multi: convert the existing channeldb.ChannelType uint8 into a bit field

In this commit, we convert the existing `channeldb.ChannelType` type
into a _bit field_. This doesn't require us to change the current
serialization or interpretation or the type as it is, since all the
current defined values us a distinct bit. This PR lays the ground work
for any future changes that may introduce new channel types (like anchor
outputs), and also any changes that may modify the existing invariants
around channels (if we're the initiator, we always have the funding
transaction).

channeldb/channel.go

@@ -125,39 +125,45 @@ var (
 // ChannelType is an enum-like type that describes one of several possible
 // channel types. Each open channel is associated with a particular type as the
 // channel type may determine how higher level operations are conducted such as
-// fee negotiation, channel closing, the format of HTLCs, etc.
-// TODO(roasbeef): split up per-chain?
+// fee negotiation, channel closing, the format of HTLCs, etc. Structure-wise,
+// a ChannelType is a bit field, with each bit denoting a modification from the
+// base channel type of single funder.
 type ChannelType uint8
 
 const (
 	// NOTE: iota isn't used here for this enum needs to be stable
 	// long-term as it will be persisted to the database.
 
-	// SingleFunder represents a channel wherein one party solely funds the
-	// entire capacity of the channel.
-	SingleFunder ChannelType = 0
+	// SingleFunderBit represents a channel wherein one party solely funds
+	// the entire capacity of the channel.
+	SingleFunderBit ChannelType = 0
 
-	// DualFunder represents a channel wherein both parties contribute
+	// DualFunderBit represents a channel wherein both parties contribute
 	// funds towards the total capacity of the channel. The channel may be
 	// funded symmetrically or asymmetrically.
-	DualFunder ChannelType = 1
+	DualFunderBit ChannelType = 1 << 0
 
 	// SingleFunderTweakless is similar to the basic SingleFunder channel
 	// type, but it omits the tweak for one's key in the commitment
 	// transaction of the remote party.
-	SingleFunderTweakless ChannelType = 2
+	SingleFunderTweaklessBit ChannelType = 1 << 1
 )
 
 // IsSingleFunder returns true if the channel type if one of the known single
 // funder variants.
 func (c ChannelType) IsSingleFunder() bool {
-	return c == SingleFunder || c == SingleFunderTweakless
+	return c&DualFunderBit == 0
+}
+
+// IsDualFunder returns true if the ChannelType has the DualFunderBit set.
+func (c ChannelType) IsDualFunder() bool {
+	return c&DualFunderBit == DualFunderBit
 }
 
 // IsTweakless returns true if the target channel uses a commitment that
 // doesn't tweak the key for the remote party.
 func (c ChannelType) IsTweakless() bool {
-	return c == SingleFunderTweakless
+	return c&SingleFunderTweaklessBit == SingleFunderTweaklessBit
 }
 
 // ChannelConstraints represents a set of constraints meant to allow a node to

channeldb/channel_test.go

@@ -187,7 +187,7 @@ func createTestChannelState(cdb *DB) (*OpenChannel, error) {
 	chanID := lnwire.NewShortChanIDFromInt(uint64(rand.Int63()))
 
 	return &OpenChannel{
-		ChanType:          SingleFunder,
+		ChanType:          SingleFunderBit,
 		ChainHash:         key,
 		FundingOutpoint:   wire.OutPoint{Hash: key, Index: rand.Uint32()},
 		ShortChannelID:    chanID,