channeldb: add BalanceAboveReserve helper

We add this helper which will be used in a following commit. For the
noop htlcs that we are going to add we need to know whether the
"potential" receiver's balance is above reserve, so we add this
specific helper that helps us safely perform this check. To account for
entries that have already been processed, and to avoid bursts of HTLC
going way over the reserve, we also accept the delta parameter which
keeps track of amounts that will be credited to the receiver.

Author: GeorgeTsagk

channeldb/channel.go

@@ -2127,6 +2127,32 @@ func (c *OpenChannel) clearChanStatus(status ChannelStatus) error {
 	return nil
 }
 
+// BalanceAboveReserve checks if the balance for the provided party is above the
+// configured reserve. It also uses the balance delta for the party, to account
+// for entry amounts that have been processed already.
+func (c *OpenChannel) BalanceAboveReserve(
+	party lntypes.ChannelParty, delta int64) bool {
+
+	c.Lock()
+	defer c.Unlock()
+
+	// We need to add the delta to the balance we're checking against the
+	// reserve.
+	deltaAmt := lnwire.MilliSatoshi(delta).ToSatoshis()
+
+	switch {
+	case party.IsLocal():
+		return c.LocalCommitment.LocalBalance.ToSatoshis()+deltaAmt >
+			c.LocalChanCfg.ChanReserve
+
+	case party.IsRemote():
+		return c.RemoteCommitment.RemoteBalance.ToSatoshis()+deltaAmt >
+			c.RemoteChanCfg.ChanReserve
+	}
+
+	return false
+}
+
 // putOpenChannel serializes, and stores the current state of the channel in its
 // entirety.
 func putOpenChannel(chanBucket kvdb.RwBucket, channel *OpenChannel) error {