pay: add absolute "maxfee" parameter.

This is what LND does, and it's better for upper layers than trying to
twist our maxfeepercent / exemptfee heuristics to suit.

(I don't remember who complained about this, sorry!)

I'm doing this now because I want to add YA parameter next!

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `pay` has new parameter `maxfee` for setting absolute fee (instead of using `maxfeepercent` and/or `exemptfee`)

contrib/pyln-client/pyln/client/lightning.py

@@ -997,7 +997,8 @@ class LightningRpc(UnixDomainSocketRpc):
 
     def pay(self, bolt11, msatoshi=None, label=None, riskfactor=None,
             maxfeepercent=None, retry_for=None,
-            maxdelay=None, exemptfee=None, localofferid=None, exclude=None):
+            maxdelay=None, exemptfee=None, localofferid=None, exclude=None,
+            maxfee=None):
         """
         Send payment specified by {bolt11} with {msatoshi}
         (ignored if {bolt11} has an amount), optional {label}
@@ -1014,6 +1015,7 @@ class LightningRpc(UnixDomainSocketRpc):
             "exemptfee": exemptfee,
             "localofferid": localofferid,
             "exclude": exclude,
+            "maxfee": maxfee,
         }
         return self.call("pay", payload)
 

doc/lightning-pay.7.md

@@ -5,8 +5,8 @@ SYNOPSIS
 --------
 
 **pay** *bolt11* [*msatoshi*] [*label*] [*riskfactor*]
-[*maxfeepercent*] [*retry\_for*] [*maxdelay*] [*exemptfee*]
-[*localofferid*] [*exclude*]
+[*maxfeepercent*] [*retry_for*] [*maxdelay*] [*exemptfee*]
+[*localofferid*] [*exclude*] [*maxfee*]
 
 DESCRIPTION
 -----------
@@ -37,6 +37,12 @@ leveraged by forwarding nodes. Setting `exemptfee` allows the
 that we only make a single payment for an offer, and that the offer is
 marked `used` once paid.
 
+*maxfee* overrides both *maxfeepercent* and *exemptfee* defaults (and
+if you specify *maxfee* you cannot specify either of those), and
+creates an absolute limit on what fee we will pay.  This allows you to
+implement your own heuristics rather than the primitive ones used
+here.
+
 The response will occur when the payment fails or succeeds. Once a
 payment has succeeded, calls to **pay** with the same *bolt11* will
 succeed immediately.

doc/schemas/pay.request.json

@@ -45,6 +45,9 @@
           }
         ]
       }
+    },
+    "maxfee": {
+      "type": "msat"
     }
   }
 }

plugins/pay.c

@@ -925,7 +925,7 @@ static struct command_result *json_pay(struct command *cmd,
 	char *b11_fail, *b12_fail;
 	u64 *maxfee_pct_millionths;
 	u32 *maxdelay;
-	struct amount_msat *exemptfee, *msat;
+	struct amount_msat *exemptfee, *msat, *maxfee;
 	const char *label;
 	unsigned int *retryfor;
 	u64 *riskfactor_millionths;
@@ -950,14 +950,15 @@ static struct command_result *json_pay(struct command *cmd,
 		   p_opt("label", param_string, &label),
 		   p_opt_def("riskfactor", param_millionths,
 			     &riskfactor_millionths, 10000000),
-		   p_opt_def("maxfeepercent", param_millionths,
-			     &maxfee_pct_millionths, 500000),
+		   p_opt("maxfeepercent", param_millionths,
+			 &maxfee_pct_millionths),
 		   p_opt_def("retry_for", param_number, &retryfor, 60),
 		   p_opt_def("maxdelay", param_number, &maxdelay,
 			     maxdelay_default),
-		   p_opt_def("exemptfee", param_msat, &exemptfee, AMOUNT_MSAT(5000)),
+		   p_opt("exemptfee", param_msat, &exemptfee),
 		   p_opt("localofferid", param_sha256, &local_offer_id),
 		   p_opt("exclude", param_route_exclusion_array, &exclusions),
+		   p_opt("maxfee", param_msat, &maxfee),
 #if DEVELOPER
 		   p_opt_def("use_shadow", param_bool, &use_shadow, true),
 #endif
@@ -1103,17 +1104,35 @@ static struct command_result *json_pay(struct command *cmd,
 	p->getroute->riskfactorppm = *riskfactor_millionths;
 	tal_free(riskfactor_millionths);
 
