df: add notification for receiving peer's funding tx sigs

This will allow us to build complex, multi-peer transactions, with
easeTM!

Changelog-Added: EXPERIMENTAL, Plugins: `openchannel_peer_sigs` notification, which contains a peer's signatures for the funding transaction (`opt_dual_fund`)

doc/PLUGINS.md

@@ -653,6 +653,22 @@ at the time lightningd broadcasts the notification.
 
 `coin_type` is the BIP173 name for the coin which moved.
 
+### `openchannel_peer_sigs`
+
+When opening a channel with a peer using the collaborative transaction protocol
+(`opt_dual_fund`), this notification is fired when the peer sends us their funding
+transaction signatures, `tx_signatures`. We update the in-progress PSBT and return it
+here, with the peer's signatures attached.
+
+```json
+{
+	"openchannel_peer_sigs": {
+		"channel_id": "<hex of a channel id (note, v2 format)>",
+		"signed_psbt": "<Base64 serialized PSBT of funding transaction,
+				with peer's sigs>"
+	}
+}
+```
 
 ## Hooks
 

lightningd/channel_control.c

@@ -25,6 +25,7 @@
 #include <lightningd/jsonrpc.h>
 #include <lightningd/lightningd.h>
 #include <lightningd/log.h>
+#include <lightningd/notification.h>
 #include <lightningd/onion_message.h>
 #include <lightningd/peer_control.h>
 #include <lightningd/subd.h>
@@ -449,6 +450,10 @@ static void peer_tx_sigs_msg(struct channel *channel, const u8 *msg)
 	}
 
 	wallet_channel_save(ld->wallet, channel);
+
+	/* Send notification with peer's signed PSBT */
+	notify_openchannel_peer_sigs(ld, &channel->cid,
+				     channel->psbt);
 }
 
 void forget_channel(struct channel *channel, const char *why)

lightningd/notification.c

@@ -467,3 +467,31 @@ void notify_coin_mvt(struct lightningd *ld,
 	jsonrpc_notification_end(n);
 	plugins_notify(ld->plugins, take(n));
 }
+
+static void openchannel_peer_sigs_serialize(struct json_stream *stream,
+					    const struct channel_id *cid,
+					    const struct wally_psbt *psbt)
+{
+	json_object_start(stream, "openchannel_peer_sigs");
+	json_add_channel_id(stream, "channel_id", cid);
+	json_add_psbt(stream, "signed_psbt", psbt);
+	json_object_end(stream);
+}
+
+REGISTER_NOTIFICATION(openchannel_peer_sigs,
+		      openchannel_peer_sigs_serialize);
+
+void notify_openchannel_peer_sigs(struct lightningd *ld,
+				  const struct channel_id *cid,
+				  const struct wally_psbt *psbt)
+{
+	void (*serialize)(struct json_stream *,
+			  const struct channel_id *cid,
+			  const struct wally_psbt *) = openchannel_peer_sigs_notification_gen.serialize;
+
+	struct jsonrpc_notification *n =
+		jsonrpc_notification_start(NULL, "openchannel_peer_sigs");
+	serialize(n->stream, cid, psbt);
+	jsonrpc_notification_end(n);
+	plugins_notify(ld->plugins, take(n));
+}

lightningd/notification.h

@@ -19,9 +19,11 @@
 #include <lightningd/pay.h>
 #include <lightningd/plugin.h>
 #include <wallet/wallet.h>
+#include <wally_psbt.h>
 #include <wire/onion_wire.h>
 
 struct onionreply;
+struct wally_psbt;
 
 bool notifications_have_topic(const char *topic);
 
@@ -86,4 +88,8 @@ void notify_sendpay_failure(struct lightningd *ld,
 
 void notify_coin_mvt(struct lightningd *ld,
 		     const struct coin_mvt *mvt);
+
+void notify_openchannel_peer_sigs(struct lightningd *ld,
+				  const struct channel_id *cid,
+				  const struct wally_psbt *psbt);
 #endif /* LIGHTNING_LIGHTNINGD_NOTIFICATION_H */