Use validator set precompiles in Attestations (#1248)

packages/protocol/contracts/common/UsingPrecompiles.sol

@@ -0,0 +1,38 @@
+pragma solidity ^0.5.3;
+
+
+contract UsingPrecompiles {
+  /**
+   * @notice Gets a validator address from the current validator set.
+   * @param index Index of requested validator in the validator set as sorted by the election.
+   * @return Address of validator at the requested index.
+   */
+  function validatorAddressFromCurrentSet(uint256 index) public view returns (address) {
+    address validatorAddress;
+    assembly {
+      let newCallDataPosition := mload(0x40)
+      mstore(newCallDataPosition, index)
+      let success := staticcall(5000, 0xfa, newCallDataPosition, 32, 0, 0)
+      returndatacopy(add(newCallDataPosition, 64), 0, 32)
+      validatorAddress := mload(add(newCallDataPosition, 64))
+    }
+
+    return validatorAddress;
+  }
+
+  /**
+   * @notice Gets the size of the current elected validator set.
+   * @return Size of the current elected validator set.
+   */
+  function numberValidatorsInCurrentSet() public view returns (uint256) {
+    uint256 numberValidators;
+    assembly {
+      let success := staticcall(5000, 0xf9, 0, 0, 0, 0)
+      let returnData := mload(0x40)
+      returndatacopy(returnData, 0, 32)
+      numberValidators := mload(returnData)
+    }
+
+    return numberValidators;
+  }
+}

packages/protocol/contracts/identity/Attestations.sol

@@ -12,12 +12,20 @@ import "../governance/interfaces/IValidators.sol";
 import "../common/Initializable.sol";
 import "../common/UsingRegistry.sol";
 import "../common/Signatures.sol";
+import "../common/UsingPrecompiles.sol";
 
 
 /**
  * @title Contract mapping identifiers to accounts
  */
-contract Attestations is IAttestations, Ownable, Initializable, UsingRegistry, ReentrancyGuard {
+contract Attestations is
+  IAttestations,
+  Ownable,
+  Initializable,
+  UsingRegistry,
+  ReentrancyGuard,
+  UsingPrecompiles
+{
 
   using SafeMath for uint256;
   using SafeMath for uint128;
@@ -739,15 +747,16 @@ contract Attestations is IAttestations, Ownable, Initializable, UsingRegistry, R
     IRandom random = IRandom(registry.getAddressForOrDie(RANDOM_REGISTRY_ID));
 
     bytes32 seed = random.random();
-    address[] memory validators = getElection().electValidators();
+    uint256 numberValidators = numberValidatorsInCurrentSet();
 
     uint256 currentIndex = 0;
     address validator;
     address issuer;
 
     while (currentIndex < n) {
       seed = keccak256(abi.encodePacked(seed));
-      validator = validators[uint256(seed) % validators.length];
+      validator = validatorAddressFromCurrentSet(uint256(seed) % numberValidators);
+
       issuer = getLockedGold().getAccountFromValidator(validator);
       Attestation storage attestations =
         state.issuedAttestations[issuer];

packages/protocol/contracts/identity/test/TestAttestations.sol

@@ -0,0 +1,25 @@
+pragma solidity ^0.5.3;
+
+import "../Attestations.sol";
+
+
+/*
+ * We need a test contract that behaves like the actual Attestations contract,
+ * but mocks the implementations of the validator set getters. Otherwise we
+ * couldn't test `request` with the current ganache local testnet.
+ */
+contract TestAttestations is Attestations {
+  address[] private __testValidators;
+
+  function __setValidators(address[] memory validators) public {
+    __testValidators = validators;
+  }
+
+  function numberValidatorsInCurrentSet() public view returns (uint256) {
+    return __testValidators.length;
+  }
+
+  function validatorAddressFromCurrentSet(uint256 index) public view returns (address) {
+    return __testValidators[index];
+  }
+}

packages/protocol/migrationsConfig.js

@@ -115,7 +115,7 @@ const linkedLibraries = {
   AddressSortedLinkedList: ['Election'],
   IntegerSortedLinkedList: ['Governance', 'IntegerSortedLinkedListTest'],
   AddressSortedLinkedListWithMedian: ['SortedOracles', 'AddressSortedLinkedListWithMedianTest'],
-  Signatures: ['Attestations', 'LockedGold', 'Escrow'],
+  Signatures: ['TestAttestations', 'Attestations', 'LockedGold', 'Escrow'],
 }
 
 const argv = minimist(process.argv.slice(2), {

packages/protocol/test/identity/attestations.ts

@@ -13,8 +13,8 @@ import { getPhoneHash } from '@celo/utils/lib/phoneNumbers'
 import BigNumber from 'bignumber.js'
 import { uniq } from 'lodash'
 import {
-  AttestationsContract,
-  AttestationsInstance,
+  TestAttestationsContract,
+  TestAttestationsInstance,
   MockLockedGoldContract,
   MockLockedGoldInstance,
   MockStableTokenContract,
@@ -28,7 +28,12 @@ import {
 } from 'types'
 import { getParsedSignatureOfAddress } from '../../lib/signing-utils'
 
-const Attestations: AttestationsContract = artifacts.require('Attestations')
+/* We use a contract that behaves like the actual Attestations contract, but
+ * mocks the implementations of validator set getters. These rely on precompiled
+ * contracts, which are not available in our current ganache fork, which we use
+ * for Truffle unit tests.
+ */
+const Attestations: TestAttestationsContract = artifacts.require('TestAttestations')
 const MockStableToken: MockStableTokenContract = artifacts.require('MockStableToken')
 const MockElection: MockElectionContract = artifacts.require('MockElection')
 const MockLockedGold: MockLockedGoldContract = artifacts.require('MockLockedGold')
@@ -41,7 +46,7 @@ const longDataEncryptionKey =
   '02f2f48ee19680706196e2e339e5da3491186e0c4c5030670656b0e01611111111'
 
 contract('Attestations', (accounts: string[]) => {
-  let attestations: AttestationsInstance
+  let attestations: TestAttestationsInstance
   let mockStableToken: MockStableTokenInstance
   let otherMockStableToken: MockStableTokenInstance
   let random: RandomInstance
@@ -157,6 +162,7 @@ contract('Attestations', (accounts: string[]) => {
       [mockStableToken.address, otherMockStableToken.address],
       [attestationFee, attestationFee]
     )
+    await attestations.__setValidators(accounts)
   })
 
   describe('#initialize()', () => {