lnwire: add new FundingFlag type for OpenChannel

In this commit we add a new type to the lnwire package: FundingFlag.
This type will serve as an enum to describe the possible flags that can
be used within the ChannelFlags field in the OpenChannel struct.

We also define the first assigned flag: FFAnnounceChannel, which
indicates if the initiator of the funding flow wishes to announce the
channel to the greater network.

lnwire/lnwire.go

@@ -86,6 +86,12 @@ func writeElement(w io.Writer, element interface{}) error {
 		if _, err := w.Write(b[:]); err != nil {
 			return err
 		}
+	case FundingFlag:
+		var b [1]byte
+		b[0] = uint8(e)
+		if _, err := w.Write(b[:]); err != nil {
+			return err
+		}
 	case uint16:
 		var b [2]byte
 		binary.BigEndian.PutUint16(b[:], e)
@@ -388,6 +394,12 @@ func readElement(r io.Reader, element interface{}) error {
 			return err
 		}
 		*e = b[0]
+	case *FundingFlag:
+		var b [1]uint8
+		if _, err := r.Read(b[:]); err != nil {
+			return err
+		}
+		*e = FundingFlag(b[0])
 	case *uint16:
 		var b [2]byte
 		if _, err := io.ReadFull(r, b[:]); err != nil {

lnwire/lnwire_test.go

@@ -154,7 +154,7 @@ func TestLightningWireProtocol(t *testing.T) {
 				FeePerKiloWeight: uint32(r.Int63()),
 				CsvDelay:         uint16(r.Int31()),
 				MaxAcceptedHTLCs: uint16(r.Int31()),
-				ChannelFlags:     byte(r.Int31()),
+				ChannelFlags:     FundingFlag(uint8(r.Int31())),
 			}
 
 			if _, err := r.Read(req.ChainHash[:]); err != nil {

lnwire/open_channel.go

@@ -8,6 +8,17 @@ import (
 	"github.com/roasbeef/btcutil"
 )
 
+// FundingFlag represents the possible bit mask values for the ChannelFlags
+// field within the OpenChannel struct.
+type FundingFlag uint8
+
+const (
+	// FFAnnounceChannel is a FundingFlag that when set, indicates the
+	// initiator of a funding flow wishes to announce the channel to the
+	// greater network.
+	FFAnnounceChannel FundingFlag = 1 << iota
+)
+
 // OpenChannel is the message Alice sends to Bob if we should like to create a
 // channel with Bob where she's the sole provider of funds to the channel.
 // Single funder channels simplify the initial funding workflow, are supported
@@ -101,7 +112,7 @@ type OpenChannel struct {
 	// channel to specify further behavior surrounding the channel.
 	// Currently, the least significant bit of this bit field indicates the
 	// initiator of the channel wishes to advertise this channel publicly.
-	ChannelFlags byte
+	ChannelFlags FundingFlag
 }
 
 // A compile time check to ensure OpenChannel implements the lnwire.Message