Populate Channel::interactive_tx_signing_session on read

We allow persisting `Channel`s in a `ChannelState::FundingNegotiated`
only if `Channel::interactive_tx_signing_session` is Some.

When reading a persisted `Channel` in `ChannelState::FundingNegotiate`,
we populate `Channel::interactive_tx_signing_session` with appropriate
values for accepting dual-funded channels without contributing.
Currently we don't need persistence, but we will need to persist
some information when we allow contributing funds to dual-funded
channels.

lightning/src/ln/channel.rs

@@ -31,8 +31,9 @@ use crate::ln::types::ChannelId;
 use crate::types::payment::{PaymentPreimage, PaymentHash};
 use crate::types::features::{ChannelTypeFeatures, InitFeatures};
 use crate::ln::interactivetxs::{
-	get_output_weight, HandleTxCompleteResult, InteractiveTxConstructor, InteractiveTxConstructorArgs,
-	InteractiveTxSigningSession, InteractiveTxMessageSendResult, TX_COMMON_FIELDS_WEIGHT,
+	ConstructedTransaction, get_output_weight, HandleTxCompleteResult, InteractiveTxConstructor,
+	InteractiveTxConstructorArgs, InteractiveTxSigningSession, InteractiveTxMessageSendResult,
+	TX_COMMON_FIELDS_WEIGHT,
 };
 use crate::ln::msgs;
 use crate::ln::msgs::{ClosingSigned, ClosingSignedFeeRange, DecodeError};
@@ -9244,7 +9245,16 @@ impl<SP: Deref> Writeable for Channel<SP> where SP::Target: SignerProvider {
 		self.context.channel_id.write(writer)?;
 		{
 			let mut channel_state = self.context.channel_state;
-			if matches!(channel_state, ChannelState::AwaitingChannelReady(_)|ChannelState::ChannelReady(_)) {
+			if matches!(channel_state, ChannelState::AwaitingChannelReady(_)|ChannelState::ChannelReady(_))
+			{
+				channel_state.set_peer_disconnected();
+			} else if self.interactive_tx_signing_session.is_some() {
+				if !matches!(channel_state, ChannelState::FundingNegotiated) {
+					debug_assert!(false, "V2 channels in pre-signing state should not be written");
+				}
+				// This is a V2 session which has an active signing session
+				// TODO(dual_funding): When we allow contributing funds to dual-funded channels,
+				// we will need to handle persisting appropriate signing session state.
 				channel_state.set_peer_disconnected();
 			} else {
 				debug_assert!(false, "Pre-funded/shutdown channels should not be written");
@@ -9898,6 +9908,28 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 		let mut next_holder_commitment_point_opt: Option<PublicKey> = None;
 		let mut is_manual_broadcast = None;
 
+		let interactive_tx_signing_session =  if matches!(channel_state, ChannelState::FundingNegotiated) {
+			if let Some(ref funding_tx) = funding_transaction {
+				// TODO(dual_funding): When we allow contributing funds to dual-funded channels,
+				// we will need to handle persisting appropriate signing session state.
+				Some(InteractiveTxSigningSession {
+					unsigned_tx: ConstructedTransaction::default(),
+					counterparty_sent_tx_signatures: false,
+					holder_sends_tx_signatures_first: true, // TODO(dual_funding): Fixed to true as we currently don't contribute inputs.
+					received_commitment_signed: true, // We only enter the FundingNegotiated state once we have persisted the initial channel monitor.
+					holder_tx_signatures: Some(msgs::TxSignatures {
+						channel_id,
+						tx_hash: funding_tx.compute_txid(),
+						witnesses: vec![], // TODO(dual_funding): Fixed to true as we currently don't contribute inputs.
+						shared_input_signature: None, // TODO(splicing): Only relevant to splicing
+					})
+				})
+			} else {
+				debug_assert!(false, "Tried to read V2 channel with signing session but there is no funding transaction");
+				None
+			}
+		} else { None };
+
 		read_tlv_fields!(reader, {
 			(0, announcement_sigs, option),
 			(1, minimum_depth, option),
@@ -10194,7 +10226,7 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 				blocked_monitor_updates: blocked_monitor_updates.unwrap(),
 				is_manual_broadcast: is_manual_broadcast.unwrap_or(false),
 			},
-			interactive_tx_signing_session: None,
+			interactive_tx_signing_session,
 		})
 	}
 }

lightning/src/ln/interactivetxs.rs

@@ -169,6 +169,23 @@ pub(crate) struct ConstructedTransaction {
 	holder_sends_tx_signatures_first: bool,
 }
 
+impl Default for ConstructedTransaction {
+	fn default() -> Self {
+		ConstructedTransaction {
+			holder_is_initiator: false,
+			inputs: vec![],
+			outputs: vec![],
+			local_inputs_value_satoshis: 0,
+			local_outputs_value_satoshis: 0,
+			remote_inputs_value_satoshis: 0,
+			remote_outputs_value_satoshis: 0,
+			lock_time: AbsoluteLockTime::from_height(0)
+				.expect("0 is a valid block height for a locktime"),
+			holder_sends_tx_signatures_first: true,
+		}
+	}
+}
+
 impl ConstructedTransaction {
 	fn new(context: NegotiationContext) -> Self {
 		let local_inputs_value_satoshis = context