pay: Let sendpay modify the recorded msatoshi of payments.

doc/lightning-sendpay.7

@@ -2,12 +2,12 @@
 .\"     Title: lightning-sendpay
 .\"    Author: [see the "AUTHOR" section]
 .\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
-.\"      Date: 03/14/2018
+.\"      Date: 03/22/2018
 .\"    Manual: \ \&
 .\"    Source: \ \&
 .\"  Language: English
 .\"
-.TH "LIGHTNING\-SENDPAY" "7" "03/14/2018" "\ \&" "\ \&"
+.TH "LIGHTNING\-SENDPAY" "7" "03/22/2018" "\ \&" "\ \&"
 .\" -----------------------------------------------------------------
 .\" * Define some portability stuff
 .\" -----------------------------------------------------------------
@@ -31,7 +31,7 @@
 lightning-sendpay \- Protocol for sending a payment via a route\&.
 .SH "SYNOPSIS"
 .sp
-\fBsendpay\fR \fIroute\fR \fIhash\fR
+\fBsendpay\fR \fIroute\fR \fIhash\fR [\fImsatoshi\fR]
 .SH "DESCRIPTION"
 .sp
 The \fBsendpay\fR RPC command attempts to send funds associated with the given \fIhash\fR, along a route to the final destination in the route\&.
@@ -40,7 +40,9 @@ Generally, a client would call getroute(7) to resolve a route, then use \fBsendp
 .sp
 The response will occur when the payment is on its way to the destination\&. The \fBsendpay\fR RPC command does not wait for definite success or definite failure of the payment\&. Instead, use the \fBwaitsendpay\fR RPC command to poll or wait for definite success or definite failure\&.
 .sp
-Once a payment has succeeded, calls to \fBsendpay\fR with the same \fIhash\fR but a different amount or destination will fail; this prevents accidental multiple payments\&. Calls to \fBsendpay\fR with the same \fIhash\fR, amount, and destination as a previous successful payment (even if a different route) will return immediately with success\&.
+The \fImsatoshi\fR amount, if provided, is the amount that will be recorded as the target payment value\&. If not specified, it will be the final amount to the destination\&. If specified, then the final amount at the destination must be from the specified \fImsatoshi\fR to twice the specified \fImsatoshi\fR, inclusive\&. This is intended to obscure payments by overpaying slightly at the destination; the actual target payment is what should be specified as the \fImsatoshi\fR argument\&.
+.sp
+Once a payment has succeeded, calls to \fBsendpay\fR with the same \fIhash\fR but a different \fImsatoshi\fR or destination will fail; this prevents accidental multiple payments\&. Calls to \fBsendpay\fR with the same \fIhash\fR, \fImsatoshi\fR, and destination as a previous successful payment (even if a different route) will return immediately with success\&.
 .SH "RETURN VALUE"
 .sp
 On success, an object similar to the output of \fBlistpayments\fR will be returned\&. This object will have a \fIstatus\fR field that is typically the string \fI"pending"\fR, but may be \fI"complete"\fR if the payment was already performed successfully\&.

doc/lightning-sendpay.7.txt

@@ -8,7 +8,7 @@ lightning-sendpay - Protocol for sending a payment via a route.
 
 SYNOPSIS
 --------
-*sendpay* 'route' 'hash'
+*sendpay* 'route' 'hash' ['msatoshi']
 
 DESCRIPTION
 -----------
@@ -27,10 +27,21 @@ definite failure of the payment.
 Instead, use the *waitsendpay* RPC command to poll or wait for
 definite success or definite failure.
 
+The 'msatoshi' amount, if provided, is the amount that will be
+recorded as the target payment value.
+If not specified, it will be the final amount to the destination.
+If specified, then the final amount at the destination must be
+from the specified 'msatoshi' to twice the specified 'msatoshi',
+inclusive.
+This is intended to obscure payments by overpaying slightly at
+the destination;
+the actual target payment is what should be specified as the
+'msatoshi' argument.
+
 Once a payment has succeeded, calls to *sendpay* with the same 'hash'
-but a different amount or destination will fail; this prevents
+but a different 'msatoshi' or destination will fail; this prevents
 accidental multiple payments.
-Calls to *sendpay* with the same 'hash', amount, and destination as a
+Calls to *sendpay* with the same 'hash', 'msatoshi', and destination as a
 previous successful payment (even if a different route) will return
 immediately with success.
 

