Change serialization backwards compat in Channel to use new version

Instead of interpreting the backwards compatibility data in Channel
serialization, use the serialization version bump present in 0.0.99
as the flag to indicate if a channel should be read in backwards
compatibility.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -4650,10 +4650,15 @@ impl<Signer: Sign> Writeable for Channel<Signer> {
 		self.counterparty_dust_limit_satoshis.write(writer)?;
 		self.holder_dust_limit_satoshis.write(writer)?;
 		self.counterparty_max_htlc_value_in_flight_msat.write(writer)?;
+
+		// Note that this field is ignored by 0.0.99+ as the TLV Optional variant is used instead.
 		self.counterparty_selected_channel_reserve_satoshis.unwrap_or(0).write(writer)?;
+
 		self.counterparty_htlc_minimum_msat.write(writer)?;
 		self.holder_htlc_minimum_msat.write(writer)?;
 		self.counterparty_max_accepted_htlcs.write(writer)?;
+
+		// Note that this field is ignored by 0.0.99+ as the TLV Optional variant is used instead.
 		self.minimum_depth.unwrap_or(0).write(writer)?;
 
 		match &self.counterparty_forwarding_info {
@@ -4844,20 +4849,25 @@ impl<'a, Signer: Sign, K: Deref> ReadableArgs<&'a K> for Channel<Signer>
 		let counterparty_dust_limit_satoshis = Readable::read(reader)?;
 		let holder_dust_limit_satoshis = Readable::read(reader)?;
 		let counterparty_max_htlc_value_in_flight_msat = Readable::read(reader)?;
-		let mut counterparty_selected_channel_reserve_satoshis = Some(Readable::read(reader)?);
-		if counterparty_selected_channel_reserve_satoshis == Some(0) {
-			// Versions up to 0.0.98 had counterparty_selected_channel_reserve_satoshis as a
-			// non-option, writing 0 for what we now consider None.
-			counterparty_selected_channel_reserve_satoshis = None;
+		let mut counterparty_selected_channel_reserve_satoshis = None;
+		if ver == 1 {
+			// Read the old serialization from version 0.0.98.
+			counterparty_selected_channel_reserve_satoshis = Some(Readable::read(reader)?);
+		} else {
+			// Read the 8 bytes of backwards-compatibility data.
+			let _dummy: u64 = Readable::read(reader)?;
 		}
 		let counterparty_htlc_minimum_msat = Readable::read(reader)?;
 		let holder_htlc_minimum_msat = Readable::read(reader)?;
 		let counterparty_max_accepted_htlcs = Readable::read(reader)?;
-		let mut minimum_depth = Some(Readable::read(reader)?);
-		if minimum_depth == Some(0) {
-			// Versions up to 0.0.98 had minimum_depth as a non-option, writing 0 for what we now
-			// consider None.
-			minimum_depth = None;
+
+		let mut minimum_depth = None;
+		if ver == 1 {
+			// Read the old serialization from version 0.0.98.
+			minimum_depth = Some(Readable::read(reader)?);
+		} else {
+			// Read the 4 bytes of backwards-compatibility data.
+			let _dummy: u32 = Readable::read(reader)?;
 		}
 
 		let counterparty_forwarding_info = match <u8 as Readable>::read(reader)? {