Move DiscardFunding generation into finish_close_channel

Currently the channel shutdown sequence has a number of steps which
all the shutdown callsites have to call. Because many shutdown
cases are rare error cases, its relatively easy to miss a call and
leave users without `Event`s or miss some important cleanup.

One of those steps, calling `issue_channel_close_events`, is rather
easy to remove, as it only generates two events, which can simply
be moved to another shutdown step.

Here we move the first of the two events, `DiscardFunding`, into
`finish_force_close_channel`.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -823,6 +823,7 @@ pub(crate) struct ShutdownResult {
 	pub(crate) unbroadcasted_batch_funding_txid: Option<Txid>,
 	pub(crate) channel_id: ChannelId,
 	pub(crate) counterparty_node_id: PublicKey,
+	pub(crate) unbroadcasted_funding_tx: Option<Transaction>,
 }
 
 /// If the majority of the channels funds are to the fundee and the initiator holds only just
@@ -2399,6 +2400,7 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider  {
 			} else { None }
 		} else { None };
 		let unbroadcasted_batch_funding_txid = self.unbroadcasted_batch_funding_txid();
+		let unbroadcasted_funding_tx = self.unbroadcasted_funding();
 
 		self.channel_state = ChannelState::ShutdownComplete;
 		self.update_time_counter += 1;
@@ -2408,6 +2410,7 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider  {
 			unbroadcasted_batch_funding_txid,
 			channel_id: self.channel_id,
 			counterparty_node_id: self.counterparty_node_id,
+			unbroadcasted_funding_tx,
 		}
 	}
 
@@ -4940,6 +4943,7 @@ impl<SP: Deref> Channel<SP> where
 					unbroadcasted_batch_funding_txid: self.context.unbroadcasted_batch_funding_txid(),
 					channel_id: self.context.channel_id,
 					counterparty_node_id: self.context.counterparty_node_id,
+					unbroadcasted_funding_tx: self.context.unbroadcasted_funding(),
 				};
 				let tx = self.build_signed_closing_transaction(&mut closing_tx, &msg.signature, &sig);
 				self.context.channel_state = ChannelState::ShutdownComplete;
@@ -4970,6 +4974,7 @@ impl<SP: Deref> Channel<SP> where
 								unbroadcasted_batch_funding_txid: self.context.unbroadcasted_batch_funding_txid(),
 								channel_id: self.context.channel_id,
 								counterparty_node_id: self.context.counterparty_node_id,
+								unbroadcasted_funding_tx: self.context.unbroadcasted_funding(),
 							};
 							self.context.channel_state = ChannelState::ShutdownComplete;
 							self.context.update_time_counter += 1;

lightning/src/ln/channelmanager.rs

@@ -2704,14 +2704,6 @@ where
 	/// Helper function that issues the channel close events
 	fn issue_channel_close_events(&self, context: &ChannelContext<SP>, closure_reason: ClosureReason) {
 		let mut pending_events_lock = self.pending_events.lock().unwrap();
-		match context.unbroadcasted_funding() {
-			Some(transaction) => {
-				pending_events_lock.push_back((events::Event::DiscardFunding {
-					channel_id: context.channel_id(), transaction
-				}, None));
-			},
-			None => {},
-		}
 		pending_events_lock.push_back((events::Event::ChannelClosed {
 			channel_id: context.channel_id(),
 			user_channel_id: context.get_user_id(),
@@ -2897,6 +2889,15 @@ where
 				"Closing a batch where all channels have completed initial monitor update",
 			);
 		}
+
+		{
+			let mut pending_events = self.pending_events.lock().unwrap();
+			if let Some(transaction) = shutdown_res.unbroadcasted_funding_tx {
+				pending_events.push_back((events::Event::DiscardFunding {
+					channel_id: shutdown_res.channel_id, transaction
+				}, None));
+			}
+		}
 		for shutdown_result in shutdown_results.drain(..) {
 			self.finish_close_channel(shutdown_result);
 		}