Merge pull request #3588 from wpaulino/quiescence

Implement quiescence protocol

fuzz/src/chanmon_consistency.rs

@@ -978,7 +978,9 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 	lock_fundings!(nodes);
 
 	let chan_a = nodes[0].list_usable_channels()[0].short_channel_id.unwrap();
+	let chan_a_id = nodes[0].list_usable_channels()[0].channel_id;
 	let chan_b = nodes[2].list_usable_channels()[0].short_channel_id.unwrap();
+	let chan_b_id = nodes[2].list_usable_channels()[0].channel_id;
 
 	let mut p_id: u8 = 0;
 	let mut p_idx: u64 = 0;
@@ -1039,6 +1041,10 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							if Some(*node_id) == expect_drop_id { panic!("peer_disconnected should drop msgs bound for the disconnected peer"); }
 							*node_id == a_id
 						},
+						events::MessageSendEvent::SendStfu { ref node_id, .. } => {
+							if Some(*node_id) == expect_drop_id { panic!("peer_disconnected should drop msgs bound for the disconnected peer"); }
+							*node_id == a_id
+						},
 						events::MessageSendEvent::SendChannelReady { .. } => continue,
 						events::MessageSendEvent::SendAnnouncementSignatures { .. } => continue,
 						events::MessageSendEvent::SendChannelUpdate { ref node_id, ref msg } => {
@@ -1101,7 +1107,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							for (idx, dest) in nodes.iter().enumerate() {
 								if dest.get_our_node_id() == node_id {
 									for update_add in update_add_htlcs.iter() {
-										out.locked_write(format!("Delivering update_add_htlc to node {}.\n", idx).as_bytes());
+										out.locked_write(format!("Delivering update_add_htlc from node {} to node {}.\n", $node, idx).as_bytes());
 										if !$corrupt_forward {
 											dest.handle_update_add_htlc(nodes[$node].get_our_node_id(), update_add);
 										} else {
@@ -1116,19 +1122,19 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 										}
 									}
 									for update_fulfill in update_fulfill_htlcs.iter() {
-										out.locked_write(format!("Delivering update_fulfill_htlc to node {}.\n", idx).as_bytes());
+										out.locked_write(format!("Delivering update_fulfill_htlc from node {} to node {}.\n", $node, idx).as_bytes());
 										dest.handle_update_fulfill_htlc(nodes[$node].get_our_node_id(), update_fulfill);
 									}
 									for update_fail in update_fail_htlcs.iter() {
-										out.locked_write(format!("Delivering update_fail_htlc to node {}.\n", idx).as_bytes());
+										out.locked_write(format!("Delivering update_fail_htlc from node {} to node {}.\n", $node, idx).as_bytes());
 										dest.handle_update_fail_htlc(nodes[$node].get_our_node_id(), update_fail);
 									}
 									for update_fail_malformed in update_fail_malformed_htlcs.iter() {
-										out.locked_write(format!("Delivering update_fail_malformed_htlc to node {}.\n", idx).as_bytes());
+										out.locked_write(format!("Delivering update_fail_malformed_htlc from node {} to node {}.\n", $node, idx).as_bytes());
 										dest.handle_update_fail_malformed_htlc(nodes[$node].get_our_node_id(), update_fail_malformed);
 									}
 									if let Some(msg) = update_fee {
-										out.locked_write(format!("Delivering update_fee to node {}.\n", idx).as_bytes());
+										out.locked_write(format!("Delivering update_fee from node {} to node {}.\n", $node, idx).as_bytes());
 										dest.handle_update_fee(nodes[$node].get_our_node_id(), &msg);
 									}
 									let processed_change = !update_add_htlcs.is_empty() || !update_fulfill_htlcs.is_empty() ||
@@ -1145,7 +1151,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 										} });
 										break;
 									}
-									out.locked_write(format!("Delivering commitment_signed to node {}.\n", idx).as_bytes());
+									out.locked_write(format!("Delivering commitment_signed from node {} to node {}.\n", $node, idx).as_bytes());
 									dest.handle_commitment_signed(nodes[$node].get_our_node_id(), &commitment_signed);
 									break;
 								}
@@ -1154,7 +1160,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 						events::MessageSendEvent::SendRevokeAndACK { ref node_id, ref msg } => {
 							for (idx, dest) in nodes.iter().enumerate() {
 								if dest.get_our_node_id() == *node_id {
-									out.locked_write(format!("Delivering revoke_and_ack to node {}.\n", idx).as_bytes());
+									out.locked_write(format!("Delivering revoke_and_ack from node {} to node {}.\n", $node, idx).as_bytes());
 									dest.handle_revoke_and_ack(nodes[$node].get_our_node_id(), msg);
 								}
 							}
@@ -1162,11 +1168,19 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 						events::MessageSendEvent::SendChannelReestablish { ref node_id, ref msg } => {
 							for (idx, dest) in nodes.iter().enumerate() {
 								if dest.get_our_node_id() == *node_id {
-									out.locked_write(format!("Delivering channel_reestablish to node {}.\n", idx).as_bytes());
+									out.locked_write(format!("Delivering channel_reestablish from node {} to node {}.\n", $node, idx).as_bytes());
 									dest.handle_channel_reestablish(nodes[$node].get_our_node_id(), msg);
 								}
 							}
 						},
+						events::MessageSendEvent::SendStfu { ref node_id, ref msg } => {
+							for (idx, dest) in nodes.iter().enumerate() {
+								if dest.get_our_node_id() == *node_id {
+									out.locked_write(format!("Delivering stfu from node {} to node {}.\n", $node, idx).as_bytes());
+									dest.handle_stfu(nodes[$node].get_our_node_id(), msg);
+								}
+							}
+						}
 						events::MessageSendEvent::SendChannelReady { .. } => {
 							// Can be generated as a reestablish response
 						},
@@ -1219,6 +1233,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							events::MessageSendEvent::UpdateHTLCs { .. } => {},
 							events::MessageSendEvent::SendRevokeAndACK { .. } => {},
 							events::MessageSendEvent::SendChannelReestablish { .. } => {},
+							events::MessageSendEvent::SendStfu { .. } => {},
 							events::MessageSendEvent::SendChannelReady { .. } => {},
 							events::MessageSendEvent::SendAnnouncementSignatures { .. } => {},
 							events::MessageSendEvent::SendChannelUpdate { ref msg, .. } => {
@@ -1245,6 +1260,7 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 							events::MessageSendEvent::UpdateHTLCs { .. } => {},
 							events::MessageSendEvent::SendRevokeAndACK { .. } => {},
 							events::MessageSendEvent::SendChannelReestablish { .. } => {},
+							events::MessageSendEvent::SendStfu { .. } => {},
 							events::MessageSendEvent::SendChannelReady { .. } => {},
 							events::MessageSendEvent::SendAnnouncementSignatures { .. } => {},
 							events::MessageSendEvent::SendChannelUpdate { ref msg, .. } => {
@@ -1688,6 +1704,19 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 				nodes[2].maybe_update_chan_fees();
 			},
 
+			0xa0 => {
+				nodes[0].maybe_propose_quiescence(&nodes[1].get_our_node_id(), &chan_a_id).unwrap()
+			},
+			0xa1 => {
+				nodes[1].maybe_propose_quiescence(&nodes[0].get_our_node_id(), &chan_a_id).unwrap()
+			},
+			0xa2 => {
+				nodes[1].maybe_propose_quiescence(&nodes[2].get_our_node_id(), &chan_b_id).unwrap()
+			},
+			0xa3 => {
+				nodes[2].maybe_propose_quiescence(&nodes[1].get_our_node_id(), &chan_b_id).unwrap()
+			},
+
 			0xf0 => complete_monitor_update(&monitor_a, &chan_1_id, &complete_first),
 			0xf1 => complete_monitor_update(&monitor_a, &chan_1_id, &complete_second),
 			0xf2 => complete_monitor_update(&monitor_a, &chan_1_id, &Vec::pop),
@@ -1753,34 +1782,49 @@ pub fn do_test<Out: Output>(data: &[u8], underlying_out: Out, anchors: bool) {
 					chan_b_disconnected = false;
 				}
 
-				for i in 0..std::usize::MAX {
-					if i == 100 {
-						panic!("It may take may iterations to settle the state, but it should not take forever");
-					}
-					// Then, make sure any current forwards make their way to their destination
-					if process_msg_events!(0, false, ProcessMessages::AllMessages) {
-						continue;
-					}
-					if process_msg_events!(1, false, ProcessMessages::AllMessages) {
-						continue;
-					}
-					if process_msg_events!(2, false, ProcessMessages::AllMessages) {
-						continue;
-					}
-					// ...making sure any pending PendingHTLCsForwardable events are handled and
-					// payments claimed.
-					if process_events!(0, false) {
-						continue;
-					}
-					if process_events!(1, false) {
-						continue;
-					}
-					if process_events!(2, false) {
-						continue;
-					}
-					break;
+				macro_rules! process_all_events {
+					() => {
+						for i in 0..std::usize::MAX {
+							if i == 100 {
+								panic!("It may take may iterations to settle the state, but it should not take forever");
+							}
+							// Then, make sure any current forwards make their way to their destination
+							if process_msg_events!(0, false, ProcessMessages::AllMessages) {
+								continue;
+							}
+							if process_msg_events!(1, false, ProcessMessages::AllMessages) {
+								continue;
+							}
+							if process_msg_events!(2, false, ProcessMessages::AllMessages) {
+								continue;
+							}
+							// ...making sure any pending PendingHTLCsForwardable events are handled and
+							// payments claimed.
+							if process_events!(0, false) {
+								continue;
+							}
+							if process_events!(1, false) {
+								continue;
+							}
+							if process_events!(2, false) {
+								continue;
+							}
+							break;
+						}
+					};
 				}
 
+				// At this point, we may be pending quiescence, so we'll process all messages to
+				// ensure we can complete its handshake. We'll then exit quiescence and process all
+				// messages again, to resolve any pending HTLCs (only irrevocably committed ones)
+				// before attempting to send more payments.
+				process_all_events!();
+				nodes[0].exit_quiescence(&nodes[1].get_our_node_id(), &chan_a_id).unwrap();
+				nodes[1].exit_quiescence(&nodes[0].get_our_node_id(), &chan_a_id).unwrap();
+				nodes[1].exit_quiescence(&nodes[2].get_our_node_id(), &chan_b_id).unwrap();
+				nodes[2].exit_quiescence(&nodes[1].get_our_node_id(), &chan_b_id).unwrap();
+				process_all_events!();
+
 				// Finally, make sure that at least one end of each channel can make a substantial payment
 				assert!(
 					send_payment(&nodes[0], &nodes[1], chan_a, 10_000_000, &mut p_id, &mut p_idx)

