funding+rpc: raise min channel size to 20k satoshis

In this commit, we raise the min channel size to 20k satoshis. This
will be evaluated before we check for dusty commitments. The goal of
this is to ensure ample room for fees at current, and future fee
levels.

fundingmanager.go

@@ -60,6 +60,10 @@ const (
 	// for the funding transaction to be confirmed before forgetting about
 	// the channel. 288 blocks is ~48 hrs
 	maxWaitNumBlocksFundingConf = 288
+
+	// minChanFundingSize is the smallest channel that we'll allow to be
+	// created over the RPC interface.
+	minChanFundingSize = btcutil.Amount(20000)
 )
 
 // reservationWithCtx encapsulates a pending channel reservation. This wrapper

rpcserver.go

@@ -701,17 +701,12 @@ func (r *rpcServer) OpenChannel(in *lnrpc.OpenChannelRequest,
 			"channel size is: %v", maxFundingAmount)
 	}
 
-	const minChannelSize = btcutil.Amount(6000)
-
-	// Restrict the size of the channel we'll actually open. Atm, we
-	// require the amount to be above 6k satoshis we currently hard-coded
-	// a 5k satoshi fee in several areas. As a result 6k sat is the min
-	// channel size that allows us to safely sit above the dust threshold
-	// after fees are applied
-	// TODO(roasbeef): remove after dynamic fees are in
-	if localFundingAmt < minChannelSize {
+	// 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 < minChanFundingSize {
 		return fmt.Errorf("channel is too small, the minimum channel "+
-			"size is: %v (6k sat)", minChannelSize)
+			"size is: %v SAT", int64(minChanFundingSize))
 	}
 
 	var (
@@ -864,6 +859,14 @@ func (r *rpcServer) OpenChannelSync(ctx context.Context,
 			"initial state must be below the local funding amount")
 	}
 
+	// 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 < minChanFundingSize {
+		return nil, fmt.Errorf("channel is too small, the minimum channel "+
+			"size is: %v SAT", int64(minChanFundingSize))
+	}
+
 	// Based on the passed fee related parameters, we'll determine an
 	// appropriate fee rate for the funding transaction.
 	feeRate, err := determineFeePerVSize(