fundpsbt: let caller specify locktime.

Required for dual funding where the opener sets it.

Changelog-Added: JSON-RPC: `fundpsbt` takes a new `locktime` parameter
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-client/pyln/client/lightning.py

@@ -1126,7 +1126,7 @@ class LightningRpc(UnixDomainSocketRpc):
         }
         return self.call("unreserveinputs", payload)
 
-    def fundpsbt(self, satoshi, feerate, startweight, minconf=None, reserve=True):
+    def fundpsbt(self, satoshi, feerate, startweight, minconf=None, reserve=True, locktime=None):
         """
         Create a PSBT with inputs sufficient to give an output of satoshi.
         """
@@ -1136,6 +1136,7 @@ class LightningRpc(UnixDomainSocketRpc):
             "startweight": startweight,
             "minconf": minconf,
             "reserve": reserve,
+            "locktime": locktime,
         }
         return self.call("fundpsbt", payload)
 

doc/lightning-fundpsbt.7

@@ -3,7 +3,7 @@
 lightning-fundpsbt - Command to populate PSBT inputs from the wallet
 .SH SYNOPSIS
 
-\fBfundpsbt\fR \fIsatoshi\fR \fIfeerate\fR \fIstartweight\fR [\fIminconf\fR] [\fIreserve\fR]
+\fBfundpsbt\fR \fIsatoshi\fR \fIfeerate\fR \fIstartweight\fR [\fIminconf\fR] [\fIreserve\fR] [\fIlocktime\fR]
 
 .SH DESCRIPTION
 
@@ -39,6 +39,10 @@ outputs should have\. Default is 1\.
 \fIreserve\fR is a boolean: if true (the default), then \fIreserveinputs\fR is
 called (successfully, with \fIexclusive\fR true) on the returned PSBT\.
 
+
+\fIlocktime\fR is an optional locktime: if not set, it is set to a recent
+block height\.
+
 .SH EXAMPLE USAGE
 
 Let's assume the caller is trying to produce a 100,000 satoshi output\.

doc/lightning-fundpsbt.7.md

@@ -4,7 +4,7 @@ lightning-fundpsbt -- Command to populate PSBT inputs from the wallet
 SYNOPSIS
 --------
 
-**fundpsbt** *satoshi* *feerate* *startweight* \[*minconf*\] \[*reserve*\]
+**fundpsbt** *satoshi* *feerate* *startweight* \[*minconf*\] \[*reserve*\] \[*locktime*\]
 
 DESCRIPTION
 -----------
@@ -36,6 +36,9 @@ outputs should have. Default is 1.
 *reserve* is a boolean: if true (the default), then *reserveinputs* is
 called (successfully, with *exclusive* true) on the returned PSBT.
 
+*locktime* is an optional locktime: if not set, it is set to a recent
+block height.
+
 EXAMPLE USAGE
 -------------
 

tests/test_wallet.py

@@ -508,6 +508,9 @@ def test_fundpsbt(node_factory, bitcoind, chainparams):
     # Should get one input, plus some excess
     funding = l1.rpc.fundpsbt(amount // 2, feerate, 0, reserve=False)
     psbt = bitcoind.rpc.decodepsbt(funding['psbt'])
+    # We can fuzz this up to 99 blocks back.
+    assert psbt['tx']['locktime'] > bitcoind.rpc.getblockcount() - 100
+    assert psbt['tx']['locktime'] <= bitcoind.rpc.getblockcount()
     assert len(psbt['tx']['vin']) == 1
     assert funding['excess_msat'] > Millisatoshi(0)
     assert funding['excess_msat'] < Millisatoshi(amount // 2 * 1000)
@@ -515,9 +518,10 @@ def test_fundpsbt(node_factory, bitcoind, chainparams):
     assert 'estimated_final_weight' in funding
     assert 'reservations' not in funding
 
