txprepare: return psbt serialized version of tx as well

Changelog-Added: JSON-API: `txprepare` returns a psbt version of the created transaction

bitcoin/tx.c

@@ -492,6 +492,19 @@ void bitcoin_tx_finalize(struct bitcoin_tx *tx)
 	assert(bitcoin_tx_check(tx));
 }
 
+char *bitcoin_tx_to_psbt_base64(const tal_t *ctx, struct bitcoin_tx *tx)
+{
+	char *serialized_psbt, *ret_val;
+	int ret;
+
+	ret = wally_psbt_to_base64(tx->psbt, &serialized_psbt);
+	assert(ret == WALLY_OK);
+
+	ret_val = tal_strdup(ctx, serialized_psbt);
+	wally_free_string(serialized_psbt);
+	return ret_val;
+}
+
 struct bitcoin_tx *pull_bitcoin_tx(const tal_t *ctx, const u8 **cursor,
 				   size_t *max)
 {

bitcoin/tx.h

@@ -195,4 +195,8 @@ void towire_bitcoin_txid(u8 **pptr, const struct bitcoin_txid *txid);
 void towire_bitcoin_tx(u8 **pptr, const struct bitcoin_tx *tx);
 void towire_bitcoin_tx_output(u8 **pptr, const struct bitcoin_tx_output *output);
 
+/*
+ * Get the base64 string encoded PSBT of a bitcoin transaction.
+ */
+char *bitcoin_tx_to_psbt_base64(const tal_t *ctx, struct bitcoin_tx *tx);
 #endif /* LIGHTNING_BITCOIN_TX_H */

common/json_helpers.c

@@ -240,6 +240,15 @@ void json_add_tx(struct json_stream *result,
 	json_add_hex_talarr(result, fieldname, linearize_tx(tmpctx, tx));
 }
 
+void json_add_psbt(struct json_stream *stream,
+		   const char *fieldname,
+		   struct bitcoin_tx *tx)
+{
+	const char *psbt_b64;
+	psbt_b64 = bitcoin_tx_to_psbt_base64(tx, tx);
+	json_add_string(stream, fieldname, take(psbt_b64));
+}
+
 void json_add_amount_msat_compat(struct json_stream *result,
 				 struct amount_msat msat,
 				 const char *rawfieldname,

common/json_helpers.h

@@ -137,4 +137,10 @@ void json_add_preimage(struct json_stream *result, const char *fieldname,
 void json_add_tx(struct json_stream *result,
 		 const char *fieldname,
 		 const struct bitcoin_tx *tx);
+
+/* '"fieldname" : "cHNidP8BAJoCAAAAAljo..." or "cHNidP8BAJoCAAAAAljo..." if fieldname is NULL */
+void json_add_psbt(struct json_stream *stream,
+		   const char *fieldname,
+		   struct bitcoin_tx *tx);
+
 #endif /* LIGHTNING_COMMON_JSON_HELPERS_H */

tests/test_wallet.py

@@ -302,6 +302,7 @@ def test_txprepare(node_factory, bitcoind, chainparams):
     wait_for(lambda: len(l1.rpc.listfunds()['outputs']) == 10)
 
     prep = l1.rpc.txprepare(outputs=[{addr: Millisatoshi(amount * 3 * 1000)}])
+    assert prep['psbt']
     decode = bitcoind.rpc.decoderawtransaction(prep['unsigned_tx'])
     assert decode['txid'] == prep['txid']
     # 4 inputs, 2 outputs (3 if we have a fee output).

wallet/walletrpc.c

@@ -443,6 +443,7 @@ static struct command_result *json_txprepare(struct command *cmd,
 	response = json_stream_success(cmd);
 	json_add_tx(response, "unsigned_tx", utx->tx);
 	json_add_txid(response, "txid", &utx->txid);
+	json_add_psbt(response, "psbt", utx->tx);
 	return command_success(cmd, response);
 }
 static const struct json_command txprepare_command = {