Add UserConfig::reject_inbound_splices

diff --git a/fuzz/src/full_stack.rs b/fuzz/src/full_stack.rs
index ee5f457..6b00f29 100644
--- a/fuzz/src/full_stack.rs
+++ b/fuzz/src/full_stack.rs
@@ -1048,7 +1048,7 @@ fn two_peer_forwarding_seed() -> Vec<u8> {
 	// our network key
 	ext_from_hex("0100000000000000000000000000000000000000000000000000000000000000", &mut test);
 	// config
-	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff0001000000000000", &mut test);
+	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff000100000000000000", &mut test);
 
 	// new outbound connection with id 0
 	ext_from_hex("00", &mut test);
@@ -1502,7 +1502,7 @@ fn gossip_exchange_seed() -> Vec<u8> {
 	// our network key
 	ext_from_hex("0100000000000000000000000000000000000000000000000000000000000000", &mut test);
 	// config
-	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff0001000000000000", &mut test);
+	ext_from_hex("000000000090000000000000000064000100000000000100ffff0000000000000000ffffffffffffffffffffffffffffffff0000000000000000ffffffffffffffff000000ffffffff00ffff1a000400010000020400000000040200000a08ffffffffffffffff000100000000000000", &mut test);
 
 	// new outbound connection with id 0
 	ext_from_hex("00", &mut test);
diff --git a/lightning/src/ln/channelmanager.rs b/lightning/src/ln/channelmanager.rs
index e448802..5cd5e80 100644
--- a/lightning/src/ln/channelmanager.rs
+++ b/lightning/src/ln/channelmanager.rs
@@ -11476,6 +11476,13 @@ This indicates a bug inside LDK. Please report this error at https://github.com/
 					counterparty_node_id, msg.channel_id,
 				), msg.channel_id)),
 			hash_map::Entry::Occupied(mut chan_entry) => {
+				if self.config.read().unwrap().reject_inbound_splices {
+					let err = ChannelError::WarnAndDisconnect(
+						"Inbound channel splices are currently not allowed".to_owned()
+					);
+					return Err(MsgHandleErrInternal::from_chan_no_close(err, msg.channel_id));
+				}
+
 				if let Some(ref mut funded_channel) = chan_entry.get_mut().as_funded_mut() {
 					let init_res = funded_channel.splice_init(
 						msg, our_funding_contribution, &self.signer_provider, &self.entropy_source,
@@ -15325,16 +15332,15 @@ pub fn provided_init_features(config: &UserConfig) -> InitFeatures {
 	features.set_provide_storage_optional();
 	#[cfg(simple_close)]
 	features.set_simple_close_optional();
+	features.set_quiescence_optional();
+	features.set_splicing_optional();
+
 	if config.channel_handshake_config.negotiate_anchors_zero_fee_htlc_tx {
 		features.set_anchors_zero_fee_htlc_tx_optional();
 	}
 	if config.enable_dual_funded_channels {
 		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();
 
 	if config.channel_handshake_config.negotiate_anchor_zero_fee_commitments {
 		features.set_anchor_zero_fee_commitments_optional();
diff --git a/lightning/src/ln/functional_test_utils.rs b/lightning/src/ln/functional_test_utils.rs
index fbfe320..0bf60e4 100644
--- a/lightning/src/ln/functional_test_utils.rs
+++ b/lightning/src/ln/functional_test_utils.rs
@@ -1372,7 +1372,7 @@ macro_rules! reload_node {
 	($node: expr, $chanman_encoded: expr, $monitors_encoded: expr, $persister: ident, $new_chain_monitor: ident, $new_channelmanager: ident) => {
 		reload_node!(
 			$node,
-			$crate::util::config::UserConfig::default(),
+			test_default_channel_config(),
 			$chanman_encoded,
 			$monitors_encoded,
 			$persister,
@@ -4331,6 +4331,7 @@ pub fn test_default_channel_config() -> UserConfig {
 	// feerate of 253).
 	default_config.channel_config.max_dust_htlc_exposure =
 		MaxDustHTLCExposure::FeeRateMultiplier(50_000_000 / 253);
+	default_config.reject_inbound_splices = false;
 	default_config
 }
 
diff --git a/lightning/src/ln/splicing_tests.rs b/lightning/src/ln/splicing_tests.rs
index 62e1064..ac84eea 100644
--- a/lightning/src/ln/splicing_tests.rs
+++ b/lightning/src/ln/splicing_tests.rs
@@ -507,6 +507,66 @@ fn do_test_splice_state_reset_on_disconnect(reload: bool) {
 	lock_splice_after_blocks(&nodes[0], &nodes[1], channel_id, ANTI_REORG_DELAY - 1);
 }
 