-    # This should add 99 to the weight, but otherwise be identical (might choose different inputs though!)
-    funding2 = l1.rpc.fundpsbt(amount // 2, feerate, 99, reserve=False)
+    # This should add 99 to the weight, but otherwise be identical (might choose different inputs though!) except for locktime.
+    funding2 = l1.rpc.fundpsbt(amount // 2, feerate, 99, reserve=False, locktime=bitcoind.rpc.getblockcount() + 1)
     psbt2 = bitcoind.rpc.decodepsbt(funding2['psbt'])
+    assert psbt2['tx']['locktime'] == bitcoind.rpc.getblockcount() + 1
     assert len(psbt2['tx']['vin']) == 1
     assert funding2['excess_msat'] < funding['excess_msat']
     assert funding2['feerate_per_kw'] == 7500

wallet/reservation.c

@@ -213,11 +213,11 @@ static struct command_result *finish_psbt(struct command *cmd,
 					  u32 feerate_per_kw,
 					  size_t weight,
 					  struct amount_sat excess,
-					  bool reserve)
+					  bool reserve,
+					  u32 *locktime)
 {
 	struct json_stream *response;
 	struct bitcoin_tx *tx;
-	u32 locktime;
 	u32 current_height = get_block_height(cmd->ld->topology);
 
 	/* Setting the locktime to the next block to be mined has multiple
@@ -229,18 +229,21 @@ static struct command_result *finish_psbt(struct command *cmd,
 	 *   0xFFFFFFFD by default. Other wallets are likely to implement
 	 *   this too).
 	 */
-	locktime = current_height;
+	if (!locktime) {
+		locktime = tal(cmd, u32);
+		*locktime = current_height;
 
-	/* Eventually fuzz it too. */
-	if (locktime > 100 && pseudorand(10) == 0)
-		locktime -= pseudorand(100);
+		/* Eventually fuzz it too. */
+		if (*locktime > 100 && pseudorand(10) == 0)
+			*locktime -= pseudorand(100);
+	}
 
 	/* FIXME: tx_spending_utxos does more than we need, but there
 	 * are other users right now. */
 	tx = tx_spending_utxos(cmd, chainparams,
 			       cast_const2(const struct utxo **, utxos),
 			       cmd->ld->wallet->bip32_base,
-			       false, 0, locktime,
+			       false, 0, *locktime,
 			       BITCOIN_TX_RBF_SEQUENCE);
 
 	response = json_stream_success(cmd);
@@ -264,7 +267,7 @@ static struct command_result *json_fundpsbt(struct command *cmd,
 	u32 *minconf, *weight;
 	struct amount_sat *amount, input, diff;
 	bool all, *reserve;
-	u32 maxheight;
+	u32 *locktime, maxheight;
 
 	if (!param(cmd, buffer, params,
 		   p_req("satoshi", param_sat_or_all, &amount),
@@ -272,6 +275,7 @@ static struct command_result *json_fundpsbt(struct command *cmd,
 		   p_req("startweight", param_number, &weight),
 		   p_opt_def("minconf", param_number, &minconf, 1),
 		   p_opt_def("reserve", param_bool, &reserve, true),
+		   p_opt("locktime", param_number, &locktime),
 		   NULL))
 		return command_param_failed();
 
@@ -336,7 +340,8 @@ static struct command_result *json_fundpsbt(struct command *cmd,
 					    tal_count(utxos));
 	}
 
-	return finish_psbt(cmd, utxos, *feerate_per_kw, *weight, diff, *reserve);
+	return finish_psbt(cmd, utxos, *feerate_per_kw, *weight, diff, *reserve,
+			   locktime);
 }
 
 static const struct json_command fundpsbt_command = {
@@ -474,7 +479,7 @@ static struct command_result *json_utxopsbt(struct command *cmd,
 	}
 
 	return finish_psbt(cmd, utxos, *feerate_per_kw, *weight, excess,
-			   *reserve);
+			   *reserve, NULL);
 }
 static const struct json_command utxopsbt_command = {
 	"utxopsbt",