lightning-types/src/features.rs

@@ -74,6 +74,8 @@
 //!     (see [bLIP 32](https://github.com/lightning/blips/blob/master/blip-0032.md) for more information).
 //! - `ProvideStorage` - Indicates that we offer the capability to store data of our peers
 //! 	(see https://github.com/lightning/bolts/pull/1110 for more info).
+//! - `Quiescence` - protocol to quiesce a channel by indicating that "SomeThing Fundamental is Underway"
+//!     (see [BOLT-2](https://github.com/lightning/bolts/blob/master/02-peer-protocol.md#channel-quiescence) for more information).
 //!
 //! LDK knows about the following features, but does not support them:
 //! - `AnchorsNonzeroFeeHtlcTx` - the initial version of anchor outputs, which was later found to be
@@ -152,7 +154,7 @@ mod sealed {
 			// Byte 3
 			RouteBlinding | ShutdownAnySegwit | DualFund | Taproot,
 			// Byte 4
-			OnionMessages,
+			Quiescence | OnionMessages,
 			// Byte 5
 			ProvideStorage | ChannelType | SCIDPrivacy,
 			// Byte 6
@@ -173,7 +175,7 @@ mod sealed {
 			// Byte 3
 			RouteBlinding | ShutdownAnySegwit | DualFund | Taproot,
 			// Byte 4
-			OnionMessages,
+			Quiescence | OnionMessages,
 			// Byte 5
 			ProvideStorage | ChannelType | SCIDPrivacy,
 			// Byte 6
@@ -536,6 +538,16 @@ mod sealed {
 		supports_taproot,
 		requires_taproot
 	);
+	define_feature!(
+		35,
+		Quiescence,
+		[InitContext, NodeContext],
+		"Feature flags for `option_quiesce`.",
+		set_quiescence_optional,
+		set_quiescence_required,
+		supports_quiescence,
+		requires_quiescence
+	);
 	define_feature!(
 		39,
 		OnionMessages,
@@ -1195,6 +1207,7 @@ mod tests {
 		init_features.set_channel_type_optional();
 		init_features.set_scid_privacy_optional();
 		init_features.set_zero_conf_optional();
+		init_features.set_quiescence_optional();
 
 		assert!(init_features.initial_routing_sync());
 		assert!(!init_features.supports_upfront_shutdown_script());
@@ -1215,7 +1228,7 @@ mod tests {
 			assert_eq!(node_features.flags[1], 0b01010001);
 			assert_eq!(node_features.flags[2], 0b10001010);
 			assert_eq!(node_features.flags[3], 0b00001010);
-			assert_eq!(node_features.flags[4], 0b10000000);
+			assert_eq!(node_features.flags[4], 0b10001000);
 			assert_eq!(node_features.flags[5], 0b10100000);
 			assert_eq!(node_features.flags[6], 0b00001000);
 		}

lightning/src/ln/channel.rs

@@ -474,6 +474,10 @@ mod state_flags {
 	pub const LOCAL_SHUTDOWN_SENT: u32 = 1 << 11;
 	pub const SHUTDOWN_COMPLETE: u32 = 1 << 12;
 	pub const WAITING_FOR_BATCH: u32 = 1 << 13;
+	pub const AWAITING_QUIESCENCE: u32 = 1 << 14;
+	pub const LOCAL_STFU_SENT: u32 = 1 << 15;
+	pub const REMOTE_STFU_SENT: u32 = 1 << 16;
+	pub const QUIESCENT: u32 = 1 << 17;
 }
 
 define_state_flags!(
@@ -532,7 +536,26 @@ define_state_flags!(
 			messages as we'd be unable to determine which HTLCs they included in their `revoke_and_ack` \
 			implicit ACK, so instead we have to hold them away temporarily to be sent later.",
 			AWAITING_REMOTE_REVOKE, state_flags::AWAITING_REMOTE_REVOKE,
-			is_awaiting_remote_revoke, set_awaiting_remote_revoke, clear_awaiting_remote_revoke)
+			is_awaiting_remote_revoke, set_awaiting_remote_revoke, clear_awaiting_remote_revoke),
+		("Indicates a local request has been made for the channel to become quiescent. Both nodes \
+			must send `stfu` for the channel to become quiescent. This flag will be cleared and we \
+			will no longer attempt quiescence if either node requests a shutdown.",
+			AWAITING_QUIESCENCE, state_flags::AWAITING_QUIESCENCE,
+			is_awaiting_quiescence, set_awaiting_quiescence, clear_awaiting_quiescence),
+		("Indicates we have sent a `stfu` message to the counterparty. This message can only be sent \
+			if either `AWAITING_QUIESCENCE` or `REMOTE_STFU_SENT` is set. Shutdown requests are \
+			rejected if this flag is set.",
+			LOCAL_STFU_SENT, state_flags::LOCAL_STFU_SENT,
+			is_local_stfu_sent, set_local_stfu_sent, clear_local_stfu_sent),
+		("Indicates we have received a `stfu` message from the counterparty. Shutdown requests are \
+			rejected if this flag is set.",
+			REMOTE_STFU_SENT, state_flags::REMOTE_STFU_SENT,
+			is_remote_stfu_sent, set_remote_stfu_sent, clear_remote_stfu_sent),
+		("Indicates the quiescence handshake has completed and the channel is now quiescent. \
+			Updates are not allowed while this flag is set, and any outbound updates will go \
+			directly into the holding cell.",
+			QUIESCENT, state_flags::QUIESCENT,
+			is_quiescent, set_quiescent, clear_quiescent)
 	]
 );
 
@@ -646,6 +669,8 @@ impl ChannelState {
 		match self {
 			ChannelState::ChannelReady(flags) =>
 				!flags.is_set(ChannelReadyFlags::AWAITING_REMOTE_REVOKE) &&
+					!flags.is_set(ChannelReadyFlags::LOCAL_STFU_SENT) &&
+					!flags.is_set(ChannelReadyFlags::QUIESCENT) &&
 					!flags.is_set(FundedStateFlags::MONITOR_UPDATE_IN_PROGRESS.into()) &&
 					!flags.is_set(FundedStateFlags::PEER_DISCONNECTED.into()),
 			_ => {
@@ -663,6 +688,10 @@ impl ChannelState {
 	impl_state_flag!(is_their_channel_ready, set_their_channel_ready, clear_their_channel_ready, AwaitingChannelReady);
 	impl_state_flag!(is_waiting_for_batch, set_waiting_for_batch, clear_waiting_for_batch, AwaitingChannelReady);
 	impl_state_flag!(is_awaiting_remote_revoke, set_awaiting_remote_revoke, clear_awaiting_remote_revoke, ChannelReady);
+	impl_state_flag!(is_awaiting_quiescence, set_awaiting_quiescence, clear_awaiting_quiescence, ChannelReady);
+	impl_state_flag!(is_local_stfu_sent, set_local_stfu_sent, clear_local_stfu_sent, ChannelReady);
+	impl_state_flag!(is_remote_stfu_sent, set_remote_stfu_sent, clear_remote_stfu_sent, ChannelReady);
+	impl_state_flag!(is_quiescent, set_quiescent, clear_quiescent, ChannelReady);
 }
 
 pub const INITIAL_COMMITMENT_NUMBER: u64 = (1 << 48) - 1;
@@ -713,6 +742,7 @@ pub const MIN_THEIR_CHAN_RESERVE_SATOSHIS: u64 = 1000;
 pub(super) enum ChannelError {
 	Ignore(String),
 	Warn(String),
+	WarnAndDisconnect(String),
 	Close((String, ClosureReason)),
 	SendError(String),
 }
@@ -720,10 +750,11 @@ pub(super) enum ChannelError {
 impl fmt::Debug for ChannelError {
 	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 		match self {
-			&ChannelError::Ignore(ref e) => write!(f, "Ignore : {}", e),
-			&ChannelError::Warn(ref e) => write!(f, "Warn : {}", e),
-			&ChannelError::Close((ref e, _)) => write!(f, "Close : {}", e),
-			&ChannelError::SendError(ref e) => write!(f, "Not Found : {}", e),
+			&ChannelError::Ignore(ref e) => write!(f, "Ignore: {}", e),
+			&ChannelError::Warn(ref e) => write!(f, "Warn: {}", e),
+			&ChannelError::WarnAndDisconnect(ref e) => write!(f, "Disconnecting with warning: {}", e),
+			&ChannelError::Close((ref e, _)) => write!(f, "Close: {}", e),
+			&ChannelError::SendError(ref e) => write!(f, "Not Found: {}", e),
 		}
 	}
 }
@@ -733,6 +764,7 @@ impl fmt::Display for ChannelError {
 		match self {
 			&ChannelError::Ignore(ref e) => write!(f, "{}", e),
 			&ChannelError::Warn(ref e) => write!(f, "{}", e),
+			&ChannelError::WarnAndDisconnect(ref e) => write!(f, "{}", e),
 			&ChannelError::Close((ref e, _)) => write!(f, "{}", e),
 			&ChannelError::SendError(ref e) => write!(f, "{}", e),
 		}
@@ -1112,9 +1144,8 @@ pub(crate) const MIN_AFFORDABLE_HTLC_COUNT: usize = 4;
 ///   * `EXPIRE_PREV_CONFIG_TICKS` = convergence_delay / tick_interval
 pub(crate) const EXPIRE_PREV_CONFIG_TICKS: usize = 5;
 
-/// The number of ticks that may elapse while we're waiting for a response to a
-/// [`msgs::RevokeAndACK`] or [`msgs::ChannelReestablish`] message before we attempt to disconnect
-/// them.
+/// The number of ticks that may elapse while we're waiting for a response before we attempt to
+/// disconnect them.
 ///
 /// See [`ChannelContext::sent_message_awaiting_response`] for more information.
 pub(crate) const DISCONNECT_PEER_AWAITING_RESPONSE_TICKS: usize = 2;
@@ -1842,16 +1873,14 @@ pub(super) struct ChannelContext<SP: Deref> where SP::Target: SignerProvider {
 	pub workaround_lnd_bug_4006: Option<msgs::ChannelReady>,
 
 	/// An option set when we wish to track how many ticks have elapsed while waiting for a response
-	/// from our counterparty after sending a message. If the peer has yet to respond after reaching
-	/// `DISCONNECT_PEER_AWAITING_RESPONSE_TICKS`, a reconnection should be attempted to try to
-	/// unblock the state machine.
+	/// from our counterparty after entering specific states. If the peer has yet to respond after
+	/// reaching `DISCONNECT_PEER_AWAITING_RESPONSE_TICKS`, a reconnection should be attempted to
+	/// try to unblock the state machine.
 	///
-	/// This behavior is mostly motivated by a lnd bug in which we don't receive a message we expect
-	/// to in a timely manner, which may lead to channels becoming unusable and/or force-closed. An
-	/// example of such can be found at <https://github.com/lightningnetwork/lnd/issues/7682>.
-	///
-	/// This is currently only used when waiting for a [`msgs::ChannelReestablish`] or
-	/// [`msgs::RevokeAndACK`] message from the counterparty.
+	/// This behavior was initially motivated by a lnd bug in which we don't receive a message we
+	/// expect to in a timely manner, which may lead to channels becoming unusable and/or
+	/// force-closed. An example of such can be found at
+	/// <https://github.com/lightningnetwork/lnd/issues/7682>.
 	sent_message_awaiting_response: Option<usize>,
 
 	/// This channel's type, as negotiated during channel open
@@ -1897,6 +1926,7 @@ pub(super) struct ChannelContext<SP: Deref> where SP::Target: SignerProvider {
 	/// If we can't release a [`ChannelMonitorUpdate`] until some external action completes, we
 	/// store it here and only release it to the `ChannelManager` once it asks for it.
 	blocked_monitor_updates: Vec<PendingChannelMonitorUpdate>,
+
 	// The `next_funding_txid` field allows peers to finalize the signing steps of an interactive
 	// transaction construction, or safely abort that transaction if it was not signed by one of the
 	// peers, who has thus already removed it from its state.
@@ -1912,6 +1942,10 @@ pub(super) struct ChannelContext<SP: Deref> where SP::Target: SignerProvider {
 	// TODO(dual_funding): Persist this when we actually contribute funding inputs. For now we always
 	// send an empty witnesses array in `tx_signatures` as a V2 channel acceptor
 	next_funding_txid: Option<Txid>,
+
+	/// Only set when a counterparty `stfu` has been processed to track which node is allowed to
+	/// propose "something fundamental" upon becoming quiescent.
+	is_holder_quiescence_initiator: Option<bool>,
 }
 
 /// A channel struct implementing this trait can receive an initial counterparty commitment
@@ -2603,6 +2637,8 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 			is_manual_broadcast: false,
 
 			next_funding_txid: None,
+
+			is_holder_quiescence_initiator: None,
 		};
 
