure!(value <= Self::free_balance(&sender), Error::<T>::InsufficientBalance);

This line will cause there are more Staking votes than the actual balances, which also means we might lock more funds than a user has. For instance, 5UW4LiYMc1r1X3yQVLCVi281wi5GCyRCwJZ3d9jHpbBSGW5Y has negative usable balances due to the over-staking-lock.

~/.../chainx-org/chainx-cli ❯❯❯ ./target/release/chainx-cli --url ws://116.62.46.8:8087 xstaking storage locks 5UW4LiYMc1r1X3yQVLCVi281wi5GCyRCwJZ3d9jHpbBSGW5Y --block-number 107671
d719d3036ac37da50627651e2353873f5eb9cdbce547ee5684b0618171bf7d0a (5GvjpjTY...): {
    Bonded: 2073339270000,
    BondedWithdrawal: 1264702800000,
}
# The usable is -5339.75070541PCX
~/.../chainx-org/chainx-cli ❯❯❯ ./target/release/chainx-cli --url ws://116.62.46.8:8087 system account-info 5UW4LiYMc1r1X3yQVLCVi281wi5GCyRCwJZ3d9jHpbBSGW5Y --block-number 107671
AccountInfo of d719d3036ac37da50627651e2353873f5eb9cdbce547ee5684b0618171bf7d0a (5GvjpjTY...): AccountInfo {
    nonce: 37,
    refcount: 2,
    data: AccountData {
        free: 2804066999459,
        reserved: 1000000,
        misc_frozen: 3338042070000,
        fee_frozen: 3338042070000,
    },
}

{
	"id":1,
	"jsonrpc":"2.0",
	"method":"xstaking_getNominationByAccount",
	"params":["5UW4LiYMc1r1X3yQVLCVi281wi5GCyRCwJZ3d9jHpbBSGW5Y", "0x923161c814c5d0d01c3926d36acee704a97c18cdb59ade0cbd72b3da667066c3"]
}

We expect 1 PCX 1 vote in theory, but there are more Staking votes now, the consequence is that some stakers are earning more rewards than they should. If someone's frozen balances are more than his free balance, then he has more unexpected gains due to this bug, at the same time some other people will have fewer rewards as the reward pot per validator is correct. This bug only relates to the distribution of all nominators behind a validator, some earn more and some others earn less.

Should be fixed by #408.
https://github.com/chainx-org/ChainX/pull/408/files#diff-acfc4fead7dba36919225985639fe230fc951b8b6630dc3e48d15fb5a3366076R351-R354 will ensure 1 PCX 1 vote again.

The stakers having any windfall, especially these have negative usable balance now, will have to wait for the fix using XStaking::force_reset_staking_lock().

Update: This bug is not the root cause of the staking reward drop as there only are 10 accounts with negative usable balance, the total involved money is less than 6000PCX, which is too small compared to the total stakes(6M+). The main reason is when we migrated to 2.0, we settled all the latest vote weight of 1.0. And the vote weight is measured by blocks, which means we have to spend 3 times the time to obtain the same vote weight in 2.0, so the users with legacy vote weight earns more, but once they make their first claim on 2.0, they will have no advantages.