psbt: add method to finalize + extract a psbt

will either use a temporary psbt (and not munge the passed in psbt)
or will finalize in place -- finalization erases most of the signature
metadata from the psbt struct

bitcoin/psbt.c

@@ -340,6 +340,38 @@ struct amount_sat psbt_input_get_amount(struct wally_psbt *psbt,
 	return val;
 }
 
+struct wally_tx *psbt_finalize(struct wally_psbt *psbt, bool finalize_in_place)
+{
+	struct wally_psbt *tmppsbt;
+	struct wally_tx *wtx;
+
+	/* We want the 'finalized' tx since that includes any signature
+	 * data, not the global tx. But 'finalizing' a tx destroys some fields
+	 * so we 'clone' it first and then finalize it */
+	if (!finalize_in_place) {
+		if (wally_psbt_clone(psbt, &tmppsbt) != WALLY_OK)
+			return NULL;
+	} else
+		tmppsbt = cast_const(struct wally_psbt *, psbt);
+
+	if (wally_finalize_psbt(tmppsbt) != WALLY_OK) {
+		if (!finalize_in_place)
+			wally_psbt_free(tmppsbt);
+		return NULL;
+	}
+
+	if (psbt_is_finalized(tmppsbt)
+		&& wally_extract_psbt(tmppsbt, &wtx) == WALLY_OK) {
+		if (!finalize_in_place)
+			wally_psbt_free(tmppsbt);
+		return wtx;
+	}
+
+	if (!finalize_in_place)
+		wally_psbt_free(tmppsbt);
+	return NULL;
+}
+
 bool psbt_from_b64(const char *b64str, struct wally_psbt **psbt)
 {
 	int wally_err;

bitcoin/psbt.h

@@ -32,6 +32,8 @@ struct wally_psbt *new_psbt(const tal_t *ctx,
  */
 bool psbt_is_finalized(struct wally_psbt *psbt);
 
+struct wally_tx *psbt_finalize(struct wally_psbt *psbt, bool finalize_in_place);
+
 struct wally_psbt_input *psbt_add_input(struct wally_psbt *psbt,
 					struct wally_tx_input *input,
 					size_t insert_at);

bitcoin/tx.c

@@ -501,38 +501,18 @@ char *bitcoin_tx_to_psbt_base64(const tal_t *ctx, struct bitcoin_tx *tx)
 
 struct bitcoin_tx *bitcoin_tx_with_psbt(const tal_t *ctx, struct wally_psbt *psbt STEALS)
 {
-	struct wally_psbt *tmppsbt;
 	struct bitcoin_tx *tx = bitcoin_tx(ctx, chainparams,
 					   psbt->tx->num_inputs,
 					   psbt->tx->num_outputs,
 					   psbt->tx->locktime);
 	wally_tx_free(tx->wtx);
-
-	/* We want the 'finalized' tx since that includes any signature
-	 * data, not the global tx. But 'finalizing' a tx destroys some fields
-	 * so we 'clone' it first and then finalize it */
-	if (wally_psbt_clone(psbt, &tmppsbt) != WALLY_OK)
-		return NULL;
-
-	if (wally_finalize_psbt(tmppsbt) != WALLY_OK) {
-		wally_psbt_free(tmppsbt);
-		return NULL;
-	}
-
-	if (psbt_is_finalized(tmppsbt)) {
-		if (wally_extract_psbt(tmppsbt, &tx->wtx) != WALLY_OK) {
-			wally_psbt_free(tmppsbt);
-			return NULL;
-		}
-	} else if (wally_tx_clone(psbt->tx, &tx->wtx) != WALLY_OK) {
-		wally_psbt_free(tmppsbt);
+	tx->wtx = psbt_finalize(psbt, false);
+	if (!tx->wtx && wally_tx_clone(psbt->tx, &tx->wtx) != WALLY_OK)
 		return NULL;
-	}
-
-	wally_psbt_free(tmppsbt);
 
 	tal_free(tx->psbt);
 	tx->psbt = tal_steal(tx, psbt);
+
 	return tx;
 }