 		Ok((funding, channel_context))
@@ -2832,6 +2868,8 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 			local_initiated_shutdown: None,
 			is_manual_broadcast: false,
 			next_funding_txid: None,
+
+			is_holder_quiescence_initiator: None,
 		};
 
 		Ok((funding, channel_context))
@@ -2921,6 +2959,57 @@ impl<SP: Deref> ChannelContext<SP> where SP::Target: SignerProvider {
 		}
 	}
 
+	/// Checks whether the channel has any HTLC additions, HTLC removals, or fee updates that have
+	/// been sent by either side but not yet irrevocably committed on both commitments. Holding cell
+	/// updates are not considered because they haven't been sent to the peer yet.
+	///
+	/// This can be used to satisfy quiescence's requirement when sending `stfu`:
+	///  - MUST NOT send `stfu` if any of the sender's htlc additions, htlc removals
+	///    or fee updates are pending for either peer.
+	fn has_pending_channel_update(&self) -> bool {
+		// An update from the local/remote node may be pending on the remote/local commitment since
+		// they are not tracked within our state, so we rely on whether any `commitment_signed` or
+		// `revoke_and_ack` messages are owed.
+		//
+		// We check these flags first as they are more likely to be set.
+		if self.channel_state.is_awaiting_remote_revoke() || self.expecting_peer_commitment_signed
+			|| self.monitor_pending_revoke_and_ack || self.signer_pending_revoke_and_ack
+			|| self.monitor_pending_commitment_signed || self.signer_pending_commitment_update
+		{
+			return true;
+		}
+
+		// A fee update is pending on either commitment.
+		if self.pending_update_fee.is_some() {
+			return true;
+		}
+
+		if self.pending_inbound_htlcs.iter()
+			.any(|htlc| match htlc.state {
+				InboundHTLCState::Committed => false,
+				// An HTLC removal from the local node is pending on the remote commitment.
+				InboundHTLCState::LocalRemoved(_) => true,
+				// An HTLC add from the remote node is pending on the local commitment.
+				InboundHTLCState::RemoteAnnounced(_)
+					| InboundHTLCState::AwaitingRemoteRevokeToAnnounce(_)
+					| InboundHTLCState::AwaitingAnnouncedRemoteRevoke(_) => true,
+			})
+		{
+			return true;
+		}
+
+		self.pending_outbound_htlcs.iter()
+			.any(|htlc| match htlc.state {
+				OutboundHTLCState::Committed => false,
+				// An HTLC add from the local node is pending on the remote commitment.
+				OutboundHTLCState::LocalAnnounced(_) => true,
+				// An HTLC removal from the remote node is pending on the local commitment.
+				OutboundHTLCState::RemoteRemoved(_)
+					| OutboundHTLCState::AwaitingRemoteRevokeToRemove(_)
+					| OutboundHTLCState::AwaitingRemovedRemoteRevoke(_) => true,
+			})
+	}
+
 	// Public utilities:
 
 	pub fn channel_id(&self) -> ChannelId {
@@ -5140,6 +5229,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	pub fn update_add_htlc<F: Deref>(
 		&mut self, msg: &msgs::UpdateAddHTLC, fee_estimator: &LowerBoundedFeeEstimator<F>,
 	) -> Result<(), ChannelError> where F::Target: FeeEstimator {
+		if self.context.channel_state.is_remote_stfu_sent() || self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got add HTLC message while quiescent".to_owned()));
+		}
 		if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
 			return Err(ChannelError::close("Got add HTLC message when channel was not in an operational state".to_owned()));
 		}
@@ -5284,6 +5376,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	}
 
 	pub fn update_fulfill_htlc(&mut self, msg: &msgs::UpdateFulfillHTLC) -> Result<(HTLCSource, u64, Option<u64>), ChannelError> {
+		if self.context.channel_state.is_remote_stfu_sent() || self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got fulfill HTLC message while quiescent".to_owned()));
+		}
 		if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
 			return Err(ChannelError::close("Got fulfill HTLC message when channel was not in an operational state".to_owned()));
 		}
@@ -5295,6 +5390,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	}
 
 	pub fn update_fail_htlc(&mut self, msg: &msgs::UpdateFailHTLC, fail_reason: HTLCFailReason) -> Result<(), ChannelError> {
+		if self.context.channel_state.is_remote_stfu_sent() || self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got fail HTLC message while quiescent".to_owned()));
+		}
 		if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
 			return Err(ChannelError::close("Got fail HTLC message when channel was not in an operational state".to_owned()));
 		}
@@ -5307,6 +5405,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	}
 
 	pub fn update_fail_malformed_htlc(&mut self, msg: &msgs::UpdateFailMalformedHTLC, fail_reason: HTLCFailReason) -> Result<(), ChannelError> {
+		if self.context.channel_state.is_remote_stfu_sent() || self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got fail malformed HTLC message while quiescent".to_owned()));
+		}
 		if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
 			return Err(ChannelError::close("Got fail malformed HTLC message when channel was not in an operational state".to_owned()));
 		}
@@ -5358,6 +5459,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	pub fn commitment_signed<L: Deref>(&mut self, msg: &msgs::CommitmentSigned, logger: &L) -> Result<Option<ChannelMonitorUpdate>, ChannelError>
 		where L::Target: Logger
 	{
+		if self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got commitment_signed message while quiescent".to_owned()));
+		}
 		if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
 			return Err(ChannelError::close("Got commitment signed message when channel was not in an operational state".to_owned()));
 		}
@@ -5607,7 +5711,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	) -> (Option<ChannelMonitorUpdate>, Vec<(HTLCSource, PaymentHash)>)
 	where F::Target: FeeEstimator, L::Target: Logger
 	{
+		assert!(matches!(self.context.channel_state, ChannelState::ChannelReady(_)));
 		assert!(!self.context.channel_state.is_monitor_update_in_progress());
+		assert!(!self.context.channel_state.is_quiescent());
 		if self.context.holding_cell_htlc_updates.len() != 0 || self.context.holding_cell_update_fee.is_some() {
 			log_trace!(logger, "Freeing holding cell with {} HTLC updates{} in channel {}", self.context.holding_cell_htlc_updates.len(),
 				if self.context.holding_cell_update_fee.is_some() { " and a fee update" } else { "" }, &self.context.channel_id());
@@ -5640,7 +5746,16 @@ impl<SP: Deref> FundedChannel<SP> where
 							amount_msat, *payment_hash, cltv_expiry, source.clone(), onion_routing_packet.clone(),
 							false, skimmed_fee_msat, blinding_point, fee_estimator, logger
 						) {
-							Ok(_) => update_add_count += 1,
+							Ok(update_add_msg_opt) => {
+								// `send_htlc` only returns `Ok(None)`, when an update goes into
+								// the holding cell, but since we're currently freeing it, we should
+								// always expect to see the `update_add` go out.
+								debug_assert!(
+									update_add_msg_opt.is_some(),
+									"Must generate new update if we're freeing the holding cell"
+								);
+								update_add_count += 1;
+							},
 							Err(e) => {
 								match e {
 									ChannelError::Ignore(ref msg) => {
@@ -5743,6 +5858,9 @@ impl<SP: Deref> FundedChannel<SP> where
 	) -> Result<(Vec<(HTLCSource, PaymentHash)>, Option<ChannelMonitorUpdate>), ChannelError>
 	where F::Target: FeeEstimator, L::Target: Logger,
 	{
+		if self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got revoke_and_ack message while quiescent".to_owned()));
+		}
 		if !matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
 			return Err(ChannelError::close("Got revoke/ACK message when channel was not in an operational state".to_owned()));
 		}
@@ -5808,7 +5926,7 @@ impl<SP: Deref> FundedChannel<SP> where
 		// OK, we step the channel here and *then* if the new generation fails we can fail the
 		// channel based on that, but stepping stuff here should be safe either way.
 		self.context.channel_state.clear_awaiting_remote_revoke();
-		self.context.sent_message_awaiting_response = None;
+		self.mark_response_received();
 		self.context.counterparty_prev_commitment_point = self.context.counterparty_cur_commitment_point;
 		self.context.counterparty_cur_commitment_point = Some(msg.next_per_commitment_point);
 		self.context.cur_counterparty_commitment_transaction_number -= 1;
@@ -5960,29 +6078,7 @@ impl<SP: Deref> FundedChannel<SP> where
 
 		self.context.monitor_pending_update_adds.append(&mut pending_update_adds);
 
-		if self.context.channel_state.is_monitor_update_in_progress() {
-			// We can't actually generate a new commitment transaction (incl by freeing holding
-			// cells) while we can't update the monitor, so we just return what we have.
-			if require_commitment {
-				self.context.monitor_pending_commitment_signed = true;
-				// When the monitor updating is restored we'll call
-				// get_last_commitment_update_for_send(), which does not update state, but we're
-				// definitely now awaiting a remote revoke before we can step forward any more, so
-				// set it here.
-				let mut additional_update = self.build_commitment_no_status_check(logger);
-				// build_commitment_no_status_check may bump latest_monitor_id but we want them to be
-				// strictly increasing by one, so decrement it here.
-				self.context.latest_monitor_update_id = monitor_update.update_id;
-				monitor_update.updates.append(&mut additional_update.updates);
-			}
-			self.context.monitor_pending_forwards.append(&mut to_forward_infos);
-			self.context.monitor_pending_failures.append(&mut revoked_htlcs);
-			self.context.monitor_pending_finalized_fulfills.append(&mut finalized_claimed_htlcs);
-			log_debug!(logger, "Received a valid revoke_and_ack for channel {} but awaiting a monitor update resolution to reply.", &self.context.channel_id());
-			return_with_htlcs_to_fail!(Vec::new());
-		}
-
-		match self.free_holding_cell_htlcs(fee_estimator, logger) {
+		match self.maybe_free_holding_cell_htlcs(fee_estimator, logger) {
 			(Some(mut additional_update), htlcs_to_fail) => {
 				// free_holding_cell_htlcs may bump latest_monitor_id multiple times but we want them to be
 				// strictly increasing by one, so decrement it here.
@@ -5997,6 +6093,11 @@ impl<SP: Deref> FundedChannel<SP> where
 			},
 			(None, htlcs_to_fail) => {
 				if require_commitment {
+					// We can't generate a new commitment transaction yet so we just return what we
+					// have. When the monitor updating is restored we'll call
+					// get_last_commitment_update_for_send(), which does not update state, but we're
+					// definitely now awaiting a remote revoke before we can step forward any more,
+					// so set it here.
 					let mut additional_update = self.build_commitment_no_status_check(logger);
 
 					// build_commitment_no_status_check may bump latest_monitor_id but we want them to be
@@ -6004,10 +6105,24 @@ impl<SP: Deref> FundedChannel<SP> where
 					self.context.latest_monitor_update_id = monitor_update.update_id;
 					monitor_update.updates.append(&mut additional_update.updates);
 
-					log_debug!(logger, "Received a valid revoke_and_ack for channel {}. Responding with a commitment update with {} HTLCs failed. {} monitor update.",
-						&self.context.channel_id(),
-						update_fail_htlcs.len() + update_fail_malformed_htlcs.len(),
-						release_state_str);
+					log_debug!(logger, "Received a valid revoke_and_ack for channel {}. {} monitor update.",
+						&self.context.channel_id(), release_state_str);
+					if self.context.channel_state.can_generate_new_commitment() {
+						log_debug!(logger, "Responding with a commitment update with {} HTLCs failed for channel {}",
+							update_fail_htlcs.len() + update_fail_malformed_htlcs.len(),
+							&self.context.channel_id);
+					} else {
+						debug_assert!(htlcs_to_fail.is_empty());
+						let reason = if self.context.channel_state.is_local_stfu_sent() {
+							"exits quiescence"
+						} else if self.context.channel_state.is_monitor_update_in_progress() {
+							"completes pending monitor update"
+						} else {
+							"can continue progress"
+						};
+						log_debug!(logger, "Holding back commitment update until channel {} {}",
+							&self.context.channel_id, reason);
+					}
 
 					self.monitor_updating_paused(false, true, false, to_forward_infos, revoked_htlcs, finalized_claimed_htlcs);
 					return_with_htlcs_to_fail!(htlcs_to_fail);
@@ -6144,7 +6259,10 @@ impl<SP: Deref> FundedChannel<SP> where
 			return None;
 		}
 
