psbt: method for extracting witness stacks

common/psbt_open.c

@@ -483,3 +483,57 @@ void psbt_input_set_final_witness_stack(struct wally_psbt_input *in,
 					   elements[i]->witness,
 					   tal_bytelen(elements[i]->witness));
 }
+
+const struct witness_stack **
+psbt_to_witness_stacks(const tal_t *ctx,
+		       const struct wally_psbt *psbt,
+		       enum tx_role side_to_stack)
+{
+	size_t stack_index;
+	u16 serial_id;
+	const struct witness_stack **stacks
+		= tal_arr(ctx, const struct witness_stack *, psbt->num_inputs);
+
+	stack_index = 0;
+	for (size_t i = 0; i < psbt->num_inputs; i++) {
+		if (!psbt_get_serial_id(&psbt->inputs[i].unknowns,
+					&serial_id))
+			/* FIXME: throw an error ? */
+			return NULL;
+
+		/* BOLT-78de9a79b491ae9fb84b1fdb4546bacf642dce87 #2:
+		 * - if is the `initiator`:
+		 *   - MUST send even `serial_id`s
+		 */
+		if (serial_id % 2 == side_to_stack) {
+			struct wally_tx_witness_stack *wtx_s =
+				psbt->inputs[i].final_witness;
+			struct witness_stack *stack =
+				tal(stacks, struct witness_stack);
+			/* Convert the wally_tx_witness_stack to
+			 * a witness_stack entry */
+			stack->witness_element =
+				tal_arr(stack, struct witness_element *,
+					wtx_s->num_items);
+			for (size_t j = 0; j < tal_count(stack->witness_element); j++) {
+				stack->witness_element[j] = tal(stack,
+								struct witness_element);
+				stack->witness_element[j]->witness =
+					tal_dup_arr(stack, u8,
+						    wtx_s->items[j].witness,
+						    wtx_s->items[j].witness_len,
+						    0);
+
+			}
+
+			stacks[stack_index++] = stack;
+		}
+
+	}
+
+	if (stack_index == 0)
+		return tal_free(stacks);
+
+	tal_resize(&stacks, stack_index);
+	return stacks;
+}

common/psbt_open.h

@@ -3,6 +3,7 @@
 #include "config.h"
 #include <ccan/short_types/short_types.h>
 #include <ccan/tal/tal.h>
+#include <common/tx_roles.h>
 #include <stdbool.h>
 #include <wally_psbt.h>
 #include <wally_transaction.h>
@@ -148,4 +149,16 @@ bool psbt_has_required_fields(struct wally_psbt *psbt);
  */
 void psbt_input_set_final_witness_stack(struct wally_psbt_input *in,
 					const struct witness_element **elements);
+
+/* psbt_to_witness_stacks - Take all sigs on a PSBT and copy to a
+ * 			    witness_stack
+ *
+ * @ctx - allocation context
+ * @psbt - PSBT to copy sigs from
+ * @opener - which side initiated this tx
+ */
+const struct witness_stack **
+psbt_to_witness_stacks(const tal_t *ctx,
+		       const struct wally_psbt *psbt,
+		       enum tx_role side_to_stack);
 #endif /* LIGHTNING_COMMON_PSBT_OPEN_H */