go staking: three-way fee split

[pro-wh]

We should give more to the previous block proposer, which is the block that first ran the transactions that paid the fees in the LastBlockFees. Currently they only get paid as a voter.

Proposed design

Summary

The fee for block $h$ is split into $k$ portions, where $k$ is the number of validators at block $h$, one for each of those validators.
Each portion is split by constant weights into a proposer share, a voter share, and a next proposer share.
At the end of block $h$, pay the proposer share of all portions to the proposer of block $h$.
At the beginning of block $h+1$, for each portion where its validator's vote is included in block $h+1$, pay the voter share to the validator and pay the next proposer share to the proposer of block $h+1$.

Changes to genesis document

In .staking.params:

• Changed: fee_split_* is renamed to fee_split_weight_*
• Added: fee_split_weight_next_propose, a quantity.Quantity
• Changed: fee_split_weight_propose controls the proposer share (previously controlled the next proposer share)

Setting idea:

• fee_split_weight_propose: 2
• fee_split_weight_vote: 1
• fee_split_weight_next_propose: 1

Changes to block context

• Added: proposerEntityKey, the block proposer, with the entity of the proposer (needed to remember the proposer from BeginBlock for use in EndBlock)

Changes to ABCI state

• Changed: lastBlockFeesKeyFmt, the last block fees, now stores only the voter shares and next proposer shares

Algorithms

In cases where resolving an entity ID from a node address fails, skip movements to its balance.

In BeginBlock:

• Get the proposer address $A_p$ from the request
• Get the last commit number $k$ of validators from the request
• Get the address $V$ of nodes that voted from the request
• Resolve entity ID $I_p$ from node address $A_p$
• Get $f_{v,q}$ from the last block fees
• Compute a portion size $s=f_{v,q}/k$ (rounds down) where $k$ is the number of validators
• Compute a share size $s_q=s*w_q/(w_v+w_q)$ (rounds down)
• Compute a share size $s_v=s-s_q$
• For each voter address $v_i \in V$:
  • Resolve the entity ID $I_{v_i}$ from node address $v_i$
  • Move $s_v$ from the last block fees to $I_{v_i}$'s general balance
• Move $s_q*|V|$ from the last block fees to $I_p$'s general balance
• Move the remaining amount from the last block fees to the common pool
• Store $I_p$ as the block proposer

In EndBlock:

• Compute a portion of the fee $f_{v,q}=f*(w_v+w_q)/(w_p+w_v+w_q)$ (rounds down) where $f$ is the accumulated fee, $w_p$ is the proposer share, $w_v$ is the voter share, and $w_q$ is the next proposer share
• Move $f_{v,q}$ from the fee accumulator to the last block fees
• Get $I_p$ from the block proposer
• Move the remaining amount from the fee accumulator to $I_p$'s general balance
• Move the remaining amount from the fee accumulator to the common pool (in case resolving $I_p$ failed)

[codecov]

Codecov Report

Merging #2794 into master will increase coverage by 0.06%.
The diff coverage is 44.44%.

@@            Coverage Diff             @@
##           master    #2794      +/-   ##
==========================================
+ Coverage   61.63%   61.70%   +0.06%     
==========================================
  Files         386      386              
  Lines       37095    37131      +36     
==========================================
+ Hits        22865    22912      +47     
+ Misses      11222    11213       -9     
+ Partials     3008     3006       -2     

Impacted Files	Coverage Δ
go/oasis-node/cmd/genesis/genesis.go	52.38% <0.00%> (ø)
go/staking/api/api.go	71.42% <ø> (ø)
go/staking/tests/debug/debug_stake.go	60.00% <ø> (ø)
go/staking/api/sanity_check.go	50.00% <25.00%> (-0.88%)	⬇️
go/consensus/tendermint/apps/staking/staking.go	61.24% <33.33%> (-0.67%)	⬇️
go/consensus/tendermint/apps/staking/fees.go	49.49% <45.67%> (+9.76%)	⬆️
go/consensus/tendermint/apps/staking/state/gas.go	84.44% <66.66%> (-5.56%)	⬇️
...oasis-node/cmd/debug/txsource/workload/transfer.go	0.00% <0.00%> (-63.81%)	⬇️
.../oasis-node/cmd/debug/txsource/workload/runtime.go	1.77% <0.00%> (-55.63%)	⬇️
go/worker/common/host/protocol/protocol.go	65.67% <0.00%> (-3.74%)	⬇️
... and 27 more

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