-		if self.context.channel_state.is_awaiting_remote_revoke() || self.context.channel_state.is_monitor_update_in_progress() {
+		// Some of the checks of `can_generate_new_commitment` have already been done above, but
+		// it's much more brittle to not use it in favor of checking the remaining flags left, as it
+		// gives us one less code path to update if the method changes.
+		if !self.context.channel_state.can_generate_new_commitment() {
 			force_holding_cell = true;
 		}
 
@@ -6174,6 +6292,10 @@ impl<SP: Deref> FundedChannel<SP> where
 			return Err(())
 		}
 
+		// We only clear `peer_disconnected` if we were able to reestablish the channel. We always
+ 		// reset our awaiting response in case we failed reestablishment and are disconnecting.
+		self.context.sent_message_awaiting_response = None;
+
 		if self.context.channel_state.is_peer_disconnected() {
 			// While the below code should be idempotent, it's simpler to just return early, as
 			// redundant disconnect events can fire, though they should be rare.
@@ -6234,7 +6356,14 @@ impl<SP: Deref> FundedChannel<SP> where
 			}
 		}
 
-		self.context.sent_message_awaiting_response = None;
+		// Reset any quiescence-related state as it is implicitly terminated once disconnected.
+		if matches!(self.context.channel_state, ChannelState::ChannelReady(_)) {
+			self.context.channel_state.clear_awaiting_quiescence();
+			self.context.channel_state.clear_local_stfu_sent();
+			self.context.channel_state.clear_remote_stfu_sent();
+			self.context.channel_state.clear_quiescent();
+			self.context.is_holder_quiescence_initiator.take();
+		}
 
 		self.context.channel_state.set_peer_disconnected();
 		log_trace!(logger, "Peer disconnection resulted in {} remote-announced HTLC drops on channel {}", inbound_drop_count, &self.context.channel_id());
@@ -6351,10 +6480,6 @@ impl<SP: Deref> FundedChannel<SP> where
 			commitment_update = None;
 		}
 
-		if commitment_update.is_some() {
-			self.mark_awaiting_response();
-		}
-
 		self.context.monitor_pending_revoke_and_ack = false;
 		self.context.monitor_pending_commitment_signed = false;
 		let order = self.context.resend_order.clone();
@@ -6397,6 +6522,9 @@ impl<SP: Deref> FundedChannel<SP> where
 		if self.context.channel_state.is_peer_disconnected() {
 			return Err(ChannelError::close("Peer sent update_fee when we needed a channel_reestablish".to_owned()));
 		}
+		if self.context.channel_state.is_remote_stfu_sent() || self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::WarnAndDisconnect("Got fee update message while quiescent".to_owned()));
+		}
 		FundedChannel::<SP>::check_remote_fee(&self.context.channel_type, fee_estimator, msg.feerate_per_kw, Some(self.context.feerate_per_kw), logger)?;
 
 		self.context.pending_update_fee = Some((msg.feerate_per_kw, FeeUpdateState::RemoteAnnounced));
@@ -6708,7 +6836,7 @@ impl<SP: Deref> FundedChannel<SP> where
 		// Go ahead and unmark PeerDisconnected as various calls we may make check for it (and all
 		// remaining cases either succeed or ErrorMessage-fail).
 		self.context.channel_state.clear_peer_disconnected();
-		self.context.sent_message_awaiting_response = None;
+		self.mark_response_received();
 
 		let shutdown_msg = self.get_outbound_shutdown();
 
@@ -6764,9 +6892,6 @@ impl<SP: Deref> FundedChannel<SP> where
 		// AwaitingRemoteRevoke set, which indicates we sent a commitment_signed but haven't gotten
 		// the corresponding revoke_and_ack back yet.
 		let is_awaiting_remote_revoke = self.context.channel_state.is_awaiting_remote_revoke();
-		if is_awaiting_remote_revoke && !self.is_awaiting_monitor_update() {
-			self.mark_awaiting_response();
-		}
 		let next_counterparty_commitment_number = INITIAL_COMMITMENT_NUMBER - self.context.cur_counterparty_commitment_transaction_number + if is_awaiting_remote_revoke { 1 } else { 0 };
 
 		let channel_ready = if msg.next_local_commitment_number == 1 && INITIAL_COMMITMENT_NUMBER - self.holder_commitment_point.transaction_number() == 1 {
@@ -6951,26 +7076,38 @@ impl<SP: Deref> FundedChannel<SP> where
 		Ok((closing_signed, None, None))
 	}
 
-	// Marks a channel as waiting for a response from the counterparty. If it's not received
-	// [`DISCONNECT_PEER_AWAITING_RESPONSE_TICKS`] after sending our own to them, then we'll attempt
-	// a reconnection.
-	fn mark_awaiting_response(&mut self) {
-		self.context.sent_message_awaiting_response = Some(0);
+	fn mark_response_received(&mut self) {
+		self.context.sent_message_awaiting_response = None;
 	}
 
 	/// Determines whether we should disconnect the counterparty due to not receiving a response
 	/// within our expected timeframe.
 	///
-	/// This should be called on every [`super::channelmanager::ChannelManager::timer_tick_occurred`].
+	/// This should be called for peers with an active socket on every
+	/// [`super::channelmanager::ChannelManager::timer_tick_occurred`].
+	#[allow(clippy::assertions_on_constants)]
 	pub fn should_disconnect_peer_awaiting_response(&mut self) -> bool {
-		let ticks_elapsed = if let Some(ticks_elapsed) = self.context.sent_message_awaiting_response.as_mut() {
-			ticks_elapsed
+		if let Some(ticks_elapsed) = self.context.sent_message_awaiting_response.as_mut() {
+			*ticks_elapsed += 1;
+			*ticks_elapsed >= DISCONNECT_PEER_AWAITING_RESPONSE_TICKS
+		} else if
+			// Cleared upon receiving `channel_reestablish`.
+			self.context.channel_state.is_peer_disconnected()
+			// Cleared upon receiving `stfu`.
+			|| self.context.channel_state.is_local_stfu_sent()
+			// Cleared upon receiving a message that triggers the end of quiescence.
+			|| self.context.channel_state.is_quiescent()
+			// Cleared upon receiving `revoke_and_ack`.
+			|| self.context.has_pending_channel_update()
+		{
+			// This is the first tick we've seen after expecting to make forward progress.
+			self.context.sent_message_awaiting_response = Some(1);
+			debug_assert!(DISCONNECT_PEER_AWAITING_RESPONSE_TICKS > 1);
+			false
 		} else {
 			// Don't disconnect when we're not waiting on a response.
-			return false;
-		};
-		*ticks_elapsed += 1;
-		*ticks_elapsed >= DISCONNECT_PEER_AWAITING_RESPONSE_TICKS
+			false
+		}
 	}
 
 	pub fn shutdown(
@@ -6993,6 +7130,14 @@ impl<SP: Deref> FundedChannel<SP> where
 		}
 		assert!(!matches!(self.context.channel_state, ChannelState::ShutdownComplete));
 
+		// TODO: The spec is pretty vague regarding the handling of shutdown within quiescence.
+		if self.context.channel_state.is_local_stfu_sent()
+			|| self.context.channel_state.is_remote_stfu_sent()
+			|| self.context.channel_state.is_quiescent()
+		{
+			return Err(ChannelError::WarnAndDisconnect("Got shutdown request while quiescent".to_owned()));
+		}
+
 		if !script::is_bolt2_compliant(&msg.scriptpubkey, their_features) {
 			return Err(ChannelError::Warn(format!("Got a nonstandard scriptpubkey ({}) from remote peer", msg.scriptpubkey.to_hex_string())));
 		}
@@ -7029,6 +7174,11 @@ impl<SP: Deref> FundedChannel<SP> where
 		// From here on out, we may not fail!
 
 		self.context.channel_state.set_remote_shutdown_sent();
+		if self.context.channel_state.is_awaiting_quiescence() {
+			// We haven't been able to send `stfu` yet, and there's no point in attempting
+			// quiescence anymore since the counterparty wishes to close the channel.
+			self.context.channel_state.clear_awaiting_quiescence();
+		}
 		self.context.update_time_counter += 1;
 
 		let monitor_update = if update_shutdown_script {
@@ -8120,7 +8270,6 @@ impl<SP: Deref> FundedChannel<SP> where
 			log_info!(logger, "Sending a data_loss_protect with no previous remote per_commitment_secret for channel {}", &self.context.channel_id());
 			[0;32]
 		};
-		self.mark_awaiting_response();
 		msgs::ChannelReestablish {
 			channel_id: self.context.channel_id(),
 			// The protocol has two different commitment number concepts - the "commitment
@@ -8481,6 +8630,12 @@ impl<SP: Deref> FundedChannel<SP> where
 		target_feerate_sats_per_kw: Option<u32>, override_shutdown_script: Option<ShutdownScript>)
 	-> Result<(msgs::Shutdown, Option<ChannelMonitorUpdate>, Vec<(HTLCSource, PaymentHash)>), APIError>
 	{
+		if self.context.channel_state.is_local_stfu_sent()
+			|| self.context.channel_state.is_remote_stfu_sent()
+			|| self.context.channel_state.is_quiescent()
+		{
+			return Err(APIError::APIMisuseError { err: "Cannot begin shutdown while quiescent".to_owned() });
+		}
 		for htlc in self.context.pending_outbound_htlcs.iter() {
 			if let OutboundHTLCState::LocalAnnounced(_) = htlc.state {
 				return Err(APIError::APIMisuseError{err: "Cannot begin shutdown with pending HTLCs. Process pending events first".to_owned()});
@@ -8525,6 +8680,9 @@ impl<SP: Deref> FundedChannel<SP> where
 		// From here on out, we may not fail!
 		self.context.target_closing_feerate_sats_per_kw = target_feerate_sats_per_kw;
 		self.context.channel_state.set_local_shutdown_sent();
+		if self.context.channel_state.is_awaiting_quiescence() {
+			self.context.channel_state.clear_awaiting_quiescence();
+		}
 		self.context.local_initiated_shutdown = Some(());
 		self.context.update_time_counter += 1;
 
