Consider maximum path length during path finding.

Author: tnull

lightning/src/routing/router.rs

@@ -68,7 +68,7 @@ pub struct Route {
 	/// last RouteHop in each path must be the same.
 	/// Each entry represents a list of hops, NOT INCLUDING our own, where the last hop is the
 	/// destination. Thus, this must always be at least length one. While the maximum length of any
-	/// given path is variable, keeping the length of any path to less than 20 should currently
+	/// given path is variable, keeping the length of any path to less than 40 should currently
 	/// ensure it is viable.
 	pub paths: Vec<Vec<RouteHop>>,
 	/// The `payment_params` parameter passed to [`find_route`].
@@ -1476,6 +1476,21 @@ where L::Target: Logger {
 	}
 
 	// Step (5).
+	// We only consider paths shorter than our maximum length estimate. 
+	// In the legacy onion format, the maximum number of hops used to be a fixed value of 20.
+	// However, in the TLV onion format, there is no fixed maximum length, but the `hop_payloads`
+	// field is always 1300 bytes. As the `tlv_payload` for each hop may vary in length, we have to
+	// estimate how many hops the route may have so that it actually fits the `hop_payloads` field.
+	//
+	// We estimate 2+8 (amt_to_forward) + 2+4 (outgoing_cltv_value) + 2+8 (short_channel_id) = 26
+	// bytes for each intermediate hop and 2+8 (amt_to_forward) + 2+4 (outgoing_cltv_value) +
+	// 2+32+8 (payment_secret and total_msat) = 58 bytes for the final hop.
+	// Since the length of the potentially included `payment_metadata` is unkown to us, we
+	// generously round down from (1300-58) / 26 = 47 to arrive at a conservative estimate of a
+	// feasible maximum path length of 40 hops.
+	const MAX_PATH_LENGTH_ESTIMATE: usize = 40;
+	payment_paths.retain(|path| path.hops.len() <= MAX_PATH_LENGTH_ESTIMATE);
+	
 	if payment_paths.len() == 0 {
 		return Err(LightningError{err: "Failed to find a path to the given destination".to_owned(), action: ErrorAction::IgnoreError});
 	}