lnd: handles the FundMax field inside parseOpenChannelReq

Adds handling of the `FundMax` field when parsing an
`OpenChannelRequest` with `rpcServer.parseOpenChannelReq`.

funding/manager.go

@@ -266,6 +266,15 @@ type InitFundingMsg struct {
 	// peer.
 	MaxLocalCsv uint16
 
+	// FundUpToMaxAmt is the maximum amount to try to commit to. If set, the
+	// MinFundAmt field denotes the acceptable minimum amount to commit to,
+	// while trying to commit as many coins as possible up to this value.
+	FundUpToMaxAmt btcutil.Amount
+
+	// MinFundAmt must be set iff FundUpToMaxAmt is set. It denotes the
+	// minimum amount to commit to.
+	MinFundAmt btcutil.Amount
+
 	// ChanFunder is an optional channel funder that allows the caller to
 	// control exactly how the channel funding is carried out. If not
 	// specified, then the default chanfunding.WalletAssembler will be

rpcserver.go

@@ -1934,6 +1934,62 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 
 	localFundingAmt := btcutil.Amount(in.LocalFundingAmount)
 	remoteInitialBalance := btcutil.Amount(in.PushSat)
+
+	// If we are not committing the maximum viable balance towards a channel
+	// then the local funding amount must be specified. In case FundMax is
+	// set the funding amount is specified as the interval between minimum
+	// funding amount and by the configured maximum channel size.
+	if !in.FundMax && localFundingAmt == 0 {
+		return nil, fmt.Errorf("local funding amount must be non-zero")
+	}
+
+	// Ensure that the initial balance of the remote party (if pushing
+	// satoshis) does not exceed the amount the local party has requested
+	// for funding. This is only checked if we are not committing the
+	// maximum viable amount towards the channel balance. If we do commit
+	// the maximum then the remote balance is checked in a dedicated FundMax
+	// check.
+	if !in.FundMax && remoteInitialBalance >= localFundingAmt {
+		return nil, fmt.Errorf("amount pushed to remote peer for " +
+			"initial state must be below the local funding amount")
+	}
+
+	// We either allow the fundmax or the psbt flow hence we return an error
+	// if both are set.
+	if in.FundingShim != nil && in.FundMax {
+		return nil, fmt.Errorf("cannot provide a psbt funding shim " +
+			"while committing the maxium wallet balance towards " +
+			"the channel opening")
+	}
+
+	// If the FundMax flag is set, ensure that the acceptable minimum local
+	// amount adheres to the amount to be pushed to the remote, and to
+	// current rules, while also respecting the settings for the maximum
+	// channel size.
+	var minFundAmt, fundUpToMaxAmt btcutil.Amount
+	if in.FundMax {
+		// We assume the configured maximum channel size to be the upper
+		// bound of our "maxed" out funding attempt.
+		fundUpToMaxAmt = btcutil.Amount(r.cfg.MaxChanSize)
+
+		// Since the standard non-fundmax flow requires the minimum
+		// funding amount to be at least in the amount of the initial
+		// remote balance(push amount) we need to adjust the minimum
+		// funding amount accordingly. We initially assume the minimum
+		// allowed channel size as minimum funding amount.
+		minFundAmt = funding.MinChanFundingSize
+
+		// If minFundAmt is less than the initial remote balance we
+		// simply assign the initial remote balance to minFundAmt in
+		// order to fullfil the criterion. Whether or not this so
+		// determined minimum amount is actually available is
+		// ascertained downstream in the lnwallet's reservation
+		// workflow.
+		if remoteInitialBalance >= minFundAmt {
+			minFundAmt = remoteInitialBalance
+		}
+	}
+
 	minHtlcIn := lnwire.MilliSatoshi(in.MinHtlcMsat)
 	remoteCsvDelay := uint16(in.RemoteCsvDelay)
 	maxValue := lnwire.MilliSatoshi(in.RemoteMaxValueInFlightMsat)
@@ -1942,20 +1998,6 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 
 	globalFeatureSet := r.server.featureMgr.Get(feature.SetNodeAnn)
 
