rewards: _pow function can revert in extreme conditions, ensure we test this function under normal boundaries

Author: abarmat

.soliumignore

@@ -5,6 +5,7 @@ contracts/discovery/erc1056
 contracts/token/IGraphToken.sol
 contracts/upgrades/GraphProxy.sol
 contracts/rewards/RewardsManager.sol
+contracts/tests/RewardsManagerMock.sol
 contracts/tests/ens
 contracts/curation/IGraphCurationToken.sol
 contracts/staking/libs/LibFixedMath.sol

contracts/tests/RewardsManagerMock.sol

@@ -0,0 +1,68 @@
+pragma solidity ^0.6.12;
+pragma experimental ABIEncoderV2;
+
+// Mock contract used for testing rewards
+contract RewardsManagerMock {
+    /**
+     * @dev Raises x to the power of n with scaling factor of base.
+     * Based on: https://github.com/makerdao/dss/blob/master/src/pot.sol#L81
+     * @param x Base of the exponentation
+     * @param n Exponent
+     * @param base Scaling factor
+     * @return z Exponential of n with base x
+     */
+    function pow(
+        uint256 x,
+        uint256 n,
+        uint256 base
+    ) public pure returns (uint256 z) {
+        assembly {
+            switch x
+                case 0 {
+                    switch n
+                        case 0 {
+                            z := base
+                        }
+                        default {
+                            z := 0
+                        }
+                }
+                default {
+                    switch mod(n, 2)
+                        case 0 {
+                            z := base
+                        }
+                        default {
+                            z := x
+                        }
+                    let half := div(base, 2) // for rounding.
+                    for {
+                        n := div(n, 2)
+                    } n {
+                        n := div(n, 2)
+                    } {
+                        let xx := mul(x, x)
+                        if iszero(eq(div(xx, x), x)) {
+                            revert(0, 0)
+                        }
+                        let xxRound := add(xx, half)
+                        if lt(xxRound, xx) {
+                            revert(0, 0)
+                        }
+                        x := div(xxRound, base)
+                        if mod(n, 2) {
+                            let zx := mul(z, x)
+                            if and(iszero(iszero(x)), iszero(eq(div(zx, x), z))) {
+                                revert(0, 0)
+                            }
+                            let zxRound := add(zx, half)
+                            if lt(zxRound, zx) {
+                                revert(0, 0)
+                            }
+                            z := div(zxRound, base)
+                        }
+                    }
+                }
+        }
+    }
+}

test/rewards/rewards.test.ts

@@ -2,12 +2,14 @@ import { expect } from 'chai'
 import { constants, BigNumber } from 'ethers'
 import { BigNumber as BN } from 'bignumber.js'
 
+import { deployContract } from '../lib/deployment'
 import { NetworkFixture } from '../lib/fixtures'
 
 import { Curation } from '../../build/typechain/contracts/Curation'
 import { EpochManager } from '../../build/typechain/contracts/EpochManager'
 import { GraphToken } from '../../build/typechain/contracts/GraphToken'
 import { RewardsManager } from '../../build/typechain/contracts/RewardsManager'
+import { RewardsManagerMock } from '../../build/typechain/contracts/RewardsManagerMock'
 import { Staking } from '../../build/typechain/contracts/Staking'
 
 import {
@@ -45,6 +47,7 @@ describe('Rewards', () => {
   let epochManager: EpochManager
   let staking: Staking
   let rewardsManager: RewardsManager
+  let rewardsManagerMock: RewardsManagerMock
 
   const subgraphDeploymentID1 = randomHexBytes()
   const subgraphDeploymentID2 = randomHexBytes()
@@ -128,6 +131,11 @@ describe('Rewards', () => {
       governor.signer,
     ))
 
+    rewardsManagerMock = ((await deployContract(
+      'RewardsManagerMock',
+      governor.signer,
+    )) as unknown) as RewardsManagerMock
+
     // 5% minute rate (4 blocks)
     await rewardsManager.connect(governor.signer).setIssuanceRate(ISSUANCE_RATE_PER_BLOCK)
 
@@ -606,4 +614,16 @@ describe('Rewards', () => {
         .withArgs(indexer1.address, allocationID, await epochManager.currentEpoch())
     })
   })
+
+  describe('pow', function () {
+    it('exponentiation works under normal boundaries (annual rate from 1% to 700%, 90 days period)', async function () {
+      const baseRatio = toGRT('0.000000004641377923') // 1% annual rate
+      const timePeriods = (60 * 60 * 24 * 10) / 15 // 90 days in blocks
+      for (let i = 0; i < 50; i = i + 4) {
+        const r = baseRatio.mul(i * 4).add(toGRT('1'))
+        const h = await rewardsManagerMock.pow(r, timePeriods, toGRT('1'))
+        console.log('\tr:', formatGRT(r), '=> c:', formatGRT(h))
+      }
+    })
+  })
 })