plugin: A new notification type, 'sendpay_success'

`sendpay_success`

A notification for topic `sendpay_success` is sent every time a sendpay
success(with `complete` status). The json is same as the return value of
command `sendpay`/`waitsendpay` when these cammand succeeds.

```json
{
	"sendpay_success": {
  "id": 1,
  "payment_hash": "5c85bf402b87d4860f4a728e2e58a2418bda92cd7aea0ce494f11670cfbfb206",
  "destination": "035d2b1192dfba134e10e540875d366ebc8bc353d5aa766b80c090b39c3a5d885d",
  "msatoshi": 100000000,
  "amount_msat": "100000000msat",
  "msatoshi_sent": 100001001,
  "amount_sent_msat": "100001001msat",
  "created_at": 1561390572,
  "status": "complete",
  "payment_preimage": "9540d98095fd7f37687ebb7759e733934234d4f934e34433d4998a37de3733ee"
  }
}
```
`sendpay` doesn't wait for the result of sendpay and `waitsendpay`
returns the result of sendpay in specified time or timeout, but
`sendpay_success` will always return the result anytime when sendpay
successes if is was subscribed.

lightningd/notification.c

@@ -227,3 +227,27 @@ void notify_forward_event(struct lightningd *ld,
 	jsonrpc_notification_end(n);
 	plugins_notify(ld->plugins, take(n));
 }
+
+static void sendpay_success_notification_serialize(struct json_stream *stream,
+						   const struct wallet_payment *payment)
+{
+	json_object_start(stream, "sendpay_success");
+	json_add_payment_fields(stream, payment);
+	json_object_end(stream); /* .sendpay_success */
+}
+
+REGISTER_NOTIFICATION(sendpay_success,
+		      sendpay_success_notification_serialize);
+
+void notify_sendpay_success(struct lightningd *ld,
+			    const struct wallet_payment *payment)
+{
+	void (*serialize)(struct json_stream *,
+			  const struct wallet_payment *) = sendpay_success_notification_gen.serialize;
+
+	struct jsonrpc_notification *n =
+	    jsonrpc_notification_start(NULL, "sendpay_success");
+	serialize(n->stream, payment);
+	jsonrpc_notification_end(n);
+	plugins_notify(ld->plugins, take(n));
+}

lightningd/notification.h

@@ -12,6 +12,7 @@
 #include <lightningd/jsonrpc.h>
 #include <lightningd/lightningd.h>
 #include <lightningd/log.h>
+#include <lightningd/pay.h>
 #include <lightningd/plugin.h>
 #include <wallet/wallet.h>
 #include <wire/gen_onion_wire.h>
@@ -54,4 +55,7 @@ void notify_forward_event(struct lightningd *ld,
 			  enum onion_type failcode,
 			  struct timeabs *resolved_time);
 
+void notify_sendpay_success(struct lightningd *ld,
+			    const struct wallet_payment *payment);
+
 #endif /* LIGHTNING_LIGHTNINGD_NOTIFICATION_H */

lightningd/pay.c

@@ -73,8 +73,7 @@ add_waitsendpay_waiter(struct lightningd *ld,
 }
 
 /* Outputs fields, not a separate object*/
-static void
-json_add_payment_fields(struct json_stream *response,
+void json_add_payment_fields(struct json_stream *response,
 			     const struct wallet_payment *t)
 {
 	json_add_u64(response, "id", t->id);

lightningd/pay.h

@@ -6,6 +6,8 @@ struct htlc_out;
 struct lightningd;
 struct preimage;
 struct sha256;
+struct json_stream;
+struct wallet_payment;
 
 void payment_succeeded(struct lightningd *ld, struct htlc_out *hout,
 		       const struct preimage *rval);
@@ -16,4 +18,8 @@ void payment_failed(struct lightningd *ld, const struct htlc_out *hout,
 /* Inform payment system to save the payment. */
 void payment_store(struct lightningd *ld, const struct sha256 *payment_hash);
 
+/* This json will be also used in 'sendpay_success' notifictaion. */
+void json_add_payment_fields(struct json_stream *response,
+			     const struct wallet_payment *t);
+
 #endif /* LIGHTNING_LIGHTNINGD_PAY_H */