fundchannel: provide outnum

It's currently always 0, but it won't be once we replace txprepare.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Changelog-Added: JSON-RPC: `fundchannel` has new `outnum` field indicating which output of the transaction funds the channel.

doc/lightning-fundchannel.7

@@ -69,6 +69,7 @@ unrecoverable once pushed\.
 .SH RETURN VALUE
 
 On success, the \fItx\fR and \fItxid\fR of the transaction is returned, as well
+as the \fIoutnum\fR indicating the output index which creates the channel, as well
 as the \fIchannel_id\fR of the newly created channel\. On failure, an error
 is reported and the channel is not funded\.
 

doc/lightning-fundchannel.7.md

@@ -64,6 +64,7 @@ RETURN VALUE
 ------------
 
 On success, the *tx* and *txid* of the transaction is returned, as well
+as the *outnum* indicating the output index which creates the channel, as well
 as the *channel\_id* of the newly created channel. On failure, an error
 is reported and the channel is not funded.
 

plugins/fundchannel.c

@@ -34,6 +34,7 @@ struct funding_req {
 
 	/* The prepared tx id */
 	struct bitcoin_txid tx_id;
+	u32 outnum;
 
 	const char *chanstr;
 	const u8 *out_script;
@@ -86,6 +87,7 @@ static struct command_result *finish(struct command *cmd,
 	json_add_tok(out, "tx", json_get_member(buf, result, "tx"), buf);
 	json_add_string(out, "txid",
 			type_to_string(tmpctx, struct bitcoin_txid, &fr->tx_id));
+	json_add_u32(out, "outnum", fr->outnum);
 	json_add_string(out, "channel_id", fr->chanstr);
 
 	return command_finished(cmd, out);
@@ -130,7 +132,6 @@ static struct command_result *tx_prepare_done(struct command *cmd,
 	struct out_req *req;
 	const struct bitcoin_tx *tx;
 	const char *hex;
-	u32 outnum;
 	bool outnum_found;
 
 	txid_tok = json_get_member(buf, result, "txid");
@@ -151,7 +152,7 @@ static struct command_result *tx_prepare_done(struct command *cmd,
 	for (size_t i = 0; i < tx->wtx->num_outputs; i++) {
 		const u8 *output_script = bitcoin_tx_output_get_script(fr, tx, i);
 		if (scripteq(output_script, fr->out_script)) {
-			outnum = i;
+			fr->outnum = i;
 			outnum_found = true;
 			break;
 		}
@@ -171,7 +172,7 @@ static struct command_result *tx_prepare_done(struct command *cmd,
 	json_add_string(req->js, "id", node_id_to_hexstr(tmpctx, fr->id));
 	/* Note that hex is reused from above */
 	json_add_string(req->js, "txid", hex);
-	json_add_u32(req->js, "txout", outnum);
+	json_add_u32(req->js, "txout", fr->outnum);
 
 	return send_outreq(cmd->plugin, req);
 }

tests/test_plugin.py

@@ -1211,13 +1211,16 @@ def test_bcli(node_factory, bitcoind, chainparams):
 
     l1.fundwallet(10**5)
     l1.connect(l2)
-    txid = l1.rpc.fundchannel(l2.info["id"], 10**4)["txid"]
-    txo = l1.rpc.call("getutxout", {"txid": txid, "vout": 0})
+    fc = l1.rpc.fundchannel(l2.info["id"], 10**4)
+    txo = l1.rpc.call("getutxout", {"txid": fc['txid'], "vout": fc['outnum']})
     assert (Millisatoshi(txo["amount"]) == Millisatoshi(10**4 * 10**3)
             and txo["script"].startswith("0020"))
     l1.rpc.close(l2.info["id"])
     # When output is spent, it should give us null !
-    wait_for(lambda: l1.rpc.call("getutxout", {"txid": txid, "vout": 0})['amount'] is None)
+    wait_for(lambda: l1.rpc.call("getutxout", {
+        "txid": fc['txid'],
+        "vout": fc['outnum']
+    })['amount'] is None)
 
     resp = l1.rpc.call("sendrawtransaction", {"tx": "dummy"})
     assert not resp["success"] and "decode failed" in resp["errmsg"]