Track funding negotiation state with an enum

PendingSplice holds a FundingScope being negotiated. However, when
implementing funding negotiation, other states are possible depending on
which party initiated the splice. Using an enum prevents needing various
Option fields which may result in invalid states.

When the user initiates the splice, the FundingNegotiationContext must
be held until the counterparty responds with splice_ack. At that point
enough information becomes available to create a new FundingScope and an
InteractiveTxConstructor.

When the counterparty initiates the splice, both a new FundingScope and
an InteractiveTxConstructor can be created immediately when responding
with splice_ack.

After the transaction is constructed, those are no longer needed. At
that point an InteractiveTxSigningSession is tracked until signatures
are exchanged.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -1821,7 +1821,8 @@ where
 			ChannelPhase::Funded(mut funded_channel) => {
 				#[cfg(splicing)]
 				let has_negotiated_pending_splice = funded_channel.pending_splice.as_ref()
-					.map(|pending_splice| pending_splice.funding.is_some())
+					.and_then(|pending_splice| pending_splice.funding_negotiation.as_ref())
+					.map(|funding_negotiation| funding_negotiation.as_funding().is_some())
 					.unwrap_or(false);
 				#[cfg(splicing)]
 				let session_received_commitment_signed = funded_channel
@@ -2171,7 +2172,7 @@ impl FundingScope {
 #[cfg(splicing)]
 struct PendingSplice {
 	pub our_funding_contribution: i64,
-	funding: Option<FundingScope>,
+	funding_negotiation: Option<FundingNegotiation>,
 
 	/// The funding txid used in the `splice_locked` sent to the counterparty.
 	sent_funding_txid: Option<Txid>,
@@ -2180,6 +2181,24 @@ struct PendingSplice {
 	received_funding_txid: Option<Txid>,
 }
 
+#[cfg(splicing)]
+enum FundingNegotiation {
+	AwaitingAck(FundingNegotiationContext),
+	Pending(FundingScope, InteractiveTxConstructor),
+	AwaitingSignatures(FundingScope),
+}
+
+#[cfg(splicing)]
+impl FundingNegotiation {
+	fn as_funding(&self) -> Option<&FundingScope> {
+		match self {
+			FundingNegotiation::AwaitingAck(_) => None,
+			FundingNegotiation::Pending(funding, _) => Some(funding),
+			FundingNegotiation::AwaitingSignatures(funding) => Some(funding),
+		}
+	}
+}
+
 #[cfg(splicing)]
 impl PendingSplice {
 	fn check_get_splice_locked<SP: Deref>(
@@ -6808,7 +6827,8 @@ where
 		let pending_splice_funding = self
 			.pending_splice
 			.as_ref()
-			.and_then(|pending_splice| pending_splice.funding.as_ref())
+			.and_then(|pending_splice| pending_splice.funding_negotiation.as_ref())
+			.and_then(|funding_negotiation| funding_negotiation.as_funding())
 			.expect("Funding must exist for negotiated pending splice");
 		let (holder_commitment_tx, _) = self.context.validate_commitment_signed(
 			pending_splice_funding,
@@ -10436,7 +10456,7 @@ where
 
 		self.pending_splice = Some(PendingSplice {
 			our_funding_contribution: our_funding_contribution_satoshis,
-			funding: None,
+			funding_negotiation: None,
 			sent_funding_txid: None,
 			received_funding_txid: None,
 		});