+#[test]
+fn test_config_reject_inbound_splices() {
+	// Tests that nodes with `reject_inbound_splices` properly reject inbound splices but still
+	// allow outbound ones.
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let mut config = test_default_channel_config();
+	config.reject_inbound_splices = true;
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, Some(config)]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	let node_id_0 = nodes[0].node.get_our_node_id();
+	let node_id_1 = nodes[1].node.get_our_node_id();
+
+	let (_, _, channel_id, _) =
+		create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 100_000, 50_000_000);
+
+	let contribution = SpliceContribution::SpliceOut {
+		outputs: vec![TxOut {
+			value: Amount::from_sat(1_000),
+			script_pubkey: nodes[0].wallet_source.get_change_script().unwrap(),
+		}],
+	};
+	nodes[0]
+		.node
+		.splice_channel(
+			&channel_id,
+			&node_id_1,
+			contribution.clone(),
+			FEERATE_FLOOR_SATS_PER_KW,
+			None,
+		)
+		.unwrap();
+
+	let stfu = get_event_msg!(nodes[0], MessageSendEvent::SendStfu, node_id_1);
+	nodes[1].node.handle_stfu(node_id_0, &stfu);
+	let stfu = get_event_msg!(nodes[1], MessageSendEvent::SendStfu, node_id_0);
+	nodes[0].node.handle_stfu(node_id_1, &stfu);
+
+	let splice_init = get_event_msg!(nodes[0], MessageSendEvent::SendSpliceInit, node_id_1);
+	nodes[1].node.handle_splice_init(node_id_0, &splice_init);
+
+	let msg_events = nodes[1].node.get_and_clear_pending_msg_events();
+	assert_eq!(msg_events.len(), 1);
+	if let MessageSendEvent::HandleError { action, .. } = &msg_events[0] {
+		assert!(matches!(action, msgs::ErrorAction::DisconnectPeerWithWarning { .. }));
+	} else {
+		panic!("Expected MessageSendEvent::HandleError");
+	}
+
+	nodes[0].node.peer_disconnected(node_id_1);
+	nodes[1].node.peer_disconnected(node_id_0);
+	let mut reconnect_args = ReconnectArgs::new(&nodes[0], &nodes[1]);
+	reconnect_args.send_channel_ready = (true, true);
+	reconnect_args.send_announcement_sigs = (true, true);
+	reconnect_nodes(reconnect_args);
+
+	let _ = splice_channel(&nodes[1], &nodes[0], channel_id, contribution);
+}
+
 #[test]
 fn test_splice_in() {
 	let chanmon_cfgs = create_chanmon_cfgs(2);
diff --git a/lightning/src/util/config.rs b/lightning/src/util/config.rs
index 079ed19..500c0b7 100644
--- a/lightning/src/util/config.rs
+++ b/lightning/src/util/config.rs
@@ -956,6 +956,18 @@ pub struct UserConfig {
 	///
 	/// [`StaticInvoice`]: crate::offers::static_invoice::StaticInvoice
 	pub hold_outbound_htlcs_at_next_hop: bool,
+	/// If this is set to `true`, then inbound channel splice requests will be rejected. This
+	/// ensures backwards compatibility is not broken with LDK versions < 0.2 while a splice is
+	/// pending.
+	///
+	/// Outbound channel splice requests (via [`ChannelManager::splice_channel`], an opt-in API) are
+	/// still allowed as users should be aware of the backwards compatibility risk prior to using
+	/// the functionality.
+	///
+	/// Default value: `true`
+	///
+	/// [`ChannelManager::splice_channel`]: crate::ln::channelmanager::ChannelManager::splice_channel
+	pub reject_inbound_splices: bool,
 }
 
 impl Default for UserConfig {
@@ -972,6 +984,7 @@ impl Default for UserConfig {
 			enable_dual_funded_channels: false,
 			enable_htlc_hold: false,
 			hold_outbound_htlcs_at_next_hop: false,
+			reject_inbound_splices: true,
 		}
 	}
 }
@@ -994,6 +1007,7 @@ impl Readable for UserConfig {
 			enable_dual_funded_channels: Readable::read(reader)?,
 			hold_outbound_htlcs_at_next_hop: Readable::read(reader)?,
 			enable_htlc_hold: Readable::read(reader)?,
+			reject_inbound_splices: Readable::read(reader)?,
 		})
 	}
 }