@@ -8590,6 +8748,200 @@ impl<SP: Deref> FundedChannel<SP> where
 			self.context.counterparty_max_htlc_value_in_flight_msat
 		);
 	}
+
+	#[cfg(any(test, fuzzing))]
+	pub fn propose_quiescence<L: Deref>(
+		&mut self, logger: &L,
+	) -> Result<Option<msgs::Stfu>, ChannelError>
+	where
+		L::Target: Logger,
+	{
+		log_debug!(logger, "Attempting to initiate quiescence");
+
+		if !self.context.is_live() {
+			return Err(ChannelError::Ignore(
+				"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.context.channel_state.is_awaiting_quiescence()
+			|| self.context.channel_state.is_local_stfu_sent()
+		{
+			return Ok(None);
+		}
+
+		self.context.channel_state.set_awaiting_quiescence();
+		Ok(Some(self.send_stfu(logger)?))
+	}
+
+	// Assumes we are either awaiting quiescence or our counterparty has requested quiescence.
+	pub fn send_stfu<L: Deref>(&mut self, logger: &L) -> Result<msgs::Stfu, ChannelError>
+	where
+		L::Target: Logger,
+	{
+		debug_assert!(!self.context.channel_state.is_local_stfu_sent());
+		// Either state being set implies the channel is live.
+		debug_assert!(
+			self.context.channel_state.is_awaiting_quiescence()
+				|| self.context.channel_state.is_remote_stfu_sent()
+		);
+		debug_assert!(self.context.is_live());
+
+		if self.context.has_pending_channel_update() {
+			return Err(ChannelError::Ignore(
+				"We cannot send `stfu` while state machine is pending".to_owned()
+			));
+		}
+
+		let initiator = if self.context.channel_state.is_remote_stfu_sent() {
+			// We may have also attempted to initiate quiescence.
+			self.context.channel_state.clear_awaiting_quiescence();
+			self.context.channel_state.clear_remote_stfu_sent();
+			self.context.channel_state.set_quiescent();
+			if let Some(initiator) = self.context.is_holder_quiescence_initiator.as_ref() {
+				log_debug!(
+					logger,
+					"Responding to counterparty stfu with our own, channel is now quiescent and we are{} the initiator",
+					if !initiator { " not" } else { "" }
+				);
+
+				*initiator
+			} else {
+				debug_assert!(false, "Quiescence initiator must have been set when we received stfu");
+				false
+			}
+		} else {
+			log_debug!(logger, "Sending stfu as quiescence initiator");
+			debug_assert!(self.context.channel_state.is_awaiting_quiescence());
+			self.context.channel_state.clear_awaiting_quiescence();
+			self.context.channel_state.set_local_stfu_sent();
+			true
+		};
+
+		Ok(msgs::Stfu { channel_id: self.context.channel_id, initiator })
+	}
+
+	pub fn stfu<L: Deref>(
+		&mut self, msg: &msgs::Stfu, logger: &L
+	) -> Result<Option<msgs::Stfu>, ChannelError> where L::Target: Logger {
+		if self.context.channel_state.is_quiescent() {
+			return Err(ChannelError::Warn("Channel is already quiescent".to_owned()));
+		}
+		if self.context.channel_state.is_remote_stfu_sent() {
+			return Err(ChannelError::Warn(
+				"Peer sent `stfu` when they already sent it and we've yet to become quiescent".to_owned()
+			));
+		}
+
+		if !self.context.is_live() {
+			return Err(ChannelError::Warn(
+				"Peer sent `stfu` when we were not in a live state".to_owned()
+			));
+		}
+
+		if self.context.channel_state.is_awaiting_quiescence()
+			|| !self.context.channel_state.is_local_stfu_sent()
+		{
+			if !msg.initiator {
+				return Err(ChannelError::WarnAndDisconnect(
+					"Peer sent unexpected `stfu` without signaling as initiator".to_owned()
+				));
+			}
+
+			// We don't check `has_pending_channel_update` prior to setting the flag because it
+			// considers pending updates from either node. This means we may accept a counterparty
+			// `stfu` while they had pending updates, but that's fine as we won't send ours until
+			// _all_ pending updates complete, allowing the channel to become quiescent then.
+			self.context.channel_state.set_remote_stfu_sent();
+
+			let is_holder_initiator = if self.context.channel_state.is_awaiting_quiescence() {
+				// We were also planning to propose quiescence, let the tie-breaker decide the
+				// initiator.
+				self.context.is_outbound()
+			} else {
+				false
+			};
+			self.context.is_holder_quiescence_initiator = Some(is_holder_initiator);
+
+			log_debug!(logger, "Received counterparty stfu proposing quiescence");
+			return self.send_stfu(logger).map(|stfu| Some(stfu));
+		}
+
+		// We already sent `stfu` and are now processing theirs. It may be in response to ours, or
+		// we happened to both send `stfu` at the same time and a tie-break is needed.
+		let is_holder_quiescence_initiator = !msg.initiator || self.context.is_outbound();
+		self.context.is_holder_quiescence_initiator = Some(is_holder_quiescence_initiator);
+
+		// We were expecting to receive `stfu` because we already sent ours.
+		self.mark_response_received();
+
+		if self.context.has_pending_channel_update() {
+			// Since we've already sent `stfu`, it should not be possible for one of our updates to
+			// be pending, so anything pending currently must be from a counterparty update.
+			return Err(ChannelError::WarnAndDisconnect(
+				"Received counterparty stfu while having pending counterparty updates".to_owned()
+			));
+		}
+
+		self.context.channel_state.clear_local_stfu_sent();
+		self.context.channel_state.set_quiescent();
+
+		log_debug!(
+			logger,
+			"Received counterparty stfu, channel is now quiescent and we are{} the initiator",
+			if !is_holder_quiescence_initiator { " not" } else { "" }
+		);
+
+		Ok(None)
+	}
+
+	pub fn try_send_stfu<L: Deref>(
+		&mut self, logger: &L,
+	) -> Result<Option<msgs::Stfu>, ChannelError>
+	where
+		L::Target: Logger,
+	{
+		// We must never see both stfu flags set, we always set the quiescent flag instead.
+		debug_assert!(
+			!(self.context.channel_state.is_local_stfu_sent()
+				&& self.context.channel_state.is_remote_stfu_sent())
+		);
+
+		// We need to send our `stfu`, either because we're trying to initiate quiescence, or the
+		// counterparty is and we've yet to send ours.
+		if self.context.channel_state.is_awaiting_quiescence()
+			|| (self.context.channel_state.is_remote_stfu_sent()
+				&& !self.context.channel_state.is_local_stfu_sent())
+		{
+			return self.send_stfu(logger).map(|stfu| Some(stfu));
+		}
+
+		// We're either:
+		//  - already quiescent
+		//  - in a state where quiescence is not possible
+		//  - not currently trying to become quiescent
+		Ok(None)
+	}
+
+	#[cfg(any(test, fuzzing))]
+	pub fn exit_quiescence(&mut self) -> bool {
+		// Make sure we either finished the quiescence handshake and are quiescent, or we never
+		// attempted to initiate quiescence at all.
+		debug_assert!(!self.context.channel_state.is_awaiting_quiescence());
+		debug_assert!(!self.context.channel_state.is_local_stfu_sent());
+		debug_assert!(!self.context.channel_state.is_remote_stfu_sent());
+
+		if self.context.channel_state.is_quiescent() {
+			self.mark_response_received();
+			self.context.channel_state.clear_quiescent();
+			self.context.is_holder_quiescence_initiator.take().expect("Must always be set while quiescent")
+		} else {
+			false
+		}
+	}
 }
 
 /// A not-yet-funded outbound (from holder) channel using V1 channel establishment.
@@ -9581,11 +9933,17 @@ impl<SP: Deref> Writeable for FundedChannel<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(_)) {
-				channel_state.set_peer_disconnected();
-			} else {
-				debug_assert!(false, "Pre-funded/shutdown channels should not be written");
+			match channel_state {
+				ChannelState::AwaitingChannelReady(_) => {},
+				ChannelState::ChannelReady(_) => {
+					channel_state.clear_awaiting_quiescence();
+					channel_state.clear_local_stfu_sent();
+					channel_state.clear_remote_stfu_sent();
+					channel_state.clear_quiescent();
+				},
+				_ => debug_assert!(false, "Pre-funded/shutdown channels should not be written"),
 			}
+			channel_state.set_peer_disconnected();
 			channel_state.to_u32().write(writer)?;
 		}
 		self.funding.channel_value_satoshis.write(writer)?;
@@ -10497,6 +10855,7 @@ 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),
+
 				// TODO(dual_funding): Instead of getting this from persisted value, figure it out based on the
 				// funding transaction and other channel state.
 				//
@@ -10504,6 +10863,8 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 				// during a signing session, but have not received `tx_signatures` we MUST set `next_funding_txid`
 				// to the txid of that interactive transaction, else we MUST NOT set it.
 				next_funding_txid: None,
+
+				is_holder_quiescence_initiator: None,
 			},
 			interactive_tx_signing_session: None,
 			holder_commitment_point,

lightning/src/ln/channelmanager.rs

@@ -840,6 +840,15 @@ impl MsgHandleErrInternal {
 						log_level: Level::Warn,
 					},
 				},
+				ChannelError::WarnAndDisconnect(msg) =>  LightningError {
+					err: msg.clone(),
+					action: msgs::ErrorAction::DisconnectPeerWithWarning {
+						msg: msgs::WarningMessage {
+							channel_id,
+							data: msg
+						},
+					},
+				},
 				ChannelError::Ignore(msg) => LightningError {
 					err: msg,
 					action: msgs::ErrorAction::IgnoreError,
@@ -3069,6 +3078,9 @@ macro_rules! convert_channel_err {
 			ChannelError::Warn(msg) => {
 				(false, MsgHandleErrInternal::from_chan_no_close(ChannelError::Warn(msg), *$channel_id))
 			},
+			ChannelError::WarnAndDisconnect(msg) => {
+				(false, MsgHandleErrInternal::from_chan_no_close(ChannelError::WarnAndDisconnect(msg), *$channel_id))
+			},
 			ChannelError::Ignore(msg) => {
 				(false, MsgHandleErrInternal::from_chan_no_close(ChannelError::Ignore(msg), *$channel_id))
 			},
@@ -6620,19 +6632,21 @@ where
 
 								funded_chan.context.maybe_expire_prev_config();
 
-								if funded_chan.should_disconnect_peer_awaiting_response() {
-									let logger = WithChannelContext::from(&self.logger, &funded_chan.context, None);
-									log_debug!(logger, "Disconnecting peer {} due to not making any progress on channel {}",
-											counterparty_node_id, chan_id);
-									pending_msg_events.push(MessageSendEvent::HandleError {
-										node_id: counterparty_node_id,
-										action: msgs::ErrorAction::DisconnectPeerWithWarning {
-											msg: msgs::WarningMessage {
-												channel_id: *chan_id,
-												data: "Disconnecting due to timeout awaiting response".to_owned(),
+								if peer_state.is_connected {
+									if funded_chan.should_disconnect_peer_awaiting_response() {
+										let logger = WithChannelContext::from(&self.logger, &funded_chan.context, None);
+										log_debug!(logger, "Disconnecting peer {} due to not making any progress on channel {}",
+												counterparty_node_id, chan_id);
+										pending_msg_events.push(MessageSendEvent::HandleError {
+											node_id: counterparty_node_id,
+											action: msgs::ErrorAction::DisconnectPeerWithWarning {
+												msg: msgs::WarningMessage {
+													channel_id: *chan_id,
+													data: "Disconnecting due to timeout awaiting response".to_owned(),
+												},
 											},
-										},
-									});
+										});
+									}
 								}
 
 								true
@@ -9214,6 +9228,58 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 		Ok(())
 	}
 
