From cead5e16c0c556308fce3a54f74fd5dc3e8ea992 Mon Sep 17 00:00:00 2001
From: niftynei <niftynei@gmail.com>
Date: Tue, 1 Jun 2021 14:38:55 -0500
Subject: [PATCH] openingd: return more informative error message for v2 opens

Fixes #4562

Reported-By: William Casarin @jb55

New error message:

	zircon:lightning (nifty/fix-4562)$ l1-cli fundchannel_start 03fce775508719e4064b7f19d4e884ddaf51db23bbfc560286ce872f9ed106fee0 10000 253perkw
	{
	   "code": 312,
	   "message": "Peer negotiated `option_dual_fund`, must use `openchannel_init` not `fundchannel_start`."
	}
---
 lightningd/opening_control.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/lightningd/opening_control.c b/lightningd/opening_control.c
index 3d26add0f..df6336c1f 100644
--- a/lightningd/opening_control.c
+++ b/lightningd/opening_control.c
@@ -1177,6 +1177,15 @@ static struct command_result *json_fundchannel_start(struct command *cmd,
 	}
 
 	if (!peer->uncommitted_channel) {
+		if (feature_negotiated(cmd->ld->our_features,
+				       peer->their_features,
+				       OPT_DUAL_FUND))
+			return command_fail(cmd, FUNDING_STATE_INVALID,
+					    "Peer negotiated"
+					    " `option_dual_fund`,"
+					    " must use `openchannel_init` not"
+					    " `fundchannel_start`.");
+
 		return command_fail(cmd, FUNDING_PEER_NOT_CONNECTED,
 				    "Peer not connected");
 	}