disable gas burning for window post messages

While over-estimation fees and miner tips are still paid, gas is no longer burnt
for direct, successful window PoSt messages.

Usually, gas is burnt to prevent an attacker from spamming the network and to
allow clients to "price" messages (using the base fee cap) based on how urgently
they need them to be processed. However:

1. Window PoSt is already a "proof of work".
2. Miners need to submit WindowedPoSts on-time so all window post messages are urgent.
3. Work is already under way to move window post verification off-chain (making
it effectively free). This change simply introduces the "free" part a bit earlier.

build/params_2k.go

@@ -24,6 +24,7 @@ var UpgradeLiftoffHeight = abi.ChainEpoch(-5)
 const UpgradeKumquatHeight = 15
 const UpgradeCalicoHeight = 20
 const UpgradePersianHeight = 25
+const UpgradeClausHeight = 30
 
 var DrandSchedule = map[abi.ChainEpoch]DrandEnum{
 	0: DrandMainnet,

build/params_mainnet.go

@@ -41,6 +41,9 @@ const UpgradeKumquatHeight = 170000
 const UpgradeCalicoHeight = 265200
 const UpgradePersianHeight = UpgradeCalicoHeight + (builtin2.EpochsInHour * 60)
 
+// 2020-12-22T02:00:00Z
+const UpgradeClausHeight = 343200
+
 func init() {
 	policy.SetConsensusMinerMinPower(abi.NewStoragePower(10 << 40))
 

build/params_testground.go

@@ -90,6 +90,7 @@ var (
 	UpgradeKumquatHeight  abi.ChainEpoch = -6
 	UpgradeCalicoHeight   abi.ChainEpoch = -7
 	UpgradePersianHeight  abi.ChainEpoch = -8
+	UpgradeClausHeight    abi.ChainEpoch = -9
 
 	DrandSchedule = map[abi.ChainEpoch]DrandEnum{
 		0: DrandMainnet,

chain/vm/burn.go

@@ -67,7 +67,7 @@ func ComputeGasOverestimationBurn(gasUsed, gasLimit int64) (int64, int64) {
 	return gasLimit - gasUsed - gasToBurn.Int64(), gasToBurn.Int64()
 }
 
-func ComputeGasOutputs(gasUsed, gasLimit int64, baseFee, feeCap, gasPremium abi.TokenAmount) GasOutputs {
+func ComputeGasOutputs(gasUsed, gasLimit int64, baseFee, feeCap, gasPremium abi.TokenAmount, chargeNetworkFee bool) GasOutputs {
 	gasUsedBig := big.NewInt(gasUsed)
 	out := ZeroGasOutputs()
 
@@ -76,7 +76,12 @@ func ComputeGasOutputs(gasUsed, gasLimit int64, baseFee, feeCap, gasPremium abi.
 		baseFeeToPay = feeCap
 		out.MinerPenalty = big.Mul(big.Sub(baseFee, feeCap), gasUsedBig)
 	}
-	out.BaseFeeBurn = big.Mul(baseFeeToPay, gasUsedBig)
+
+	// If chargeNetworkFee is disabled, just skip computing the BaseFeeBurn. However,
+	// we charge all the other fees regardless.
+	if chargeNetworkFee {
+		out.BaseFeeBurn = big.Mul(baseFeeToPay, gasUsedBig)
+	}
 
 	minerTip := gasPremium
 	if big.Cmp(big.Add(baseFeeToPay, minerTip), feeCap) > 0 {

chain/vm/burn_test.go

@@ -63,7 +63,7 @@ func TestGasOutputs(t *testing.T) {
 	for _, test := range tests {
 		test := test
 		t.Run(fmt.Sprintf("%v", test), func(t *testing.T) {
-			output := ComputeGasOutputs(test.used, test.limit, baseFee, types.NewInt(test.feeCap), types.NewInt(test.premium))
+			output := ComputeGasOutputs(test.used, test.limit, baseFee, types.NewInt(test.feeCap), types.NewInt(test.premium), true)
 			i2s := func(i uint64) string {
 				return fmt.Sprintf("%d", i)
 			}

chain/vm/vm.go

@@ -32,6 +32,7 @@ import (
 	"github.com/filecoin-project/lotus/chain/actors/adt"
 	"github.com/filecoin-project/lotus/chain/actors/aerrors"
 	"github.com/filecoin-project/lotus/chain/actors/builtin/account"
+	"github.com/filecoin-project/lotus/chain/actors/builtin/miner"
 	"github.com/filecoin-project/lotus/chain/actors/builtin/reward"
 	"github.com/filecoin-project/lotus/chain/state"
 	"github.com/filecoin-project/lotus/chain/types"
@@ -561,7 +562,13 @@ func (vm *VM) ApplyMessage(ctx context.Context, cmsg types.ChainMsg) (*ApplyRet,
 	if gasUsed < 0 {
 		gasUsed = 0
 	}
-	gasOutputs := ComputeGasOutputs(gasUsed, msg.GasLimit, vm.baseFee, msg.GasFeeCap, msg.GasPremium)
+
+	burn, err := vm.shouldBurn(st, msg, errcode)
+	if err != nil {
+		return nil, xerrors.Errorf("deciding whether should burn failed: %w", err)
+	}
+
+	gasOutputs := ComputeGasOutputs(gasUsed, msg.GasLimit, vm.baseFee, msg.GasFeeCap, msg.GasPremium, burn)
 
 	if err := vm.transferFromGasHolder(builtin.BurntFundsActorAddr, gasHolder,
 		gasOutputs.BaseFeeBurn); err != nil {
@@ -599,6 +606,29 @@ func (vm *VM) ApplyMessage(ctx context.Context, cmsg types.ChainMsg) (*ApplyRet,
 	}, nil
 }
 
+func (vm *VM) shouldBurn(st *state.StateTree, msg *types.Message, errcode exitcode.ExitCode) (bool, error) {
+	// Check to see if we should burn funds. We avoid burning on successful
+	// window post. This won't catch _indirect_ window post calls, but this
+	// is the best we can get for now.
+	if vm.blockHeight > build.UpgradeClausHeight && errcode == exitcode.Ok && msg.Method == miner.Methods.SubmitWindowedPoSt {
+		// Ok, we've checked the _method_, but we still need to check
+		// the target actor. It would be nice if we could just look at
+		// the trace, but I'm not sure if that's safe?
+		if toActor, err := st.GetActor(msg.To); err != nil {
+			// If the actor wasn't found, we probably deleted it or something. Move on.
+			if !xerrors.Is(err, types.ErrActorNotFound) {
+				// Otherwise, this should never fail and something is very wrong.
+				return false, xerrors.Errorf("failed to lookup target actor: %w", err)
+			}
+		} else if builtin.IsStorageMinerActor(toActor.Code) {
+			// Ok, this is a storage miner and we've processed a window post. Remove the burn.
+			return false, nil
+		}
+	}
+
+	return true, nil
+}
+
 func (vm *VM) ActorBalance(addr address.Address) (types.BigInt, aerrors.ActorError) {
 	act, err := vm.cstate.GetActor(addr)
 	if err != nil {