renepay: limit arc capacities by htlc_max

In the Min. Cost Flow solver we put a constraint on arcs capacities,
such that the total flow that can be allocated through a channel does
not exceed htlc_max. This solves two previous problem of the htlc_max
constraint at the MCF level.

plugins/renepay/mcf.c

@@ -483,11 +483,18 @@ static bool linearize_channel(const struct pay_parameters *params,
 	a = MAX(a,0);
 	b = MAX(a+1,b);
 
+	/* An extra bound on capacity, here we use it to reduce the flow such
+	 * that it does not exceed htlcmax. */
+	s64 cap_on_capacity =
+	 channel_htlc_max(c, dir).millisatoshis/1000; /* Raw: linearize_channel */
+
 	capacity[0]=a;
 	cost[0]=0;
 	for(size_t i=1;i<CHANNEL_PARTS;++i)
 	{
-		capacity[i] = params->cap_fraction[i]*(b-a);
+		capacity[i] = MIN(params->cap_fraction[i]*(b-a), cap_on_capacity);
+		cap_on_capacity -= capacity[i];
+		assert(cap_on_capacity>=0);
 
 		cost[i] = params->cost_fraction[i]
 		          *params->amount.millisatoshis /* Raw: linearize_channel */

plugins/renepay/pay.c

@@ -483,9 +483,9 @@ static void gossmod_cb(struct gossmap_localmods *mods,
 	}
 
 	/* FIXME: features? */
-	gossmap_local_addchan(mods, self, peer, &scidd->scid, NULL);
+	gossmap_local_addchan(mods, self, peer, scidd->scid, NULL);
 
-	gossmap_local_updatechan(mods, &scidd->scid, min, max,
+	gossmap_local_updatechan(mods, scidd->scid, min, max,
 				 fee_base.millisatoshis, /* Raw: gossmap */
 				 fee_proportional,
 				 cltv_delta,

tests/test_renepay.py

@@ -489,12 +489,12 @@ def test_htlc_max(node_factory):
         ]
     )
 
-    inv = l6.rpc.invoice("1800000sat", "inv", "description")
+    inv = l6.rpc.invoice("800000sat", "inv", "description")
 
     l1.rpc.call("renepay", {"invstring": inv["bolt11"]})
     l1.wait_for_htlcs()
     invoice = only_one(l6.rpc.listinvoices("inv")["invoices"])
-    assert invoice["amount_received_msat"] >= Millisatoshi("1800000sat")
+    assert invoice["amount_received_msat"] >= Millisatoshi("800000sat")
 
 
 def test_previous_sendpays(node_factory, bitcoind):
@@ -635,3 +635,49 @@ def test_local_htlcmax0(node_factory):
     assert details["status"] == "complete"
     assert details["amount_msat"] == Millisatoshi(123000)
     assert details["destination"] == l3.info["id"]
+
+
+def test_htlcmax0(node_factory):
+    """
+    Topology:
+    1----2----4
+    |         |
+    3----5----6
+    Tests the plugin when some routes have htlc_max=0.
+    """
+    opts = [
+        {"disable-mpp": None, "fee-base": 0, "fee-per-satoshi": 0},
+        {"disable-mpp": None, "fee-base": 0, "fee-per-satoshi": 0},
+        {"disable-mpp": None, "fee-base": 0, "fee-per-satoshi": 0},
+        {
+            "disable-mpp": None,
+            "fee-base": 0,
+            "fee-per-satoshi": 0,
+            "htlc-maximum-msat": 0,
+        },
+        {
+            "disable-mpp": None,
+            "fee-base": 0,
+            "fee-per-satoshi": 0,
+            "htlc-maximum-msat": 800000000,
+        },
+        {"disable-mpp": None, "fee-base": 0, "fee-per-satoshi": 0},
+    ]
+    l1, l2, l3, l4, l5, l6 = node_factory.get_nodes(6, opts=opts)
+    start_channels(
+        [
+            (l1, l2, 10000000),
+            (l2, l4, 1000000),
+            (l4, l6, 1000000),
+            (l1, l3, 10000000),
+            (l3, l5, 1000000),
+            (l5, l6, 1000000),
+        ]
+    )
+
+    inv = l6.rpc.invoice("600000sat", "inv", "description")
+
+    l1.rpc.call("renepay", {"invstring": inv["bolt11"]})
+    l1.wait_for_htlcs()
+    invoice = only_one(l6.rpc.listinvoices("inv")["invoices"])
+    assert invoice["amount_received_msat"] >= Millisatoshi("600000sat")