lnwallet: fix bug with funding channels with 50/50 balances

This commit fixes a prior bug in the wallet triggered by the creation
of a channel using the single funder workflow, but pushing exactly
*half* of the channel over to the other side. The prior logic to
determine who the initiator would result in a disagreement over who
created the channel initially. This wouldn’t manifest until the channel
was attempted to be closed cooperatively. As both side disagreed about
who created the channel they would apply the closing fee to different
outputs, thereby creating mismatched closing transaction. The signature
would fail to validate as the closer will create a different
transaction from that of the responder.

This commit fixes the issue by properly detecting who initially created
the channel.

lnwallet/reservation.go

@@ -146,6 +146,7 @@ func NewChannelReservation(capacity, fundingAmt btcutil.Amount, minFeeRate btcut
 	var (
 		ourBalance   btcutil.Amount
 		theirBalance btcutil.Amount
+		initiator    bool
 	)
 
 	// If we're the responder to a single-funder reservation, then we have
@@ -154,6 +155,7 @@ func NewChannelReservation(capacity, fundingAmt btcutil.Amount, minFeeRate btcut
 	if fundingAmt == 0 {
 		ourBalance = pushSat
 		theirBalance = capacity - commitFee - pushSat
+		initiator = false
 	} else {
 		// TODO(roasbeef): need to rework fee structure in general and
 		// also when we "unlock" dual funder within the daemon
@@ -172,34 +174,21 @@ func NewChannelReservation(capacity, fundingAmt btcutil.Amount, minFeeRate btcut
 		}
 
 		theirBalance = capacity - fundingAmt - commitFee + pushSat
+		initiator = true
 	}
 
-	var (
+	// Next we'll set the channel type based on what we can ascertain about
-		initiator bool
+	// the balances/push amount within the channel.
-		chanType  channeldb.ChannelType
+	var chanType channeldb.ChannelType
-	)
-	switch {
-	// If our balance is zero, or we're being pushed our entire balance in
-	// the first state,  then we're the responder to a single funder
-	// channel workflow.
-	case pushSat != 0 && ourBalance-pushSat == 0:
-		fallthrough
-	case ourBalance == 0:
-		initiator = false
-		chanType = channeldb.SingleFunder
 
-	// If their balance is zero, or being pushed entirely to them, then
+	// If either of the balances are zero at this point, or we have a
-	// we're the initiator to a single funder channel workflow.
+	// non-zero push amt (there's no pushing for dual funder), then this is
-	case pushSat != 0 && theirBalance-pushSat == 0:
+	// a single-funder channel.
-		fallthrough
+	if ourBalance == 0 || theirBalance == 0 || pushSat != 0 {
-	case theirBalance == 0:
-		initiator = true
 		chanType = channeldb.SingleFunder
-
+	} else {
-	// Otherwise, if both sides are starting out with a non-zero balance,
+		// Otherwise, this is a dual funder channel, and no side is
-	// then neither of us is technically the "initiator" and this is a dual
+		// technically the "initiator"
-	// funder channel.
-	default:
 		initiator = false
 		chanType = channeldb.DualFunder
 	}