-	/* We free unneeded params as we use them, to keep memleak happy. */
-	if (!amount_msat_fee(&p->constraints.fee_budget, p->amount, 0,
-			     *maxfee_pct_millionths / 100)) {
-		return command_fail(
-		    cmd, JSONRPC2_INVALID_PARAMS,
-		    "Overflow when computing fee budget, fee rate too high.");
-	}
-	tal_free(maxfee_pct_millionths);
+	if (maxfee) {
+		if (maxfee_pct_millionths || exemptfee) {
+			return command_fail(
+				cmd, JSONRPC2_INVALID_PARAMS,
+				"If you specify maxfee, cannot specify maxfeepercent or exemptfee.");
+		}
+		p->constraints.fee_budget = *maxfee;
+		payment_mod_exemptfee_get_data(p)->amount = AMOUNT_MSAT(0);
+	} else {
+		u64 maxppm;
+
+		if (maxfee_pct_millionths)
+			maxppm = *maxfee_pct_millionths / 100;
+		else
+			maxppm = 500000 / 100;
+		if (!amount_msat_fee(&p->constraints.fee_budget, p->amount, 0,
+				     maxppm)) {
+			return command_fail(
+				cmd, JSONRPC2_INVALID_PARAMS,
+				"Overflow when computing fee budget, fee rate too high.");
+		}
+		payment_mod_exemptfee_get_data(p)->amount
+			= exemptfee ? *exemptfee : AMOUNT_MSAT(5000);
+
+		/* We free unneeded params now to keep memleak happy. */
+		tal_free(maxfee_pct_millionths);
+		tal_free(exemptfee);
+	}
 
-	payment_mod_exemptfee_get_data(p)->amount = *exemptfee;
-	tal_free(exemptfee);
 	shadow_route = payment_mod_shadowroute_get_data(p);
 	payment_mod_presplit_get_data(p)->disable = disablempp;
 	payment_mod_adaptive_splitter_get_data(p)->disable = disablempp;

tests/test_pay.py

@@ -128,10 +128,15 @@ def test_pay_limits(node_factory):
     assert(len(status) == 2)
     assert(status[0]['failure']['code'] == 205)
 
+    # This fails!
+    err = r'Fee exceeds our fee budget: 2msat > 1msat, discarding route'
+    with pytest.raises(RpcError, match=err) as err:
+        l1.rpc.pay(bolt11=inv['bolt11'], msatoshi=100000, maxfee=1)
+
     # This works, because fee is less than exemptfee.
     l1.dev_pay(inv['bolt11'], msatoshi=100000, maxfeepercent=0.0001,
                exemptfee=2000, use_shadow=False)
-    status = l1.rpc.call('paystatus', {'bolt11': inv['bolt11']})['pay'][2]['attempts']
+    status = l1.rpc.call('paystatus', {'bolt11': inv['bolt11']})['pay'][3]['attempts']
     assert len(status) == 1
     assert status[0]['strategy'] == "Initial attempt"
 

tests/test_plugin.py

@@ -397,7 +397,8 @@ def test_pay_plugin(node_factory):
 
     # Make sure usage messages are present.
     msg = 'pay bolt11 [msatoshi] [label] [riskfactor] [maxfeepercent] '\
-          '[retry_for] [maxdelay] [exemptfee] [localofferid] [exclude]'
+          '[retry_for] [maxdelay] [exemptfee] [localofferid] [exclude] '\
+          '[maxfee]'
     if DEVELOPER:
         msg += ' [use_shadow]'
     assert only_one(l1.rpc.help('pay')['help'])['command'] == msg