rpc: prevent spending unconfirmed funds within OpenChannel by default

In this commit, we address a slight regression in the defaults provided
by the OpenChannel RPC with regards to spending unconfirmed outputs to
fund funding transactions. We now add a new boolean parameter to
OpenChannelRequest: SpendUnconfirmed. If set, we'll use this to
indicate to the server that the funding transaction of the channel to be
created can spend the wallet's unconfirmed outputs to fund it. This
addition is needed because otherwise, if the caller doesn't specify a
MinConfs parameter to the request, then a default of 0 will be used,
which is not ideal.

rpcserver.go

@@ -710,6 +710,42 @@ func (r *rpcServer) DisconnectPeer(ctx context.Context,
 	return &lnrpc.DisconnectPeerResponse{}, nil
 }
 
+// extractOpenChannelMinConfs extracts the minimum number of confirmations from
+// the OpenChannelRequest that each output used to fund the channel's funding
+// transaction should satisfy.
+func extractOpenChannelMinConfs(in *lnrpc.OpenChannelRequest) (int32, error) {
+	switch {
+	// Ensure that the MinConfs parameter is non-negative.
+	case in.MinConfs < 0:
+		return 0, errors.New("minimum number of confirmations must " +
+			"be a non-negative number")
+
+	// The funding transaction should not be funded with unconfirmed outputs
+	// unless explicitly specified by SpendUnconfirmed. We do this to
+	// provide sane defaults to the OpenChannel RPC, as otherwise, if the
+	// MinConfs field isn't explicitly set by the caller, we'll use
+	// unconfirmed outputs without the caller being aware.
+	case in.MinConfs == 0 && !in.SpendUnconfirmed:
+		return 1, nil
+
+	// In the event that the caller set MinConfs > 0 and SpendUnconfirmed to
+	// true, we'll return an error to indicate the conflict.
+	case in.MinConfs > 0 && in.SpendUnconfirmed:
+		return 0, errors.New("SpendUnconfirmed set to true with " +
+			"MinConfs > 0")
+
+	// The funding transaction of the new channel to be created can be
+	// funded with unconfirmed outputs.
+	case in.SpendUnconfirmed:
+		return 0, nil
+
+	// If none of the above cases matched, we'll return the value set
+	// explicitly by the caller.
+	default:
+		return in.MinConfs, nil
+	}
+}
+
 // OpenChannel attempts to open a singly funded channel specified in the
 // request to a remote peer.
 func (r *rpcServer) OpenChannel(in *lnrpc.OpenChannelRequest,
@@ -755,16 +791,17 @@ func (r *rpcServer) OpenChannel(in *lnrpc.OpenChannelRequest,
 			"size is: %v SAT", int64(minChanFundingSize))
 	}
 
-	// Ensure that the MinConfs parameter is non-negative.
-	if in.MinConfs < 0 {
-		return errors.New("minimum number of confirmations must be a " +
-			"non-negative number")
+	// Then, we'll extract the minimum number of confirmations that each
+	// output we use to fund the channel's funding transaction should
+	// satisfy.
+	minConfs, err := extractOpenChannelMinConfs(in)
+	if err != nil {
+		return err
 	}
 
 	var (
 		nodePubKey      *btcec.PublicKey
 		nodePubKeyBytes []byte
-		err             error
 	)
 
 	// TODO(roasbeef): also return channel ID?
@@ -813,7 +850,7 @@ func (r *rpcServer) OpenChannel(in *lnrpc.OpenChannelRequest,
 		fundingFeePerKw: feeRate,
 		private:         in.Private,
 		remoteCsvDelay:  remoteCsvDelay,
-		minConfs:        in.MinConfs,
+		minConfs:        minConfs,
 	}
 
 	updateChan, errChan := r.server.OpenChannel(req)
@@ -928,10 +965,12 @@ func (r *rpcServer) OpenChannelSync(ctx context.Context,
 			"size is: %v SAT", int64(minChanFundingSize))
 	}
 
-	// Ensure that the MinConfs parameter is non-negative.
-	if in.MinConfs < 0 {
-		return nil, errors.New("minimum number of confirmations must " +
-			"be a non-negative number")
+	// Then, we'll extract the minimum number of confirmations that each
+	// output we use to fund the channel's funding transaction should
+	// satisfy.
+	minConfs, err := extractOpenChannelMinConfs(in)
+	if err != nil {
+		return nil, err
 	}
 
 	// Based on the passed fee related parameters, we'll determine an
@@ -955,7 +994,7 @@ func (r *rpcServer) OpenChannelSync(ctx context.Context,
 		fundingFeePerKw: feeRate,
 		private:         in.Private,
 		remoteCsvDelay:  remoteCsvDelay,
-		minConfs:        in.MinConfs,
+		minConfs:        minConfs,
 	}
 
 	updateChan, errChan := r.server.OpenChannel(req)