+	fn internal_stfu(&self, counterparty_node_id: &PublicKey, msg: &msgs::Stfu) -> Result<bool, MsgHandleErrInternal> {
+		let per_peer_state = self.per_peer_state.read().unwrap();
+		let peer_state_mutex = per_peer_state.get(counterparty_node_id).ok_or_else(|| {
+			debug_assert!(false);
+			MsgHandleErrInternal::send_err_msg_no_close(
+				format!("Can't find a peer matching the passed counterparty node_id {}", counterparty_node_id),
+				msg.channel_id
+			)
+		})?;
+		let mut peer_state_lock = peer_state_mutex.lock().unwrap();
+		let peer_state = &mut *peer_state_lock;
+
+		if !self.init_features().supports_quiescence() {
+			return Err(MsgHandleErrInternal::from_chan_no_close(
+				ChannelError::Warn("Quiescense not supported".to_string()), msg.channel_id
+			));
+		}
+
+		let mut sent_stfu = false;
+		match peer_state.channel_by_id.entry(msg.channel_id) {
+			hash_map::Entry::Occupied(mut chan_entry) => {
+				if let Some(chan) = chan_entry.get_mut().as_funded_mut() {
+					let logger = WithContext::from(
+						&self.logger, Some(*counterparty_node_id), Some(msg.channel_id), None
+					);
+
+					if let Some(stfu) = try_channel_entry!(
+						self, peer_state, chan.stfu(&msg, &&logger), chan_entry
+					) {
+						sent_stfu = true;
+						peer_state.pending_msg_events.push(MessageSendEvent::SendStfu {
+							node_id: *counterparty_node_id,
+							msg: stfu,
+						});
+					}
+				} else {
+					let msg = "Peer sent `stfu` for an unfunded channel";
+					let err = Err(ChannelError::Close(
+						(msg.into(), ClosureReason::ProcessingError { err: msg.into() })
+					));
+					return try_channel_entry!(self, peer_state, err, chan_entry);
+				}
+			},
+			hash_map::Entry::Vacant(_) => return Err(MsgHandleErrInternal::send_err_msg_no_close(
+				format!("Got a message for a channel from the wrong node! No such channel for the passed counterparty_node_id {}", counterparty_node_id),
+				msg.channel_id
+			))
+		}
+
+		Ok(sent_stfu)
+	}
+
 	fn internal_announcement_signatures(&self, counterparty_node_id: &PublicKey, msg: &msgs::AnnouncementSignatures) -> Result<(), MsgHandleErrInternal> {
 		let per_peer_state = self.per_peer_state.read().unwrap();
 		let peer_state_mutex = per_peer_state.get(counterparty_node_id)
@@ -9739,6 +9805,118 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 		has_update
 	}
 
+	fn maybe_send_stfu(&self) {
+		let per_peer_state = self.per_peer_state.read().unwrap();
+		for (counterparty_node_id, peer_state_mutex) in per_peer_state.iter() {
+			let mut peer_state_lock = peer_state_mutex.lock().unwrap();
+			let peer_state = &mut *peer_state_lock;
+			let pending_msg_events = &mut peer_state.pending_msg_events;
+			for (channel_id, chan) in &mut peer_state.channel_by_id {
+				if let Some(funded_chan) = chan.as_funded_mut() {
+					let logger = WithContext::from(
+						&self.logger, Some(*counterparty_node_id), Some(*channel_id), None
+					);
+					match funded_chan.try_send_stfu(&&logger) {
+						Ok(None) => {},
+						Ok(Some(stfu)) => {
+							pending_msg_events.push(events::MessageSendEvent::SendStfu {
+								node_id: chan.context().get_counterparty_node_id(),
+								msg: stfu,
+							});
+						},
+						Err(e) => {
+							log_debug!(logger, "Could not advance quiescence handshake: {}", e);
+						}
+					}
+				}
+			}
+		}
+	}
+
+	#[cfg(any(test, fuzzing))]
+	pub fn maybe_propose_quiescence(&self, counterparty_node_id: &PublicKey, channel_id: &ChannelId) -> Result<(), APIError> {
+		let mut result = Ok(());
+		PersistenceNotifierGuard::optionally_notify(self, || {
+			let mut notify = NotifyOption::SkipPersistNoEvents;
+
+			let per_peer_state = self.per_peer_state.read().unwrap();
+			let peer_state_mutex_opt = per_peer_state.get(counterparty_node_id);
+			if peer_state_mutex_opt.is_none() {
+				result = Err(APIError::ChannelUnavailable {
+					err: format!("Can't find a peer matching the passed counterparty node_id {}", counterparty_node_id)
+				});
+				return notify;
+			}
+
+			let mut peer_state = peer_state_mutex_opt.unwrap().lock().unwrap();
+			if !peer_state.latest_features.supports_quiescence() {
+				result = Err(APIError::ChannelUnavailable { err: "Peer does not support quiescence".to_owned() });
+				return notify;
+			}
+
+			match peer_state.channel_by_id.entry(channel_id.clone()) {
+				hash_map::Entry::Occupied(mut chan_entry) => {
+					if let Some(chan) = chan_entry.get_mut().as_funded_mut() {
+						let logger = WithContext::from(
+							&self.logger, Some(*counterparty_node_id), Some(*channel_id), None
+						);
+
+						match chan.propose_quiescence(&&logger) {
+							Ok(None) => {},
+							Ok(Some(stfu)) => {
+								peer_state.pending_msg_events.push(MessageSendEvent::SendStfu {
+									node_id: *counterparty_node_id, msg: stfu
+								});
+								notify = NotifyOption::SkipPersistHandleEvents;
+							},
+							Err(msg) => log_trace!(logger, "{}", msg),
+						}
+					} else {
+						result = Err(APIError::APIMisuseError {
+							err: format!("Unfunded channel {} cannot be quiescent", channel_id),
+						});
+					}
+				},
+				hash_map::Entry::Vacant(_) => {
+					result = Err(APIError::ChannelUnavailable {
+						err: format!("Channel with id {} not found for the passed counterparty node_id {}",
+							channel_id, counterparty_node_id),
+					});
+				},
+			}
+
+			notify
+		});
+
+		result
+	}
+
+	#[cfg(any(test, fuzzing))]
+	pub fn exit_quiescence(&self, counterparty_node_id: &PublicKey, channel_id: &ChannelId) -> Result<bool, APIError> {
+		let per_peer_state = self.per_peer_state.read().unwrap();
+		let peer_state_mutex = per_peer_state.get(counterparty_node_id)
+			.ok_or_else(|| APIError::ChannelUnavailable {
+				err: format!("Can't find a peer matching the passed counterparty node_id {}", counterparty_node_id)
+			})?;
+		let mut peer_state = peer_state_mutex.lock().unwrap();
+		let initiator = match peer_state.channel_by_id.entry(*channel_id) {
+			hash_map::Entry::Occupied(mut chan_entry) => {
+				if let Some(chan) = chan_entry.get_mut().as_funded_mut() {
+					chan.exit_quiescence()
+				} else {
+					return Err(APIError::APIMisuseError {
+						err: format!("Unfunded channel {} cannot be quiescent", channel_id),
+					})
+				}
+			},
+			hash_map::Entry::Vacant(_) => return Err(APIError::ChannelUnavailable {
+				err: format!("Channel with id {} not found for the passed counterparty node_id {}",
+					channel_id, counterparty_node_id),
+			}),
+		};
+		Ok(initiator)
+	}
+
 	/// Utility for creating a BOLT11 invoice that can be verified by [`ChannelManager`] without
 	/// storing any additional state. It achieves this by including a [`PaymentSecret`] in the
 	/// invoice which it uses to verify that the invoice has not expired and the payment amount is
@@ -10927,6 +11105,9 @@ where
 				result = NotifyOption::DoPersist;
 			}
 
+			// Quiescence is an in-memory protocol, so we don't have to persist because of it.
+			self.maybe_send_stfu();
+
 			let mut is_any_peer_connected = false;
 			let mut pending_events = Vec::new();
 			let per_peer_state = self.per_peer_state.read().unwrap();
@@ -11547,9 +11728,20 @@ where
 	}
 
 	fn handle_stfu(&self, counterparty_node_id: PublicKey, msg: &msgs::Stfu) {
-		let _: Result<(), _> = handle_error!(self, Err(MsgHandleErrInternal::send_err_msg_no_close(
-			"Quiescence not supported".to_owned(),
-			msg.channel_id.clone())), counterparty_node_id);
+		let _persistence_guard = PersistenceNotifierGuard::optionally_notify(self, || {
+			let res = self.internal_stfu(&counterparty_node_id, msg);
+			let persist = match &res {
+				Err(e) if e.closes_channel() => NotifyOption::DoPersist,
+				Err(_) => NotifyOption::SkipPersistHandleEvents,
+				Ok(sent_stfu) => if *sent_stfu {
+					NotifyOption::SkipPersistHandleEvents
+				} else {
+					NotifyOption::SkipPersistNoEvents
+				},
+			};
+			let _ = handle_error!(self, res, counterparty_node_id);
+			persist
+		});
 	}
 
 	#[cfg(splicing)]
@@ -12558,6 +12750,10 @@ pub fn provided_init_features(config: &UserConfig) -> InitFeatures {
 	}
 	#[cfg(dual_funding)]
 	features.set_dual_fund_optional();
+	// Only signal quiescence support in tests for now, as we don't yet support any
+	// quiescent-dependent protocols (e.g., splicing).
+	#[cfg(any(test, fuzzing))]
+	features.set_quiescence_optional();
 	features
 }
 

lightning/src/ln/mod.rs

@@ -89,6 +89,8 @@ mod monitor_tests;
 #[allow(unused_mut)]
 mod shutdown_tests;
 #[cfg(test)]
+mod quiescence_tests;
+#[cfg(test)]
 #[allow(unused_mut)]
 mod async_signer_tests;
 #[cfg(test)]

lightning/src/ln/msgs.rs

@@ -455,8 +455,8 @@ pub type SerialId = u64;
 pub struct Stfu {
 	/// The channel ID where quiescence is intended
 	pub channel_id: ChannelId,
-	/// Initiator flag, 1 if initiating, 0 if replying to an stfu.
-	pub initiator: u8,
+	/// Initiator flag, true if initiating, false if replying to an stfu.
+	pub initiator: bool,
 }
 
 /// A `splice_init` message to be sent by or received from the stfu initiator (splice initiator).
@@ -4112,10 +4112,17 @@ mod tests {
 	fn encoding_stfu() {
 		let stfu = msgs::Stfu {
 			channel_id: ChannelId::from_bytes([2; 32]),
-			initiator: 1,
+			initiator: true,
 		};
 		let encoded_value = stfu.encode();
 		assert_eq!(encoded_value.as_hex().to_string(), "020202020202020202020202020202020202020202020202020202020202020201");
+
+		let stfu = msgs::Stfu {
+			channel_id: ChannelId::from_bytes([3; 32]),
+			initiator: false,
+		};
+		let encoded_value = stfu.encode();
+		assert_eq!(encoded_value.as_hex().to_string(), "030303030303030303030303030303030303030303030303030303030303030300");
 	}
 
