lightningd: fix spurious "more than twice final" error.

Bastien TEINTURIER <bastien@acinq.fr> writes:
> It looks like the split on c-lightning side is quite limited at the moment:
> the only option is to split a payment in exactly its two halves,
> otherwise I get rejected because of the rule of overpaying more than
> twice the amount?

We only tested exactly two equal-size payments; indeed, our finalhop
test was backwards.  We only complain if the final hop pays more than
twice msat (technically, this test is still too loose for mpp: the
spec says we should sum to the exact amount).

Reported-by: @t-bast
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/pay.c

@@ -1292,26 +1292,26 @@ static struct command_result *json_sendpay(struct command *cmd,
 		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
 				    "Must specify msatoshi with partid");
 
-	/* if not: finalhop.amount <= 2 * msatoshi, fail. */
+	/* finalhop.amount > 2 * msatoshi, fail. */
 	if (msat) {
-		struct amount_msat limit = route[routetok->size-1].amount;
+		struct amount_msat limit;
 
-		if (!amount_msat_add(&limit, limit, limit))
+		if (!amount_msat_add(&limit, *msat, *msat))
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-					    "Unbelievable final amount %s",
+					    "Unbelievable msatoshi %s",
 					    type_to_string(tmpctx,
 							   struct amount_msat,
-							   &route[routetok->size-1].amount));
+							   msat));
 
-		if (amount_msat_greater(*msat, limit))
+		if (amount_msat_greater(route[routetok->size-1].amount, limit))
 			return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-					    "msatoshi %s more than twice final %s",
+					    "final %s more than twice msatoshi %s",
 					    type_to_string(tmpctx,
 							   struct amount_msat,
-							   msat),
+							   &route[routetok->size-1].amount),
 					    type_to_string(tmpctx,
 							   struct amount_msat,
-							   &route[routetok->size-1].amount));
+							   msat));
 	}
 
 	/* It's easier to leave this in the API, then ignore it here. */

tests/test_pay.py

@@ -2593,8 +2593,8 @@ def test_partial_payment(node_factory, bitcoind, executor):
     paysecret = l4.rpc.decodepay(inv['bolt11'])['payment_secret']
 
     # Separate routes for each part of the payment.
-    r134 = l1.rpc.getroute(l4.info['id'], 500, 1, exclude=[scid24 + '/0', scid24 + '/1'])['route']
+    r134 = l1.rpc.getroute(l4.info['id'], 501, 1, exclude=[scid24 + '/0', scid24 + '/1'])['route']
-    r124 = l1.rpc.getroute(l4.info['id'], 500, 1, exclude=[scid34 + '/0', scid34 + '/1'])['route']
+    r124 = l1.rpc.getroute(l4.info['id'], 499, 1, exclude=[scid34 + '/0', scid34 + '/1'])['route']
 
     # These can happen in parallel.
     l1.rpc.call('sendpay', [r134, inv['payment_hash'], None, 1000, inv['bolt11'], paysecret, 1])
@@ -2638,7 +2638,7 @@ def test_partial_payment(node_factory, bitcoind, executor):
     for i in range(2):
         line = l4.daemon.wait_for_log('print_htlc_onion.py: Got onion')
         assert "'type': 'tlv'" in line
-        assert "'forward_amount': '500msat'" in line
+        assert "'forward_amount': '499msat'" in line or "'forward_amount': '501msat'" in line
         assert "'total_msat': '1000msat'" in line
         assert "'payment_secret': '{}'".format(paysecret) in line