psbt: finalize to-remote with option_anchor_outputs.

Until it gains miniscript support, wally doesn't know how to
finalize P2WSH.  This happens with `option_anchor_outputs` for
to-remote, which requires a 1 block delay to force use of
anchor outputs for CPFP.

So we finalize this ourselves.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

bitcoin/psbt.c

@@ -343,6 +343,44 @@ struct wally_tx *psbt_finalize(struct wally_psbt *psbt, bool finalize_in_place)
 	} else
 		tmppsbt = cast_const(struct wally_psbt *, psbt);
 
+	/* Wally doesn't know how to finalize P2WSH; this happens with
+	 * option_anchor_outputs, and finalizing is trivial. */
+	/* FIXME: miniscript! miniscript! miniscript! */
+	for (size_t i = 0; i < tmppsbt->num_inputs; i++) {
+		struct wally_psbt_input *input = &tmppsbt->inputs[i];
+		struct wally_tx_witness_stack *stack;
+
+		if (!is_anchor_witness_script(input->witness_script,
+					      input->witness_script_len))
+			continue;
+
+		if (input->signatures.num_items != 1)
+			continue;
+
+		/* BOLT-a12da24dd0102c170365124782b46d9710950ac1 #3:
+		 * #### `to_remote` Output
+		 *...
+		 *
+		 * If `option_anchor_outputs` applies to the commitment
+		 * transaction, the `to_remote` output is encumbered by a one
+		 * block csv lock.
+		 *
+		 *    <remote_pubkey> OP_CHECKSIGVERIFY 1 OP_CHECKSEQUENCEVERIFY
+		 *
+		 * The output is spent by a transaction with `nSequence` field set to `1` and witness:
+		 *
+		 *    <remote_sig>
+		 */
+		wally_tx_witness_stack_init_alloc(2, &stack);
+		wally_tx_witness_stack_add(stack,
+					   input->signatures.items[0].value,
+					   input->signatures.items[0].value_len);
+		wally_tx_witness_stack_add(stack,
+					   input->witness_script,
+					   input->witness_script_len);
+		input->final_witness = stack;
+	}
+
 	if (wally_psbt_finalize(tmppsbt) != WALLY_OK) {
 		if (!finalize_in_place)
 			wally_psbt_free(tmppsbt);

bitcoin/script.c

@@ -340,9 +340,25 @@ u8 *anchor_to_remote_redeem(const tal_t *ctx,
 	add_number(&script, 1);
 	add_op(&script, OP_CHECKSEQUENCEVERIFY);
 
+	assert(is_anchor_witness_script(script, tal_bytelen(script)));
 	return script;
 }
 
+bool is_anchor_witness_script(const u8 *script, size_t script_len)
+{
+	if (script_len != 34 + 1 + 1 + 1)
+		return false;
+	if (script[0] != OP_PUSHBYTES(33))
+		return false;
+	if (script[34] != OP_CHECKSIGVERIFY)
+		return false;
+	if (script[35] != 0x51)
+		return false;
+	if (script[36] != OP_CHECKSEQUENCEVERIFY)
+		return false;
+	return true;
+}
+
 /* Create a witness which spends the 2of2. */
 u8 **bitcoin_witness_2of2(const tal_t *ctx,
 			  const struct bitcoin_signature *sig1,

bitcoin/script.h

@@ -149,6 +149,9 @@ bool is_p2wpkh(const u8 *script, struct bitcoin_address *addr);
 /* Is this one of the four above script types? */
 bool is_known_scripttype(const u8 *script);
 
+/* Is this an anchor witness script? */
+bool is_anchor_witness_script(const u8 *script, size_t script_len);
+
 /* Are these two scripts equal? */
 bool scripteq(const u8 *s1, const u8 *s2);
 

bitcoin/test/run-bitcoin_block_from_hex.c

@@ -63,6 +63,9 @@ u16 fromwire_u16(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 /* Generated stub for fromwire_u32 */
 u32 fromwire_u32(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_u32 called!\n"); abort(); }
+/* Generated stub for is_anchor_witness_script */
+bool is_anchor_witness_script(const u8 *script UNNEEDED, size_t script_len UNNEEDED)
+{ fprintf(stderr, "is_anchor_witness_script called!\n"); abort(); }
 /* Generated stub for is_p2sh */
 bool is_p2sh(const u8 *script UNNEEDED, struct ripemd160 *addr UNNEEDED)
 { fprintf(stderr, "is_p2sh called!\n"); abort(); }

bitcoin/test/run-tx-encode.c

@@ -64,6 +64,9 @@ u16 fromwire_u16(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 /* Generated stub for fromwire_u32 */
 u32 fromwire_u32(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 { fprintf(stderr, "fromwire_u32 called!\n"); abort(); }
+/* Generated stub for is_anchor_witness_script */
+bool is_anchor_witness_script(const u8 *script UNNEEDED, size_t script_len UNNEEDED)
+{ fprintf(stderr, "is_anchor_witness_script called!\n"); abort(); }
 /* Generated stub for is_p2sh */
 bool is_p2sh(const u8 *script UNNEEDED, struct ripemd160 *addr UNNEEDED)
 { fprintf(stderr, "is_p2sh called!\n"); abort(); }