lightningd/pay.c

@@ -628,6 +628,7 @@ send_payment(const tal_t *ctx,
 	     struct lightningd* ld,
 	     const struct sha256 *rhash,
 	     const struct route_hop *route,
+	     u64 msatoshi,
 	     void (*cb)(const struct sendpay_result *, void*),
 	     void *cbarg)
 {
@@ -685,7 +686,7 @@ send_payment(const tal_t *ctx,
 		if (payment->status == PAYMENT_COMPLETE) {
 			log_add(ld->log, "... succeeded");
 			/* Must match successful payment parameters. */
-			if (payment->msatoshi != hop_data[n_hops-1].amt_forward) {
+			if (payment->msatoshi != msatoshi) {
 				char *msg = tal_fmt(tmpctx,
 						    "Already succeeded "
 						    "with amount %"PRIu64,
@@ -740,8 +741,8 @@ send_payment(const tal_t *ctx,
 				    sizeof(struct sha256), &path_secrets);
 	onion = serialize_onionpacket(tmpctx, packet);
 
-	log_info(ld->log, "Sending %u over %zu hops to deliver %u",
-		 route[0].amount, n_hops, route[n_hops-1].amount);
+	log_info(ld->log, "Sending %u over %zu hops to deliver %"PRIu64"",
+		 route[0].amount, n_hops, msatoshi);
 
 	failcode = send_htlc_out(channel, route[0].amount,
 				 base_expiry + route[0].delay,
@@ -781,7 +782,7 @@ send_payment(const tal_t *ctx,
 	payment->payment_hash = *rhash;
 	payment->destination = ids[n_hops - 1];
 	payment->status = PAYMENT_PENDING;
-	payment->msatoshi = route[n_hops-1].amount;
+	payment->msatoshi = msatoshi;
 	payment->msatoshi_sent = route[0].amount;
 	payment->timestamp = time_now().ts.tv_sec;
 	payment->payment_preimage = NULL;
@@ -912,14 +913,17 @@ static void json_sendpay(struct command *cmd,
 			 const char *buffer, const jsmntok_t *params)
 {
 	jsmntok_t *routetok, *rhashtok;
+	jsmntok_t *msatoshitok;
 	const jsmntok_t *t, *end;
 	size_t n_hops;
 	struct sha256 rhash;
 	struct route_hop *route;
+	u64 msatoshi;
 
 	if (!json_get_params(cmd, buffer, params,
 			     "route", &routetok,
 			     "payment_hash", &rhashtok,
+			     "?msatoshi", &msatoshitok,
 			     NULL)) {
 		return;
 	}
@@ -994,7 +998,29 @@ static void json_sendpay(struct command *cmd,
 		return;
 	}
 
-	if (send_payment(cmd, cmd->ld, &rhash, route,
+	if (msatoshitok) {
+		if (!json_tok_u64(buffer, msatoshitok, &msatoshi)) {
+			command_fail(cmd, "'%.*s' is not a number",
+				     msatoshitok->end - msatoshitok->start,
+				     buffer + msatoshitok->start);
+			return;
+		}
+		/* The given msatoshi is the actual payment that
+		 * the payee is requesting. The final hop amount, is
+		 * what we actually give, which can be from the
+		 * msatoshi to twice msatoshi. */
+		/* if not: msatoshi <= finalhop.amount <= 2 * msatoshi,
+		 * fail. */
+		if (!(msatoshi <= route[n_hops-1].amount &&
+		      route[n_hops-1].amount <= 2 * msatoshi)) {
+			command_fail(cmd, "msatoshi %"PRIu64" out of range",
+				     msatoshi);
+			return;
+		}
+	} else
+		msatoshi = route[n_hops-1].amount;
+
+	if (send_payment(cmd, cmd->ld, &rhash, route, msatoshi,
 			  &json_sendpay_on_resolve, cmd))
 		command_still_pending(cmd);
 }

lightningd/pay.h

@@ -57,11 +57,14 @@ struct sendpay_result {
  * sendpay_result indicating an error code of PAY_IN_PROGRESS.
  * It will only call the callback with successful sendpay_result
  * if the payment has already completed with the same amount
- * and destination before. */
+ * and destination before.
+ *
+ * The msatoshi given is what is recorded in the payment. */
 bool send_payment(const tal_t *ctx,
 		  struct lightningd* ld,
 		  const struct sha256 *rhash,
 		  const struct route_hop *route,
+		  u64 msatoshi,
 		  void (*cb)(const struct sendpay_result *, void*),
 		  void *cbarg);
 /* Wait for a previous send_payment to complete in definite

lightningd/payalgo.c

@@ -475,6 +475,7 @@ static void json_pay_getroute_reply(struct subd *gossip UNUSED,
 	pay->in_sendpay = true;
 	send_payment(pay->try_parent,
 		     pay->cmd->ld, &pay->payment_hash, route,
+		     pay->msatoshi,
 		     &json_pay_sendpay_resume, pay);
 }