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pay: Exempt a fee from the maxfeepercent rule it is still tiny

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Several users have noticed that they cannot pay satoshis.place or similar places
that have tiny payment amounts if they are not directly connected. This is due
to the forwarding fee dominating the transferred amount.

This commit adds a new option, exempting tiny fees (up to 5 satoshis by default)
from having to pass the maxfeepercent flag. While we could have told users to
tweak maxfeepercent I think it is usefull to have a default exemption.

[Squashed --RR]
This commit is contained in:
Christian Decker
2018-07-30 03:05:43 +02:00
committed by Rusty Russell
parent a235c1fa67
commit 55d450ff00
4 changed files with 20 additions and 7 deletions
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11
lightningd/payalgo.c
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@@ -128,6 +128,11 @@ struct pay {
/* Whether we are attempting payment or not. */
bool in_sendpay;
/* Maximum fee that is exempted from the maxfeepercent computation. This
* is mainly useful for tiny transfers for which the leveraged fee would
* be dominated by the forwarding fee. */
u64 exemptfee;
};
static struct routing_failure *
@@ -435,7 +440,7 @@ static void json_pay_getroute_reply(struct subd *gossip UNUSED,
* That loss will not be representable in double. So, it's Okay to
* cast u64 to double for feepercent calculation. */
feepercent = ((double) fee) * 100.0 / ((double) pay->msatoshi);
fee_too_high = (feepercent > pay->maxfeepercent);
fee_too_high = (fee > pay->exemptfee && feepercent > pay->maxfeepercent);
delay_too_high = (route[0].delay > pay->maxdelay);
/* compare fuzz to range */
if ((fee_too_high || delay_too_high) && pay->fuzz < 0.01) {
@@ -602,6 +607,7 @@ static void json_pay(struct command *cmd,
char *fail, *b11str, *desc;
unsigned int retryfor;
unsigned int maxdelay;
unsigned int exemptfee;
if (!param(cmd, buffer, params,
p_req("bolt11", json_tok_tok, &bolt11tok),
@@ -612,6 +618,7 @@ static void json_pay(struct command *cmd,
p_opt_def("retry_for", json_tok_number, &retryfor, 60),
p_opt_def("maxdelay", json_tok_number, &maxdelay,
cmd->ld->config.locktime_max),
p_opt_def("exemptfee", json_tok_number, &exemptfee, 5000),
NULL))
return;
@@ -636,6 +643,7 @@ static void json_pay(struct command *cmd,
memset(&pay->expiry, 0, sizeof(pay->expiry));
pay->expiry.ts.tv_sec = b11->timestamp + b11->expiry;
pay->min_final_cltv_expiry = b11->min_final_cltv_expiry;
pay->exemptfee = exemptfee;
if (b11->msatoshi) {
if (msatoshi) {
@@ -703,6 +711,7 @@ static const struct json_command pay_command = {
"{description} (required if {bolt11} uses description hash), "
"{riskfactor} (default 1.0), "
"{maxfeepercent} (default 0.5) the maximum acceptable fee as a percentage (e.g. 0.5 => 0.5%), "
"{exemptfee} (default 5000 msat) disables the maxfeepercent check for fees below the threshold, "
"{retry_for} (default 60) the integer number of seconds before we stop retrying, and "
"{maxdelay} (default 500) the maximum number of blocks we allow the funds to possibly get locked"
};