Relax channel count check for unannounced nodes

When creating blinded paths, introduction nodes are limited to peers
with at least three channels to prevent easily guessing the recipient.
Relax this check when the recipient is unannounced since they won't be
in the NetworkGraph.

Author: jkczyz

lightning/src/onion_message/messenger.rs

@@ -511,7 +511,9 @@ where
 			.filter_map(|peer|
 				network_graph
 					.node(&NodeId::from_pubkey(&peer.node_id))
-					.filter(|info| info.channels.len() >= MIN_PEER_CHANNELS)
+					.filter(|info|
+						!is_recipient_announced || info.channels.len() >= MIN_PEER_CHANNELS
+					)
 					.map(|info| (peer, info.is_tor_only(), info.channels.len()))
 					// Allow messages directly with the only peer when unannounced.
 					.or_else(|| (!is_recipient_announced && has_one_peer)

lightning/src/routing/router.rs

@@ -119,9 +119,10 @@ impl<G: Deref<Target = NetworkGraph<L>> + Clone, L: Deref, ES: Deref, S: Deref,
 			.filter(|details| amount_msats <= details.inbound_capacity_msat)
 			.filter(|details| amount_msats >= details.inbound_htlc_minimum_msat.unwrap_or(0))
 			.filter(|details| amount_msats <= details.inbound_htlc_maximum_msat.unwrap_or(u64::MAX))
+			// Limit to peers with announced channels unless the recipient is unannounced.
 			.filter(|details| network_graph
 					.node(&NodeId::from_pubkey(&details.counterparty.node_id))
-					.map(|node_info| node_info.channels.len() >= MIN_PEER_CHANNELS)
+					.map(|node| !is_recipient_announced || node.channels.len() >= MIN_PEER_CHANNELS)
 					// Allow payments directly with the only peer when unannounced.
 					.unwrap_or(!is_recipient_announced && has_one_peer)
 			)