 	#[test]

lightning/src/ln/quiescence_tests.rs

@@ -0,0 +1,501 @@
+use crate::chain::ChannelMonitorUpdateStatus;
+use crate::events::Event;
+use crate::events::HTLCDestination;
+use crate::events::MessageSendEvent;
+use crate::events::MessageSendEventsProvider;
+use crate::ln::channel::DISCONNECT_PEER_AWAITING_RESPONSE_TICKS;
+use crate::ln::channelmanager::PaymentId;
+use crate::ln::channelmanager::RecipientOnionFields;
+use crate::ln::functional_test_utils::*;
+use crate::ln::msgs;
+use crate::ln::msgs::{ChannelMessageHandler, ErrorAction};
+use crate::util::errors::APIError;
+use crate::util::test_channel_signer::SignerOp;
+
+#[test]
+fn test_quiescence_tie() {
+	// Test that both nodes proposing quiescence at the same time results in the channel funder
+	// becoming the quiescence initiator.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	nodes[0].node.maybe_propose_quiescence(&nodes[1].node.get_our_node_id(), &chan_id).unwrap();
+	nodes[1].node.maybe_propose_quiescence(&nodes[0].node.get_our_node_id(), &chan_id).unwrap();
+
+	let stfu_node_0 =
+		get_event_msg!(nodes[0], MessageSendEvent::SendStfu, nodes[1].node.get_our_node_id());
+	nodes[1].node.handle_stfu(nodes[0].node.get_our_node_id(), &stfu_node_0);
+
+	let stfu_node_1 =
+		get_event_msg!(nodes[1], MessageSendEvent::SendStfu, nodes[0].node.get_our_node_id());
+	nodes[0].node.handle_stfu(nodes[1].node.get_our_node_id(), &stfu_node_1);
+
+	assert!(stfu_node_0.initiator && stfu_node_1.initiator);
+
+	assert!(nodes[0].node.exit_quiescence(&nodes[1].node.get_our_node_id(), &chan_id).unwrap());
+	assert!(!nodes[1].node.exit_quiescence(&nodes[0].node.get_our_node_id(), &chan_id).unwrap());
+}
+
+#[test]
+fn test_quiescence_shutdown_ignored() {
+	// Test that a shutdown sent/received during quiescence is ignored.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	nodes[0].node.maybe_propose_quiescence(&nodes[1].node.get_our_node_id(), &chan_id).unwrap();
+	let _ = get_event_msg!(nodes[0], MessageSendEvent::SendStfu, nodes[1].node.get_our_node_id());
+
+	if let Err(e) = nodes[0].node.close_channel(&chan_id, &nodes[1].node.get_our_node_id()) {
+		assert_eq!(
+			e,
+			APIError::APIMisuseError { err: "Cannot begin shutdown while quiescent".to_owned() }
+		);
+	} else {
+		panic!("Expected shutdown to be ignored while quiescent");
+	}
+
+	nodes[1].node.close_channel(&chan_id, &nodes[0].node.get_our_node_id()).unwrap();
+	let shutdown =
+		get_event_msg!(nodes[1], MessageSendEvent::SendShutdown, nodes[0].node.get_our_node_id());
+
+	nodes[0].node.handle_shutdown(nodes[1].node.get_our_node_id(), &shutdown);
+	let msg_events = nodes[0].node.get_and_clear_pending_msg_events();
+	match msg_events[0] {
+		MessageSendEvent::HandleError {
+			action: ErrorAction::DisconnectPeerWithWarning { ref msg, .. },
+			..
+		} => {
+			assert_eq!(msg.data, "Got shutdown request while quiescent".to_owned());
+		},
+		_ => panic!(),
+	}
+}
+
+#[test]
+fn test_allow_shutdown_while_awaiting_quiescence() {
+	allow_shutdown_while_awaiting_quiescence(false);
+	allow_shutdown_while_awaiting_quiescence(true);
+}
+
+fn allow_shutdown_while_awaiting_quiescence(local_shutdown: bool) {
+	// Test that a shutdown sent/received while we're still awaiting quiescence (stfu has not been
+	// sent yet) is honored and the channel is closed cooperatively.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	let local_node = &nodes[0];
+	let remote_node = &nodes[1];
+	let local_node_id = local_node.node.get_our_node_id();
+	let remote_node_id = remote_node.node.get_our_node_id();
+
+	let payment_amount = 1_000_000;
+	let (route, payment_hash, _, payment_secret) =
+		get_route_and_payment_hash!(local_node, remote_node, payment_amount);
+	let onion = RecipientOnionFields::secret_only(payment_secret);
+	let payment_id = PaymentId(payment_hash.0);
+	local_node.node.send_payment_with_route(route, payment_hash, onion, payment_id).unwrap();
+	check_added_monitors!(local_node, 1);
+
+	// Attempt to send an HTLC, but don't fully commit it yet.
+	let update_add = get_htlc_update_msgs!(local_node, remote_node_id);
+	remote_node.node.handle_update_add_htlc(local_node_id, &update_add.update_add_htlcs[0]);
+	remote_node.node.handle_commitment_signed(local_node_id, &update_add.commitment_signed);
+	let (revoke_and_ack, commit_sig) = get_revoke_commit_msgs!(remote_node, local_node_id);
+	local_node.node.handle_revoke_and_ack(remote_node_id, &revoke_and_ack);
+	check_added_monitors(local_node, 1);
+
+	// Request the local node to propose quiescence, and immediately try to close the channel. Since
+	// we haven't sent `stfu` yet as the state machine is pending, we should forget about our
+	// quiescence attempt.
+	local_node.node.maybe_propose_quiescence(&remote_node_id, &chan_id).unwrap();
+	assert!(local_node.node.get_and_clear_pending_msg_events().is_empty());
+
+	let (closer_node, closee_node) =
+		if local_shutdown { (local_node, remote_node) } else { (remote_node, local_node) };
+	let closer_node_id = closer_node.node.get_our_node_id();
+	let closee_node_id = closee_node.node.get_our_node_id();
+
+	closer_node.node.close_channel(&chan_id, &closee_node_id).unwrap();
+	check_added_monitors(&remote_node, 1);
+	let shutdown_initiator =
+		get_event_msg!(closer_node, MessageSendEvent::SendShutdown, closee_node_id);
+	closee_node.node.handle_shutdown(closer_node_id, &shutdown_initiator);
+	let shutdown_responder =
+		get_event_msg!(closee_node, MessageSendEvent::SendShutdown, closer_node_id);
+	closer_node.node.handle_shutdown(closee_node_id, &shutdown_responder);
+
+	// Continue exchanging messages until the HTLC is irrevocably committed and eventually failed
+	// back as we are shutting down.
+	local_node.node.handle_commitment_signed(remote_node_id, &commit_sig);
+	check_added_monitors(local_node, 1);
+
+	let last_revoke_and_ack =
+		get_event_msg!(local_node, MessageSendEvent::SendRevokeAndACK, remote_node_id);
+	remote_node.node.handle_revoke_and_ack(local_node_id, &last_revoke_and_ack);
+	check_added_monitors(remote_node, 1);
+	expect_pending_htlcs_forwardable!(remote_node);
+	expect_htlc_handling_failed_destinations!(
+		remote_node.node.get_and_clear_pending_events(),
+		&[HTLCDestination::FailedPayment { payment_hash }]
+	);
+	check_added_monitors(remote_node, 1);
+
+	let update_fail = get_htlc_update_msgs!(remote_node, local_node_id);
+	local_node.node.handle_update_fail_htlc(remote_node_id, &update_fail.update_fail_htlcs[0]);
+	local_node.node.handle_commitment_signed(remote_node_id, &update_fail.commitment_signed);
+
+	let (revoke_and_ack, commit_sig) = get_revoke_commit_msgs!(local_node, remote_node_id);
+	remote_node.node.handle_revoke_and_ack(local_node_id, &revoke_and_ack);
+	check_added_monitors(remote_node, 1);
+	remote_node.node.handle_commitment_signed(local_node_id, &commit_sig);
+	check_added_monitors(remote_node, 1);
+
+	let last_revoke_and_ack =
+		get_event_msg!(remote_node, MessageSendEvent::SendRevokeAndACK, local_node_id);
+	local_node.node.handle_revoke_and_ack(remote_node_id, &last_revoke_and_ack);
+
+	expect_payment_failed_conditions(
+		local_node,
+		payment_hash,
+		true,
+		PaymentFailedConditions::new(),
+	);
+
+	// Now that the state machine is no longer pending, and `closing_signed` is ready to be sent,
+	// make sure we're still not waiting for the quiescence handshake to complete.
+	local_node.node.exit_quiescence(&remote_node_id, &chan_id).unwrap();
+
+	let _ = get_event_msg!(local_node, MessageSendEvent::SendClosingSigned, remote_node_id);
+	check_added_monitors(local_node, 2); // One for the last revoke_and_ack, another for closing_signed
+}
+
+#[test]
+fn test_quiescence_tracks_monitor_update_in_progress_and_waits_for_async_signer() {
+	// Test that quiescence:
+	//   a) considers an async signer when determining whether a pending channel update exists
+	//   b) tracks in-progress monitor updates until no longer quiescent
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	let node_id_0 = nodes[0].node.get_our_node_id();
+	let node_id_1 = nodes[1].node.get_our_node_id();
+
+	let payment_amount = 1_000_000;
+	let (preimage, payment_hash, ..) = route_payment(&nodes[0], &[&nodes[1]], payment_amount);
+	nodes[1].node.claim_funds(preimage);
+	check_added_monitors(&nodes[1], 1);
+
+	let update = get_htlc_update_msgs!(&nodes[1], node_id_0);
+	nodes[0].node.handle_update_fulfill_htlc(node_id_1, &update.update_fulfill_htlcs[0]);
+	nodes[0].node.handle_commitment_signed(node_id_1, &update.commitment_signed);
+	check_added_monitors(&nodes[0], 1);
+
+	// While settling back the payment, propose quiescence from nodes[1]. We won't see its `stfu` go
+	// out yet as the `update_fulfill` is still pending on both sides.
+	nodes[1].node.maybe_propose_quiescence(&node_id_0, &chan_id).unwrap();
+
+	// Disable releasing commitment secrets on nodes[1], to hold back their `stfu` until the
+	// `revoke_and_ack` goes out, and drive the state machine forward.
+	nodes[1].disable_channel_signer_op(&node_id_0, &chan_id, SignerOp::ReleaseCommitmentSecret);
+
+	let (revoke_and_ack, commit_sig) = get_revoke_commit_msgs!(&nodes[0], node_id_1);
+	nodes[1].node.handle_revoke_and_ack(node_id_0, &revoke_and_ack);
+	check_added_monitors(&nodes[1], 1);
+	nodes[1].node.handle_commitment_signed(node_id_0, &commit_sig);
+	check_added_monitors(&nodes[1], 1);
+
+	assert!(nodes[1].node.get_and_clear_pending_msg_events().is_empty());
+
+	// Resume the signer. We should now expect to see both messages.
+	nodes[1].enable_channel_signer_op(&node_id_0, &chan_id, SignerOp::ReleaseCommitmentSecret);
+	nodes[1].node.signer_unblocked(Some((node_id_0, chan_id)));
+
+	expect_payment_claimed!(&nodes[1], payment_hash, payment_amount);
+
+	macro_rules! find_msg {
+		($events: expr, $msg: ident) => {{
+			$events
+				.iter()
+				.find_map(|event| {
+					if let MessageSendEvent::$msg { ref msg, .. } = event {
+						Some(msg)
+					} else {
+						None
+					}
+				})
+				.unwrap()
+		}};
+	}
+	let msg_events = nodes[1].node.get_and_clear_pending_msg_events();
+	assert_eq!(msg_events.len(), 2);
+	let revoke_and_ack = find_msg!(msg_events, SendRevokeAndACK);
+	let stfu = find_msg!(msg_events, SendStfu);
+
+	// While handling the last `revoke_and_ack` on nodes[0], we'll hold the monitor update and
+	// become quiescent.
+	chanmon_cfgs[0].persister.set_update_ret(ChannelMonitorUpdateStatus::InProgress);
+	nodes[0].node.handle_revoke_and_ack(node_id_1, &revoke_and_ack);
+
+	nodes[0].node.handle_stfu(node_id_1, &stfu);
+	let stfu = get_event_msg!(&nodes[0], MessageSendEvent::SendStfu, node_id_1);
