Add a QuiescentAction to track why we're going quiescent

When we initiate quiescence, it should always be because we're
trying to accomplish something (in the short term only splicing).
In order to actually do that thing, we need to store the
instructions for that thing somewhere the splicing logic knows to
look at once we reach quiescence.

Here we add a simple enum which will eventually store such actions.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -1863,6 +1863,7 @@ where
 					holder_commitment_point,
 					#[cfg(splicing)]
 					pending_splice: None,
+					quiescent_action: None,
 				};
 				let res = funded_channel.initial_commitment_signed_v2(msg, best_block, signer_provider, logger)
 					.map(|monitor| (Some(monitor), None))
@@ -2429,6 +2430,15 @@ impl PendingSplice {
 	}
 }
 
+pub(crate) enum QuiescentAction {
+	// TODO: Make this test-only once we have another variant (as some code requires *a* variant).
+	DoNothing,
+}
+
+impl_writeable_tlv_based_enum_upgradable!(QuiescentAction,
+	(99, DoNothing) => {},
+);
+
 /// Wrapper around a [`Transaction`] useful for caching the result of [`Transaction::compute_txid`].
 struct ConfirmedTransaction<'a> {
 	tx: &'a Transaction,
@@ -6050,6 +6060,12 @@ where
 	/// Info about an in-progress, pending splice (if any), on the pre-splice channel
 	#[cfg(splicing)]
 	pending_splice: Option<PendingSplice>,
+
+	/// Once we become quiescent, if we're the initiator, there's some action we'll want to take.
+	/// This keeps track of that action. Note that if we become quiescent and we're not the
+	/// initiator we may be able to merge this action into what the counterparty wanted to do (e.g.
+	/// in the case of splicing).
+	quiescent_action: Option<QuiescentAction>,
 }
 
 #[cfg(splicing)]
@@ -11529,7 +11545,7 @@ where
 	#[cfg(any(test, fuzzing))]
 	#[rustfmt::skip]
 	pub fn propose_quiescence<L: Deref>(
-		&mut self, logger: &L,
+		&mut self, logger: &L, action: QuiescentAction,
 	) -> Result<Option<msgs::Stfu>, ChannelError>
 	where
 		L::Target: Logger,
@@ -11541,11 +11557,13 @@ where
 				"Channel is not in a live state to propose quiescence".to_owned()
 			));
 		}
-		if self.context.channel_state.is_quiescent() {
-			return Err(ChannelError::Ignore("Channel is already quiescent".to_owned()));
+		if self.quiescent_action.is_some() {
+			return Err(ChannelError::Ignore("Channel is already quiescing".to_owned()));
 		}
 
-		if self.context.channel_state.is_awaiting_quiescence()
+		self.quiescent_action = Some(action);
+		if self.context.channel_state.is_quiescent()
+			|| self.context.channel_state.is_awaiting_quiescence()
 			|| self.context.channel_state.is_local_stfu_sent()
 		{
 			return Ok(None);
@@ -11664,6 +11682,21 @@ where
 			if !is_holder_quiescence_initiator { " not" } else { "" }
 		);
 
+		if is_holder_quiescence_initiator {
+			match self.quiescent_action.take() {
+				None => {
+					debug_assert!(false);
+					return Err(ChannelError::WarnAndDisconnect(
+						"Internal Error: Didn't have anything to do after reaching quiescence".to_owned()
+					));
+				},
+				Some(QuiescentAction::DoNothing) => {
+					// In quiescence test we want to just hang out here, letting the test manually
+					// leave quiescence.
+				},
+			}
+		}
+
 		Ok(None)
 	}
 
@@ -12029,6 +12062,7 @@ where
 			holder_commitment_point,
 			#[cfg(splicing)]
 			pending_splice: None,
+			quiescent_action: None,
 		};
 
 		let need_channel_ready = channel.check_get_channel_ready(0, logger).is_some()
@@ -12315,6 +12349,7 @@ where
 			holder_commitment_point,
 			#[cfg(splicing)]
 			pending_splice: None,
+			quiescent_action: None,
 		};
 		let need_channel_ready = channel.check_get_channel_ready(0, logger).is_some()
 			|| channel.context.signer_pending_channel_ready;
@@ -13974,6 +14009,7 @@ where
 			holder_commitment_point,
 			#[cfg(splicing)]
 			pending_splice: None,
+			quiescent_action: None,
 		})
 	}
 }

lightning/src/ln/channelmanager.rs

@@ -58,8 +58,8 @@ use crate::events::{
 };
 use crate::events::{FundingInfo, PaidBolt12Invoice};
 use crate::ln::chan_utils::selected_commitment_sat_per_1000_weight;
-// Since this struct is returned in `list_channels` methods, expose it here in case users want to
-// construct one themselves.
+#[cfg(any(test, fuzzing))]
+use crate::ln::channel::QuiescentAction;
 use crate::ln::channel::{
 	self, hold_time_since, Channel, ChannelError, ChannelUpdateStatus, FundedChannel,
 	InboundV1Channel, OutboundV1Channel, PendingV2Channel, ReconnectionMsg, ShutdownResult,
@@ -11712,7 +11712,7 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 							&self.logger, Some(*counterparty_node_id), Some(*channel_id), None
 						);
 
-						match chan.propose_quiescence(&&logger) {
+						match chan.propose_quiescence(&&logger, QuiescentAction::DoNothing) {
 							Ok(None) => {},
 							Ok(Some(stfu)) => {
 								peer_state.pending_msg_events.push(MessageSendEvent::SendStfu {