df: finalize redeemscript at the same time as witness stack

libwally has a quirk where the finalize method will fail to 'completely'
finalize an input's parts if either the final_scriptsig or
final_redeemscript fields are set

since we manually set the final_witness stack here, we also need to
fully finalize the redeemscript -> final_scriptsig here as well.

channeld/channeld.c

@@ -2134,7 +2134,7 @@ static void handle_tx_sigs(struct peer *peer, const u8 *msg)
 
 		elem = cast_const2(const struct witness_element **,
 				   ws[j++]->witness_element);
-		psbt_input_set_final_witness_stack(peer->psbt, in, elem);
+		psbt_finalize_input(peer->psbt, in, elem);
 	}
 
 	/* Send to the peer controller, who will broadcast the funding_tx

common/psbt_internal.c

@@ -1,12 +1,15 @@
 #include "common/psbt_internal.h"
+#include <bitcoin/script.h>
+#include <ccan/ccan/tal/tal.h>
 #include <common/psbt_open.h>
 #include <wally_psbt.h>
 #include <wire/peer_wire.h>
 
 #if EXPERIMENTAL_FEATURES
-void psbt_input_set_final_witness_stack(const tal_t *ctx,
-					struct wally_psbt_input *in,
-					const struct witness_element **elements)
+static void
+psbt_input_set_final_witness_stack(const tal_t *ctx,
+				   struct wally_psbt_input *in,
+				   const struct witness_element **elements)
 {
 	tal_wally_start();
 	wally_tx_witness_stack_init_alloc(tal_count(elements),
@@ -19,6 +22,34 @@ void psbt_input_set_final_witness_stack(const tal_t *ctx,
 	tal_wally_end(ctx);
 }
 
+void psbt_finalize_input(const tal_t *ctx,
+			 struct wally_psbt_input *in,
+			 const struct witness_element **elements)
+{
+	psbt_input_set_final_witness_stack(ctx, in, elements);
+
+	/* There's this horrible edgecase where we set the final_witnesses
+	 * directly onto the PSBT, but the input is a P2SH-wrapped input
+	 * (which has redeemscripts that belong in the scriptsig). Because
+	 * of how the internal libwally stuff works calling 'finalize'
+	 * on these just .. ignores it!? Murder. Anyway, here we do a final
+	 * scriptsig check -- if there's a redeemscript field still around we
+	 * just go ahead and mush it into the final_scriptsig field. */
+	if (in->redeem_script) {
+		u8 *redeemscript = tal_dup_arr(NULL, u8,
+					       in->redeem_script,
+					       in->redeem_script_len, 0);
+		in->final_scriptsig =
+			bitcoin_scriptsig_redeem(NULL,
+						 take(redeemscript));
+		in->final_scriptsig_len =
+			tal_bytelen(in->final_scriptsig);
+
+		in->redeem_script = tal_free(in->redeem_script);
+		in->redeem_script_len = 0;
+	}
+}
+
 const struct witness_stack **
 psbt_to_witness_stacks(const tal_t *ctx,
 		       const struct wally_psbt *psbt,

common/psbt_internal.h

@@ -12,15 +12,17 @@ struct witness_element;
 #endif /* EXPERIMENTAL_FEATURES */
 
 #if EXPERIMENTAL_FEATURES
-/* psbt_input_set_final_witness_stack - Set the witness stack for PSBT input
+/* psbt_finalize_input - Finalize an input with a given witness stack
  *
+ * Sets the given witness elements onto the PSBT. Also finalizes
+ * the redeem_script, if any.
  * @ctx - the context to allocate onto
  * @in - input to set final_witness for
  * @witness_element - elements to add to witness stack
  */
-void psbt_input_set_final_witness_stack(const tal_t *ctx,
-					struct wally_psbt_input *in,
-					const struct witness_element **elements);
+void psbt_finalize_input(const tal_t *ctx,
+			 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
  *