[Slashing] Allow voting gold to be slashable

[lucasege]

Description

Adding function slashVotes within Election.sol that relies on another new function revokeVotes which combines revokePending and revokeActive in order to only sort internal group lists once.

Points of focus:

• Cannot make the contract onlyRegisteredContract(LOCKED_GOLD_REGISTRY_ID) because if I override mockLockedGold's address the tests revert because that contract cannot be properly called, but I also can't use mockLockedGold's address as the caller since that address is unknown to truffle/ganache. Any guidance on this would be helpful, there don't appear to be any examples of doing this anywhere else.

Tested

Added unit tests

Related issues

• Fixes Make Locked gold voting in validator elections slashable #2083

Backwards compatibility

yes

[asaj]

There seems to be a decent amount of duplicated code in the following 3 test cases. Would it be possible to define something that parses objects like the following, sets up the test case, and then asserts the expected values?

const testCases = [
  {
    votesBefore: [
       {
          group: 0x1          
          pending: 2000
          active: 500
       }, {
          group: 0x2         
          pending: 1000
          active: 0
       }
    ],
    slash: 2700
    votesAfter: [
       {
          group: 0x1          
          pending: 0
          active: 0
       }, {
          group: 0x2         
          pending: 800
          active: 0
       }
    ]
  }]
}  

[lucasege]

Up to you if you want to delay merging for this, I'll get started on it once I finish addressing reviews for #2317 since it seems lower priority, but still worth adding.

[asaj]

Not worth blocking the PR but something I suggest we do after since it will clean up the testing code quite a bit

[codecov]

Codecov Report

Merging #2302 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2302   +/-   ##
=======================================
  Coverage   74.55%   74.55%           
=======================================
  Files         278      278           
  Lines        7777     7777           
  Branches      991      991           
=======================================
  Hits         5798     5798           
  Misses       1868     1868           
  Partials      111      111

Flag	Coverage Δ
#mobile	74.55% <ø> (ø)	⬆️

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[asaj]

Emit an event?

[lucasege]

An event ValidatorGroupVoteRevoked is emitted if any votes are successfully decremented. This is the same event as in revokePending and revokeActive. I don't think we need an additional event, let me know if you think otherwise or you just didn't see the existing event