pay: don't discard high-htlc_min channels when searching for routehin…

…t starts.

As side-effect, getroute(0) is special too.

Reported-by: MiddleW4y in Discord
Fixes: #6577
Changelog-Fixed: `pay` will still use an invoice routehint if path to it doesn't take 1-msat payments.

common/route.c

@@ -13,7 +13,9 @@ bool route_can_carry_even_disabled(const struct gossmap *map,
 {
 	if (!gossmap_chan_set(c, dir))
 		return false;
-	if (!gossmap_chan_capacity(c, dir, amount))
+	/* Amount 0 is a special "ignore min" probe case */
+	if (!amount_msat_eq(amount, AMOUNT_MSAT(0))
+	    && !gossmap_chan_capacity(c, dir, amount))
 		return false;
 	return true;
 }

doc/lightning-getroute.7.md

@@ -17,7 +17,9 @@ arrive at *id* with *cltv*-blocks to spare (default 9).
 *amount\_msat* is in millisatoshi precision; it can be a whole number, or a
 whole number ending in *msat* or *sat*, or a number with three decimal
 places ending in *sat*, or a number with 1 to 11 decimal places ending
-in *btc*.
+in *btc*.  The 0 value is special: it ignores any *htlc\_minimum\_msat*
+setting on channels, and simply returns a possible route (if any) which
+is useful for simple probing.
 
 There are two considerations for how good a route is: how low the fees
 are, and how long your payment will get stuck in a delayed output if a

plugins/libplugin-pay.c

@@ -2521,7 +2521,7 @@ static struct route_info **filter_routehints(struct gossmap *map,
 		}
 
 		distance = dijkstra_distance(
-		    dijkstra(tmpctx, map, entrynode, AMOUNT_MSAT(1), 1,
+		    dijkstra(tmpctx, map, entrynode, AMOUNT_MSAT(0), 1,
 			     payment_route_can_carry_even_disabled,
 			     route_score_cheaper, p),
 		    gossmap_node_idx(map, src));
@@ -2763,12 +2763,12 @@ static void routehint_check_reachable(struct payment *p)
 	if (dst == NULL)
 		d->destination_reachable = false;
 	else if (src != NULL) {
-		dij = dijkstra(tmpctx, gossmap, dst, AMOUNT_MSAT(1000),
+		dij = dijkstra(tmpctx, gossmap, dst, AMOUNT_MSAT(0),
 			       10 / 1000000.0,
 			       payment_route_can_carry_even_disabled,
 			       route_score_cheaper, p);
 		r = route_from_dijkstra(tmpctx, gossmap, dij, src,
-					AMOUNT_MSAT(1000), 0);
+					AMOUNT_MSAT(0), 0);
 
 		/* If there was a route the destination is reachable
 		 * without routehints. */

tests/test_pay.py

@@ -5389,7 +5389,6 @@ def test_listsendpays_crash(node_factory):
     l1.rpc.listsendpays('lightning:' + inv)
 
 
-@pytest.mark.xfail(strict=True)
 @pytest.mark.developer("updates are delayed without --dev-fast-gossip")
 def test_pay_routehint_minhtlc(node_factory, bitcoind):
     # l1 -> l2 -> l3 private -> l4