Add validate_splice_ack helper function

As in `splice_init`, this helps clearly delineate `splice_ack` message
validation from the subsequent state mutations.

This is a code-move.

Author: tankyleo

lightning/src/ln/channel.rs

@@ -10821,7 +10821,7 @@ where
 		})
 	}
 
-	/// Handle splice_ack
+	/// See also [`validate_splice_ack`]
 	#[cfg(splicing)]
 	pub(crate) fn splice_ack<ES: Deref, L: Deref>(
 		&mut self, msg: &msgs::SpliceAck, signer_provider: &SP, entropy_source: &ES,
@@ -10831,42 +10831,7 @@ where
 		ES::Target: EntropySource,
 		L::Target: Logger,
 	{
-		// TODO(splicing): Add check that we are the splice (quiescence) initiator
-
-		let funding_negotiation_context = match &self
-			.pending_splice
-			.as_ref()
-			.ok_or(ChannelError::Ignore(format!("Channel is not in pending splice")))?
-			.funding_negotiation
-		{
-			Some(FundingNegotiation::AwaitingAck(context)) => context,
-			Some(FundingNegotiation::ConstructingTransaction(_, _))
-			| Some(FundingNegotiation::AwaitingSignatures(_)) => {
-				return Err(ChannelError::WarnAndDisconnect(format!(
-					"Got unexpected splice_ack; splice negotiation already in progress"
-				)));
-			},
-			None => {
-				return Err(ChannelError::Ignore(format!(
-					"Got unexpected splice_ack; no splice negotiation in progress"
-				)));
-			},
-		};
-
-		let our_funding_contribution_satoshis =
-			funding_negotiation_context.our_funding_contribution_satoshis;
-		let their_funding_contribution_satoshis = msg.funding_contribution_satoshis;
-
-		let splice_funding = FundingScope::for_splice(
-			&self.funding,
-			&self.context,
-			our_funding_contribution_satoshis,
-			their_funding_contribution_satoshis,
-			msg.funding_pubkey,
-		)?;
-
-		// TODO(splicing): Pre-check for reserve requirement
-		// (Note: It should also be checked later at tx_complete)
+		let splice_funding = self.validate_splice_ack(msg)?;
 
 		log_info!(
 			logger,
@@ -10912,6 +10877,49 @@ where
 		Ok(tx_msg_opt)
 	}
 
+	/// Checks during handling splice_ack
+	#[cfg(splicing)]
+	fn validate_splice_ack(&self, msg: &msgs::SpliceAck) -> Result<FundingScope, ChannelError> {
+		// TODO(splicing): Add check that we are the splice (quiescence) initiator
+
+		let funding_negotiation_context = match &self
+			.pending_splice
+			.as_ref()
+			.ok_or(ChannelError::Ignore(format!("Channel is not in pending splice")))?
+			.funding_negotiation
+		{
+			Some(FundingNegotiation::AwaitingAck(context)) => context,
+			Some(FundingNegotiation::ConstructingTransaction(_, _))
+			| Some(FundingNegotiation::AwaitingSignatures(_)) => {
+				return Err(ChannelError::WarnAndDisconnect(format!(
+					"Got unexpected splice_ack; splice negotiation already in progress"
+				)));
+			},
+			None => {
+				return Err(ChannelError::Ignore(format!(
+					"Got unexpected splice_ack; no splice negotiation in progress"
+				)));
+			},
+		};
+
+		let our_funding_contribution_satoshis =
+			funding_negotiation_context.our_funding_contribution_satoshis;
+		let their_funding_contribution_satoshis = msg.funding_contribution_satoshis;
+
+		let splice_funding = FundingScope::for_splice(
+			&self.funding,
+			&self.context,
+			our_funding_contribution_satoshis,
+			their_funding_contribution_satoshis,
+			msg.funding_pubkey,
+		)?;
+
+		// TODO(splicing): Pre-check for reserve requirement
+		// (Note: It should also be checked later at tx_complete)
+
+		Ok(splice_funding)
+	}
+
 	#[cfg(splicing)]
 	pub fn splice_locked<NS: Deref, L: Deref>(
 		&mut self, msg: &msgs::SpliceLocked, node_signer: &NS, chain_hash: ChainHash,