askrene: hack in some padding so we don't overflow capacities.

Of course, we still will, since spendable is for a single HTLC, but
this also shows why we should treat *minimum* as the incorrect answer
if they cross, too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Fixes: #7563

Author: rustyrussell

plugins/askrene/mcf.c

@@ -453,9 +453,17 @@ static void linearize_channel(const struct pay_parameters *params,
 
 	/* This takes into account any payments in progress. */
 	get_constraints(params->rq, c, dir, &mincap, &maxcap);
-	/* Assume if min > max, max is wrong */
+
+	/* We seem to have some rounding error (perhaps due to our use
+	 * of sats and fee interactions?).  Since it's unusual to see
+	 * a large unmber of flows, even if each overflows by 1 sat,
+	 * 5 sats should be plenty. */
+	if (!amount_msat_sub(&maxcap, maxcap, AMOUNT_MSAT(5000)))
+		maxcap = AMOUNT_MSAT(0);
+
+	/* Assume if min > max, min is wrong */
 	if (amount_msat_greater(mincap, maxcap))
-		maxcap = mincap;
+		mincap = maxcap;
 
 	u64 a = mincap.millisatoshis/1000, /* Raw: linearize_channel */
 	    b = 1 + maxcap.millisatoshis/1000; /* Raw: linearize_channel */

tests/test_askrene.py

@@ -169,8 +169,8 @@ def test_getroutes(node_factory):
                             amount_msat=1000,
                             layers=[],
                             maxfee_msat=1000,
-                            final_cltv=99) == {'probability_ppm': 999999,
-                                               'routes': [{'probability_ppm': 999999,
+                            final_cltv=99) == {'probability_ppm': 999998,
+                                               'routes': [{'probability_ppm': 999998,
                                                            'final_cltv': 99,
                                                            'amount_msat': 1000,
                                                            'path': [{'short_channel_id': '0x1x0',
@@ -184,8 +184,8 @@ def test_getroutes(node_factory):
                             amount_msat=100000,
                             layers=[],
                             maxfee_msat=5000,
-                            final_cltv=99) == {'probability_ppm': 999798,
-                                               'routes': [{'probability_ppm': 999798,
+                            final_cltv=99) == {'probability_ppm': 999797,
+                                               'routes': [{'probability_ppm': 999797,
                                                            'final_cltv': 99,
                                                            'amount_msat': 100000,
                                                            'path': [{'short_channel_id': '0x1x0',
@@ -247,11 +247,11 @@ def test_getroutes(node_factory):
                          10000000,
                          [[{'short_channel_id': '0x2x1',
                             'next_node_id': nodemap[2],
-                            'amount_msat': 500000,
+                            'amount_msat': 505000,
                             'delay': 99 + 6}],
                           [{'short_channel_id': '0x2x3',
                             'next_node_id': nodemap[2],
-                            'amount_msat': 9500009,
+                            'amount_msat': 9495009,
                             'delay': 99 + 6}]])
 
 
@@ -313,8 +313,8 @@ def test_getroutes_auto_sourcefree(node_factory):
                             amount_msat=1000,
                             layers=['auto.sourcefree'],
                             maxfee_msat=1000,
-                            final_cltv=99) == {'probability_ppm': 999999,
-                                               'routes': [{'probability_ppm': 999999,
+                            final_cltv=99) == {'probability_ppm': 999998,
+                                               'routes': [{'probability_ppm': 999998,
                                                            'final_cltv': 99,
                                                            'amount_msat': 1000,
                                                            'path': [{'short_channel_id': '0x1x0',
@@ -328,8 +328,8 @@ def test_getroutes_auto_sourcefree(node_factory):
                             amount_msat=100000,
                             layers=['auto.sourcefree'],
                             maxfee_msat=5000,
-                            final_cltv=99) == {'probability_ppm': 999798,
-                                               'routes': [{'probability_ppm': 999798,
+                            final_cltv=99) == {'probability_ppm': 999797,
+                                               'routes': [{'probability_ppm': 999797,
                                                            'final_cltv': 99,
                                                            'amount_msat': 100000,
                                                            'path': [{'short_channel_id': '0x1x0',
@@ -451,7 +451,6 @@ def test_fees_dont_exceed_constraints(node_factory):
     assert amount <= max_msat
 
 
-@pytest.mark.xfail(strict=True)
 def test_live_spendable(node_factory, bitcoind):
     """Test we don't exceed spendable limits on a real network on nodes"""
     l1, l2, l3 = node_factory.get_nodes(3)