paymod: Apply and unapply routes to the channel hints

Add/remove the HTLC amount from the channel hints so concurrent getroute calls
have the correct exclusions. This can sometimes underflow, if we're unlucky
and call getroute too closely, but that's not a big issue, it can only result
in a failed MPP attempt too many, nothing fatal, and it'll get corrected based
on the result returned by the failing node.

plugins/libplugin-pay.c

@@ -296,6 +296,60 @@ payment_constraints_update(struct payment_constraints *cons,
 	return true;
 }
 
+/* Given a route and a couple of channel hints, apply the route to the channel
+ * hints, so we have a better estimation of channel's capacity. We apply a
+ * route to a channel hint before calling `sendonion` so subsequent `route`
+ * calls don't accidentally try to use those out-of-date estimates. We unapply
+ * if the payment failed, i.e., all HTLCs we might have added have been torn
+ * down again. Finally we leave the update in place if the payment went
+ * through, since the balances really changed in that case. The `remove`
+ * argument indicates whether we want to apply (`remove=false`), or clear a
+ * prior application (`remove=true`). */
+static void payment_chanhints_apply_route(struct payment *p, bool remove)
+{
+	struct route_hop *curhop;
+	struct channel_hint *curhint;
+	struct payment *root = payment_root(p);
+	assert(p->route != NULL);
+	for (size_t i = 0; i < tal_count(p->route); i++) {
+		curhop = &p->route[i];
+		for (size_t j = 0; j < tal_count(root->channel_hints); j++) {
+			curhint = &root->channel_hints[j];
+			if (short_channel_id_eq(&curhint->scid.scid,
+						&curhop->channel_id) &&
+			    curhint->scid.dir == curhop->direction) {
+				if (remove && !amount_msat_add(
+						  &curhint->estimated_capacity,
+						  curhint->estimated_capacity,
+						  curhop->amount)) {
+					/* This should never happen, it'd mean
+					 * that we unapply a route that would
+					 * result in a msatoshi
+					 * wrap-around. */
+					abort();
+				} else if (!amount_msat_sub(
+					       &curhint->estimated_capacity,
+					       curhint->estimated_capacity,
+					       curhop->amount)) {
+					/* This can happen in case of multipl
+					 * concurrent getroute calls using the
+					 * same channel_hints, no biggy, it's
+					 * an estimation anyway. */
+					plugin_log(
+					    p->plugin, LOG_UNUSUAL,
+					    "Could not update the channel hint "
+					    "for %s. Could be a concurrent "
+					    "`getroute` call.",
+					    type_to_string(
+						tmpctx,
+						struct short_channel_id_dir,
+						&curhint->scid));
+				}
+			}
+		}
+	}
+}
+
 static struct command_result *payment_getroute_result(struct command *cmd,
 						      const char *buffer,
 						      const jsmntok_t *toks,
@@ -679,6 +733,7 @@ payment_waitsendpay_finished(struct command *cmd, const char *buffer,
 	}
 
 	root = payment_root(p);
+	payment_chanhints_apply_route(p, true);
 
 	switch (p->result->failcode) {
 	case WIRE_PERMANENT_CHANNEL_FAILURE:
@@ -777,6 +832,9 @@ static struct command_result *payment_createonion_success(struct command *cmd,
 	struct out_req *req;
 	struct route_hop *first = &p->route[0];
 	struct secret *secrets;
+
+	payment_chanhints_apply_route(p, false);
+
 	p->createonion_response = tal_createonion_response_from_json(p, buffer, toks);
 
 	req = jsonrpc_request_start(p->plugin, NULL, "sendonion",