Add message structs required for dual-funded channels

This is the first of a set of PRs to enable the experimental dual-funded
channels feature using interactive transaction construction. This allows
both the channel initiator and channel acceptor to contribute funds
towards the channel.

Author: dunxen

fuzz/src/msg_targets/gen_target.sh

@@ -33,8 +33,8 @@ GEN_TEST lightning::ln::msgs::UpdateFailHTLC test_msg_simple ""
 GEN_TEST lightning::ln::msgs::UpdateFailMalformedHTLC test_msg_simple ""
 GEN_TEST lightning::ln::msgs::UpdateFee test_msg_simple ""
 GEN_TEST lightning::ln::msgs::UpdateFulfillHTLC test_msg_simple ""
+GEN_TEST lightning::ln::msgs::ChannelReestablish test_msg_simple ""
 
-GEN_TEST lightning::ln::msgs::ChannelReestablish test_msg ""
 GEN_TEST lightning::ln::msgs::DecodedOnionErrorPacket test_msg ""
 
 GEN_TEST lightning::ln::msgs::ChannelAnnouncement test_msg_exact ""

fuzz/src/msg_targets/msg_channel_reestablish.rs

@@ -15,11 +15,11 @@ use crate::utils::test_logger;
 
 #[inline]
 pub fn msg_channel_reestablish_test<Out: test_logger::Output>(data: &[u8], _out: Out) {
-	test_msg!(lightning::ln::msgs::ChannelReestablish, data);
+	test_msg_simple!(lightning::ln::msgs::ChannelReestablish, data);
 }
 
 #[no_mangle]
 pub extern "C" fn msg_channel_reestablish_run(data: *const u8, datalen: usize) {
 	let data = unsafe { std::slice::from_raw_parts(data, datalen) };
-	test_msg!(lightning::ln::msgs::ChannelReestablish, data);
+	test_msg_simple!(lightning::ln::msgs::ChannelReestablish, data);
 }

lightning/src/ln/channel.rs

@@ -5686,6 +5686,9 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 			// overflow here.
 			next_remote_commitment_number: INITIAL_COMMITMENT_NUMBER - self.cur_counterparty_commitment_transaction_number - 1,
 			data_loss_protect,
+			// TODO: If we've sent `commtiment_signed` for an interactive transaction construction but have not received `tx_signatures`
+			// we MUST set `next_funding_txid` to the txid of that interactive transaction, else we MUST NOT set it.
+			next_funding_txid: None,
 		}
 	}