+	nodes[1].node.handle_stfu(node_id_0, &stfu);
+
+	nodes[0].node.exit_quiescence(&node_id_1, &chan_id).unwrap();
+	nodes[1].node.exit_quiescence(&node_id_0, &chan_id).unwrap();
+
+	// After exiting quiescence, we should be able to resume payments from nodes[0], but the monitor
+	// update has yet to complete. Attempting to send a payment now will be delayed until the
+	// monitor update completes.
+	{
+		let (route, payment_hash, _, payment_secret) =
+			get_route_and_payment_hash!(&nodes[0], &nodes[1], payment_amount);
+		let onion = RecipientOnionFields::secret_only(payment_secret);
+		let payment_id = PaymentId(payment_hash.0);
+		nodes[0].node.send_payment_with_route(route, payment_hash, onion, payment_id).unwrap();
+	}
+	check_added_monitors(&nodes[0], 0);
+	assert!(nodes[0].node.get_and_clear_pending_msg_events().is_empty());
+
+	// We have two updates pending:
+	{
+		let chain_monitor = &nodes[0].chain_monitor;
+		let (_, latest_update) =
+			chain_monitor.latest_monitor_update_id.lock().unwrap().get(&chan_id).unwrap().clone();
+		let chain_monitor = &nodes[0].chain_monitor.chain_monitor;
+		// One for the latest commitment transaction update from the last `revoke_and_ack`
+		chain_monitor.channel_monitor_updated(chan_id, latest_update - 1).unwrap();
+		expect_payment_sent(&nodes[0], preimage, None, true, true);
+		// One for the commitment secret update from the last `revoke_and_ack`
+		chain_monitor.channel_monitor_updated(chan_id, latest_update).unwrap();
+	}
+
+	// With the pending monitor updates complete, we'll see a new monitor update go out when freeing
+	// the holding cells to send out the new HTLC.
+	nodes[0].chain_monitor.complete_sole_pending_chan_update(&chan_id);
+	let _ = get_htlc_update_msgs!(&nodes[0], node_id_1);
+	check_added_monitors(&nodes[0], 1);
+}
+
+#[test]
+fn test_quiescence_updates_go_to_holding_cell() {
+	quiescence_updates_go_to_holding_cell(false);
+	quiescence_updates_go_to_holding_cell(true);
+}
+
+fn quiescence_updates_go_to_holding_cell(fail_htlc: bool) {
+	// Test that any updates made to a channel while quiescent go to the holding cell.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	let node_id_0 = nodes[0].node.get_our_node_id();
+	let node_id_1 = nodes[1].node.get_our_node_id();
+
+	// Send enough to be able to pay from both directions.
+	let payment_amount = 1_000_000;
+	send_payment(&nodes[0], &[&nodes[1]], payment_amount * 4);
+
+	// Propose quiescence from nodes[1], and immediately try to send a payment. Since its `stfu` has
+	// already gone out first, the outbound HTLC will go into the holding cell.
+	nodes[1].node.maybe_propose_quiescence(&node_id_0, &chan_id).unwrap();
+	let stfu = get_event_msg!(&nodes[1], MessageSendEvent::SendStfu, node_id_0);
+
+	let (route1, payment_hash1, payment_preimage1, payment_secret1) =
+		get_route_and_payment_hash!(&nodes[1], &nodes[0], payment_amount);
+	let onion1 = RecipientOnionFields::secret_only(payment_secret1);
+	let payment_id1 = PaymentId(payment_hash1.0);
+	nodes[1].node.send_payment_with_route(route1, payment_hash1, onion1, payment_id1).unwrap();
+	check_added_monitors!(&nodes[1], 0);
+	assert!(nodes[1].node.get_and_clear_pending_msg_events().is_empty());
+
+	// Send a payment in the opposite direction. Since nodes[0] hasn't sent its own `stfu` yet, it's
+	// allowed to make updates.
+	let (route2, payment_hash2, payment_preimage2, payment_secret2) =
+		get_route_and_payment_hash!(&nodes[0], &nodes[1], payment_amount);
+	let onion2 = RecipientOnionFields::secret_only(payment_secret2);
+	let payment_id2 = PaymentId(payment_hash2.0);
+	nodes[0].node.send_payment_with_route(route2, payment_hash2, onion2, payment_id2).unwrap();
+	check_added_monitors!(&nodes[0], 1);
+
+	let update_add = get_htlc_update_msgs!(&nodes[0], node_id_1);
+	nodes[1].node.handle_update_add_htlc(node_id_0, &update_add.update_add_htlcs[0]);
+	commitment_signed_dance!(&nodes[1], &nodes[0], update_add.commitment_signed, false);
+	expect_pending_htlcs_forwardable!(&nodes[1]);
+	expect_payment_claimable!(nodes[1], payment_hash2, payment_secret2, payment_amount);
+
+	// Have nodes[1] attempt to fail/claim nodes[0]'s payment. Since nodes[1] already sent out
+	// `stfu`, the `update_fail/fulfill` will go into the holding cell.
+	if fail_htlc {
+		nodes[1].node.fail_htlc_backwards(&payment_hash2);
+		let failed_payment = HTLCDestination::FailedPayment { payment_hash: payment_hash2 };
+		expect_pending_htlcs_forwardable_and_htlc_handling_failed!(&nodes[1], vec![failed_payment]);
+	} else {
+		nodes[1].node.claim_funds(payment_preimage2);
+		check_added_monitors(&nodes[1], 1);
+	}
+	assert!(nodes[1].node.get_and_clear_pending_msg_events().is_empty());
+
+	// Finish the quiescence handshake.
+	nodes[0].node.handle_stfu(node_id_1, &stfu);
+	let stfu = get_event_msg!(&nodes[0], MessageSendEvent::SendStfu, node_id_1);
+	nodes[1].node.handle_stfu(node_id_0, &stfu);
+
+	nodes[0].node.exit_quiescence(&node_id_1, &chan_id).unwrap();
+	nodes[1].node.exit_quiescence(&node_id_0, &chan_id).unwrap();
+
+	// Now that quiescence is over, nodes are allowed to make updates again. nodes[1] will have its
+	// outbound HTLC finally go out, along with the fail/claim of nodes[0]'s payment.
+	let update = get_htlc_update_msgs!(&nodes[1], node_id_0);
+	check_added_monitors(&nodes[1], 1);
+	nodes[0].node.handle_update_add_htlc(node_id_1, &update.update_add_htlcs[0]);
+	if fail_htlc {
+		nodes[0].node.handle_update_fail_htlc(node_id_1, &update.update_fail_htlcs[0]);
+	} else {
+		nodes[0].node.handle_update_fulfill_htlc(node_id_1, &update.update_fulfill_htlcs[0]);
+	}
+	commitment_signed_dance!(&nodes[0], &nodes[1], update.commitment_signed, false);
+
+	if !fail_htlc {
+		expect_payment_claimed!(nodes[1], payment_hash2, payment_amount);
+	}
+
+	// The payment from nodes[0] should now be seen as failed/successful.
+	let events = nodes[0].node.get_and_clear_pending_events();
+	assert_eq!(events.len(), 3);
+	assert!(events.iter().find(|e| matches!(e, Event::PendingHTLCsForwardable { .. })).is_some());
+	if fail_htlc {
+		assert!(events.iter().find(|e| matches!(e, Event::PaymentFailed { .. })).is_some());
+		assert!(events.iter().find(|e| matches!(e, Event::PaymentPathFailed { .. })).is_some());
+	} else {
+		assert!(events.iter().find(|e| matches!(e, Event::PaymentSent { .. })).is_some());
+		assert!(events.iter().find(|e| matches!(e, Event::PaymentPathSuccessful { .. })).is_some());
+		check_added_monitors(&nodes[0], 1);
+	}
+	nodes[0].node.process_pending_htlc_forwards();
+	expect_payment_claimable!(nodes[0], payment_hash1, payment_secret1, payment_amount);
+
+	// Have nodes[0] fail/claim nodes[1]'s payment.
+	if fail_htlc {
+		nodes[0].node.fail_htlc_backwards(&payment_hash1);
+		let failed_payment = HTLCDestination::FailedPayment { payment_hash: payment_hash1 };
+		expect_pending_htlcs_forwardable_and_htlc_handling_failed!(&nodes[0], vec![failed_payment]);
+	} else {
+		nodes[0].node.claim_funds(payment_preimage1);
+	}
+	check_added_monitors(&nodes[0], 1);
+
+	let update = get_htlc_update_msgs!(&nodes[0], node_id_1);
+	if fail_htlc {
+		nodes[1].node.handle_update_fail_htlc(node_id_0, &update.update_fail_htlcs[0]);
+	} else {
+		nodes[1].node.handle_update_fulfill_htlc(node_id_0, &update.update_fulfill_htlcs[0]);
+	}
+	commitment_signed_dance!(&nodes[1], &nodes[0], update.commitment_signed, false);
+
+	// The payment from nodes[1] should now be seen as failed/successful.
+	if fail_htlc {
+		let conditions = PaymentFailedConditions::new();
+		expect_payment_failed_conditions(&nodes[1], payment_hash1, true, conditions);
+	} else {
+		expect_payment_claimed!(nodes[0], payment_hash1, payment_amount);
+		expect_payment_sent(&nodes[1], payment_preimage1, None, true, true);
+	}
+}
+
+#[test]
+fn test_quiescence_timeout() {
+	// Test that we'll disconnect if we remain quiescent for `DISCONNECT_PEER_AWAITING_RESPONSE_TICKS`.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	let node_id_0 = nodes[0].node.get_our_node_id();
+	let node_id_1 = nodes[1].node.get_our_node_id();
+
+	nodes[0].node.maybe_propose_quiescence(&nodes[1].node.get_our_node_id(), &chan_id).unwrap();
+
+	let stfu_initiator = get_event_msg!(nodes[0], MessageSendEvent::SendStfu, node_id_1);
+	nodes[1].node.handle_stfu(node_id_0, &stfu_initiator);
+
+	let stfu_responder = get_event_msg!(nodes[1], MessageSendEvent::SendStfu, node_id_0);
+	nodes[0].node.handle_stfu(node_id_1, &stfu_responder);
+
+	assert!(stfu_initiator.initiator && !stfu_responder.initiator);
+
+	for _ in 0..DISCONNECT_PEER_AWAITING_RESPONSE_TICKS {
+		nodes[0].node.timer_tick_occurred();
+		nodes[1].node.timer_tick_occurred();
+	}
+
+	let f = |event| {
+		if let MessageSendEvent::HandleError { action, .. } = event {
+			if let msgs::ErrorAction::DisconnectPeerWithWarning { .. } = action {
+				Some(())
+			} else {
+				None
+			}
+		} else {
+			None
+		}
+	};
+	assert!(nodes[0].node.get_and_clear_pending_msg_events().into_iter().find_map(f).is_some());
+	assert!(nodes[1].node.get_and_clear_pending_msg_events().into_iter().find_map(f).is_some());
+}
+
+#[test]
+fn test_quiescence_timeout_while_waiting_for_counterparty_stfu() {
+	// Test that we'll disconnect if the counterparty does not send their stfu within a reasonable
+	// time if we've already sent ours.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+	let chan_id = create_announced_chan_between_nodes(&nodes, 0, 1).2;
+
+	let node_id_0 = nodes[0].node.get_our_node_id();
+
+	nodes[1].node.maybe_propose_quiescence(&node_id_0, &chan_id).unwrap();
+	let _ = get_event_msg!(nodes[1], MessageSendEvent::SendStfu, node_id_0);
+
+	// Route a payment in between to ensure expecting to receive `revoke_and_ack` doesn't override
+	// the expectation of receiving `stfu` as well.
+	let _ = route_payment(&nodes[0], &[&nodes[1]], 1_000_000);
+
+	for _ in 0..DISCONNECT_PEER_AWAITING_RESPONSE_TICKS {
+		nodes[0].node.timer_tick_occurred();
+		nodes[1].node.timer_tick_occurred();
+	}
+
+	// nodes[0] hasn't received stfu from nodes[1], so it's not enforcing any timeouts.
+	assert!(nodes[0].node.get_and_clear_pending_msg_events().is_empty());
+
+	// nodes[1] didn't receive nodes[0]'s stfu within the timeout so it'll disconnect.
+	let f = |&ref event| {
+		if let MessageSendEvent::HandleError { action, .. } = event {
+			if let msgs::ErrorAction::DisconnectPeerWithWarning { .. } = action {
+				Some(())
+			} else {
+				None
+			}
+		} else {
+			None
+		}
+	};
+	assert!(nodes[1].node.get_and_clear_pending_msg_events().iter().find_map(f).is_some());
+}