-	// Ensure that a local funding amount has been specified.
-	if localFundingAmt == 0 {
-		return nil, fmt.Errorf("local funding amount must be non-zero")
-	}
-
-	// Ensure that the initial balance of the remote party (if pushing
-	// satoshis) does not exceed the amount the local party has requested
-	// for funding.
-	//
-	if remoteInitialBalance >= localFundingAmt {
-		return nil, fmt.Errorf("amount pushed to remote peer for " +
-			"initial state must be below the local funding amount")
-	}
-
 	// Determine if the user provided channel fees
 	// and if so pass them on to the funding workflow.
 	var channelBaseFee, channelFeeRate *uint64
@@ -1968,7 +2010,7 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 
 	// Ensure that the remote channel reserve does not exceed 20% of the
 	// channel capacity.
-	if remoteChanReserve >= localFundingAmt/5 {
+	if !in.FundMax && remoteChanReserve >= localFundingAmt/5 {
 		return nil, fmt.Errorf("remote channel reserve must be less " +
 			"than the %%20 of the channel capacity")
 	}
@@ -1976,18 +2018,25 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 	// Ensure that the user doesn't exceed the current soft-limit for
 	// channel size. If the funding amount is above the soft-limit, then
 	// we'll reject the request.
+	// If the FundMax flag is set the local amount is determined downstream
+	// in the wallet hence we do not check it here against the maximum
+	// funding amount. Only if the localFundingAmt is specified we can check
+	// if it exceeds the maximum funding amount.
 	wumboEnabled := globalFeatureSet.HasFeature(
 		lnwire.WumboChannelsOptional,
 	)
-	if !wumboEnabled && localFundingAmt > MaxFundingAmount {
+	if !in.FundMax && !wumboEnabled && localFundingAmt > MaxFundingAmount {
 		return nil, fmt.Errorf("funding amount is too large, the max "+
 			"channel size is: %v", MaxFundingAmount)
 	}
 
 	// Restrict the size of the channel we'll actually open. At a later
-	// level, we'll ensure that the output we create after accounting for
-	// fees that a dust output isn't created.
-	if localFundingAmt < funding.MinChanFundingSize {
+	// level, we'll ensure that the output we create, after accounting for
+	// fees, does not leave a dust output. In case of the FundMax flow
+	// dedicated checks ensure that the lower boundary of the channel size
+	// is at least in the amount of MinChanFundingSize or potentially higher
+	// if a remote balance is specified.
+	if !in.FundMax && localFundingAmt < funding.MinChanFundingSize {
 		return nil, fmt.Errorf("channel is too small, the minimum "+
 			"channel size is: %v SAT", int64(funding.MinChanFundingSize))
 	}
@@ -2128,12 +2177,14 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 	// open a new channel. A stream is returned in place, this stream will
 	// be used to consume updates of the state of the pending channel.
 	return &funding.InitFundingMsg{
-		TargetPubkey:      nodePubKey,
-		ChainHash:         *r.cfg.ActiveNetParams.GenesisHash,
-		LocalFundingAmt:   localFundingAmt,
-		BaseFee:           channelBaseFee,
-		FeeRate:           channelFeeRate,
-		PushAmt:           lnwire.NewMSatFromSatoshis(remoteInitialBalance),
+		TargetPubkey:    nodePubKey,
+		ChainHash:       *r.cfg.ActiveNetParams.GenesisHash,
+		LocalFundingAmt: localFundingAmt,
+		BaseFee:         channelBaseFee,
+		FeeRate:         channelFeeRate,
+		PushAmt: lnwire.NewMSatFromSatoshis(
+			remoteInitialBalance,
+		),
 		MinHtlcIn:         minHtlcIn,
 		FundingFeePerKw:   feeRate,
 		Private:           in.Private,
@@ -2145,6 +2196,8 @@ func (r *rpcServer) parseOpenChannelReq(in *lnrpc.OpenChannelRequest,
 		MaxHtlcs:          maxHtlcs,
 		MaxLocalCsv:       uint16(in.MaxLocalCsv),
 		ChannelType:       channelType,
+		FundUpToMaxAmt:    fundUpToMaxAmt,
+		MinFundAmt:        minFundAmt,
 	}, nil
 }