implement reward and add a base penalty

* The reward ensures that, at a minimum, the gas fees for disputing a
  proof is covered.
* The base penalty ensures that the dispute method cannot be used to
  "take an advance" on vesting rewards.

actors/builtin/miner/miner_test.go

@@ -1928,7 +1928,7 @@ func TestWindowPost(t *testing.T) {
 			result = &poStDisputeResult{
 				expectedPowerDelta:  pwr.Neg(),
 				expectedPenalty:     expectedFee,
-				expectedReward:      big.Zero(),
+				expectedReward:      miner.BaseRewardForDisputedWindowPoSt,
 				expectedPledgeDelta: big.Zero(),
 			}
 		}
@@ -2494,7 +2494,7 @@ func TestWindowPost(t *testing.T) {
 		result = &poStDisputeResult{
 			expectedPowerDelta:  pwr.Neg(),
 			expectedPenalty:     expectedFee,
-			expectedReward:      big.Zero(),
+			expectedReward:      miner.BaseRewardForDisputedWindowPoSt,
 			expectedPledgeDelta: big.Zero(),
 		}
 

actors/builtin/miner/monies.go

@@ -62,6 +62,11 @@ const ConsensusFaultFactor = 5
 var LockedRewardFactorNum = big.NewInt(75)
 var LockedRewardFactorDenom = big.NewInt(100)
 
+// Base reward for successfully disputing a window posts proofs.
+var BaseRewardForDisputedWindowPoSt = big.Lsh(big.NewInt(4), math.Precision128) // PARAM_SPEC
+// Base penalty for a successful disputed window post proof.
+var BasePenaltyForDisputedWindowPoSt = big.Lsh(big.NewInt(20), math.Precision128) // PARAM_SPEC
+
 // The projected block reward a sector would earn over some period.
 // Also known as "BR(t)".
 // BR(t) = ProjectedRewardFraction(t) * SectorQualityAdjustedPower
@@ -123,7 +128,10 @@ func PledgePenaltyForTermination(dayReward abi.TokenAmount, sectorAge abi.ChainE
 
 // The penalty for optimistically proving a sector with an invalid window PoSt.
 func PledgePenaltyForInvalidWindowPoSt(rewardEstimate, networkQAPowerEstimate smoothing.FilterEstimate, qaSectorPower abi.StoragePower) abi.TokenAmount {
-	return ExpectedRewardForPower(rewardEstimate, networkQAPowerEstimate, qaSectorPower, InvalidWindowPoStProjectionPeriod)
+	return big.Add(
+		ExpectedRewardForPower(rewardEstimate, networkQAPowerEstimate, qaSectorPower, InvalidWindowPoStProjectionPeriod),
+		BasePenaltyForDisputedWindowPoSt,
+	)
 }
 
 // Computes the PreCommit deposit given sector qa weight and current network conditions.

actors/builtin/miner/policy.go

@@ -348,7 +348,6 @@ func RewardForConsensusSlashReport(elapsedEpoch abi.ChainEpoch, collateral abi.T
 
 // The reward given for successfully disputing a window post.
 func RewardForDisputedWindowPoSt(proofType abi.RegisteredPoStProof, disputedPower PowerPair) abi.TokenAmount {
-	// This is currently zero but may be raised at some point to
-	// ensure that disputing proofs rational.
-	return big.Zero()
+	// This is currently just the base. In the future, the fee may scale based on the disputed power.
+	return BaseRewardForDisputedWindowPoSt
 }