lightningd: add preimage to forward_event notification

diff --git a/doc/developers-guide/plugin-development/event-notifications.md b/doc/developers-guide/plugin-development/event-notifications.md
index 4ee034a8..ea906c25 100644
--- a/doc/developers-guide/plugin-development/event-notifications.md
+++ b/doc/developers-guide/plugin-development/event-notifications.md
@@ -223,7 +223,8 @@ A notification for topic `forward_event` is sent every time the status of a forw
     "fee_msat": 1001,
     "status": "settled",
     "received_time": 1560696342.368,
-    "resolved_time": 1560696342.556
+    "resolved_time": 1560696342.556,
+    "preimage": "0000000000000000000000000000000000000000000000000000000000000000"
   }
 }
 ```
@@ -262,6 +263,7 @@ or
       fields;
 - `received_time` means when we received the htlc of this payment from the previous peer. It will be contained into all status case;
 - `resolved_time` means when the htlc of this payment between us and the next peer was resolved. The resolved result may success or fail, so only `settled` and `failed` case contain `resolved_time`;
+- `preimage` is the 64-hex-char payment preimage revealed when the HTLC was fulfilled. Only present when `status` is `settled`;
 - The `failcode` and `failreason` are defined in [BOLT 4](https://github.com/lightning/bolts/blob/master/04-onion-routing.md#failure-messages).
 
 ### `sendpay_success`
diff --git a/lightningd/forwards.c b/lightningd/forwards.c
index 6c86d506..4ca7c0ee 100644
--- a/lightningd/forwards.c
+++ b/lightningd/forwards.c
@@ -1,7 +1,9 @@
 #include "config.h"
+#include <bitcoin/preimage.h>
 #include <ccan/mem/mem.h>
 #include <ccan/tal/str/str.h>
 #include <common/json_command.h>
+#include <common/json_stream.h>
 #include <inttypes.h>
 #include <lightningd/forwards.h>
 #include <lightningd/jsonrpc.h>
@@ -86,7 +88,8 @@ bool string_to_forward_status(const char *status_str,
  * between 'listforwards' API and 'forward_event' notification. */
 void json_add_forwarding_fields(struct json_stream *response,
 				const struct forwarding *cur,
-				const struct sha256 *payment_hash)
+				const struct sha256 *payment_hash,
+				const struct preimage *preimage)
 {
 	/* We don't bother grabbing id from db on update. */
 	if (cur->created_index)
@@ -136,6 +139,8 @@ void json_add_forwarding_fields(struct json_stream *response,
 	json_add_timeabs(response, "received_time", cur->received_time);
 	if (cur->resolved_time)
 		json_add_timeabs(response, "resolved_time", *cur->resolved_time);
+	if (preimage)
+		json_add_preimage(response, "preimage", preimage);
 }
 
 static void listforwardings_add_forwardings(struct json_stream *response,
@@ -155,7 +160,7 @@ static void listforwardings_add_forwardings(struct json_stream *response,
 	while (stmt) {
 		const struct forwarding *cur = forwarding_details(tmpctx, wallet, stmt);
 		json_object_start(response, NULL);
-		json_add_forwarding_fields(response, cur, NULL);
+		json_add_forwarding_fields(response, cur, NULL, NULL);
 		json_object_end(response);
 		tal_free(cur);
 		stmt = forwarding_next(wallet, stmt);
diff --git a/lightningd/forwards.h b/lightningd/forwards.h
index 0436035b..0156e63d 100644
--- a/lightningd/forwards.h
+++ b/lightningd/forwards.h
@@ -56,7 +56,8 @@ struct forwarding {
  * `listforwardings_add_forwardings()`. */
 void json_add_forwarding_fields(struct json_stream *response,
 				const struct forwarding *cur,
-				const struct sha256 *payment_hash);
+				const struct sha256 *payment_hash,
+				const struct preimage *preimage);
 
 static inline const char* forward_status_name(enum forward_status status)
 {
diff --git a/lightningd/notification.c b/lightningd/notification.c
index 755a606b..9a5eac50 100644
--- a/lightningd/notification.c
+++ b/lightningd/notification.c
@@ -1,4 +1,5 @@
 #include "config.h"
+#include <bitcoin/preimage.h>
 #include <ccan/cast/cast.h>
 #include <lightningd/channel.h>
 #include <lightningd/coin_mvts.h>
@@ -389,8 +390,7 @@ static void forward_event_notification_serialize(struct json_stream *stream,
 	cur->htlc_id_in = in->key.id;
 	cur->created_index = created_index;
 	cur->updated_index = updated_index;
-
-	json_add_forwarding_fields(stream, cur, &in->payment_hash);
+	json_add_forwarding_fields(stream, cur, &in->payment_hash, in->preimage);
 }
 
 REGISTER_NOTIFICATION(forward_event);
diff --git a/tests/test_plugin.py b/tests/test_plugin.py
index 594e4ab3..75e8b51f 100644
--- a/tests/test_plugin.py
+++ b/tests/test_plugin.py
@@ -1490,9 +1490,12 @@ def test_forward_event_notification(node_factory, bitcoind, executor):
     plugin_stats = l2.rpc.call('listforwards_plugin')['forwards']
     assert len(plugin_stats) == 6
 
-    # We don't have payment_hash in listforwards any more.
+    # We don't have payment_hash in listforwards any more. We also don't have
+    # preimage in listforwards
     for p in plugin_stats:
         del p['payment_hash']
+        if p.get('preimage') is not None:
+            del p['preimage']
 
     # use stats to build what we expect went to plugin.
     expect = stats[0].copy()