bitcoin/psbt: handle anchor spends.

Turns out it's a single sig, identical to the already-handled
case where we spend a to_remote output.

We also close a temporary memleak: stack was unused, but
tallocated off the psbt, so it lives as long as the PSBT.

Signed-off-by: Rusty Russell <[email protected]>

bitcoin/psbt.c

@@ -586,18 +586,24 @@ bool psbt_finalize(struct wally_psbt *psbt)
 	tal_wally_start();
 
 	/* Wally doesn't know how to finalize P2WSH; this happens with
-	 * option_anchor_outputs, and finalizing is trivial. */
+	 * option_anchor_outputs, and finalizing those two cases is trivial. */
 	/* FIXME: miniscript! miniscript! miniscript! */
 	for (size_t i = 0; i < psbt->num_inputs; i++) {
 		struct wally_psbt_input *input = &psbt->inputs[i];
 		struct wally_tx_witness_stack *stack;
 		const struct wally_map_item *iws;
 
 		iws = wally_map_get_integer(&input->psbt_fields, /* PSBT_IN_WITNESS_SCRIPT */ 0x05);
-		if (!iws || !is_to_remote_anchored_witness_script(iws->value,
-								  iws->value_len))
+		if (!iws)
 			continue;
 
+		if (!is_to_remote_anchored_witness_script(iws->value,
+							  iws->value_len)
+		    && !is_anchor_witness_script(iws->value,
+						 iws->value_len)) {
+			continue;
+		}
+
 		if (input->signatures.num_items != 1)
 			continue;
 
@@ -615,6 +621,19 @@ bool psbt_finalize(struct wally_psbt *psbt)
 		 *
 		 *    <remote_sig>
 		 */
+		/* BOLT #3:
+		 * #### `to_local_anchor` and `to_remote_anchor` Output (option_anchors)
+		 *...
+		 *    <local_funding_pubkey/remote_funding_pubkey> OP_CHECKSIG OP_IFDUP
+		 *    OP_NOTIF
+		 *        OP_16 OP_CHECKSEQUENCEVERIFY
+		 *    OP_ENDIF
+		 *...
+		 * Spending of the output requires the following witness:
+		 *     <local_sig/remote_sig>
+		 */
+
+		/* i.e. in both cases, this is the same thing */
 		wally_tx_witness_stack_init_alloc(2, &stack);
 		wally_tx_witness_stack_add(stack,
 					   input->signatures.items[0].value,
@@ -623,6 +642,7 @@ bool psbt_finalize(struct wally_psbt *psbt)
 					   iws->value,
 					   iws->value_len);
 		wally_psbt_input_set_final_witness(input, stack);
+		wally_tx_witness_stack_free(stack);
 	}
 
 	ok = (wally_psbt_finalize(psbt, 0 /* flags */) == WALLY_OK);

bitcoin/script.c

@@ -874,9 +874,31 @@ u8 *bitcoin_wscript_anchor(const tal_t *ctx,
 	add_op(&script, OP_CHECKSEQUENCEVERIFY);
 	add_op(&script, OP_ENDIF);
 
+	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 + 1 + 1 + 1)
+		return false;
+	if (script[0] != OP_PUSHBYTES(33))
+		return false;
+	if (script[34] != OP_CHECKSIG)
+		return false;
+	if (script[35] != OP_IFDUP)
+		return false;
+	if (script[36] != OP_NOTIF)
+		return false;
+	if (script[37] != 0x50 + 16)
+		return false;
+	if (script[38] != OP_CHECKSEQUENCEVERIFY)
+		return false;
+	if (script[39] != OP_ENDIF)
+		return false;
+	return true;
+}
+
 bool scripteq(const u8 *s1, const u8 *s2)
 {
 	memcheck(s1, tal_count(s1));

bitcoin/script.h

@@ -159,6 +159,9 @@ bool is_known_scripttype(const u8 *script);
 /* Is this a to-remote witness script (used for option_anchor_outputs)? */
 bool is_to_remote_anchored_witness_script(const u8 *script, size_t script_len);
 
+/* 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

@@ -69,6 +69,9 @@ u8 fromwire_u8(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 /* Generated stub for fromwire_u8_array */
 void fromwire_u8_array(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, u8 *arr UNNEEDED, size_t num UNNEEDED)
 { fprintf(stderr, "fromwire_u8_array 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_to_remote_anchored_witness_script */
 bool is_to_remote_anchored_witness_script(const u8 *script UNNEEDED, size_t script_len UNNEEDED)
 { fprintf(stderr, "is_to_remote_anchored_witness_script called!\n"); abort(); }

bitcoin/test/run-psbt-from-tx.c

@@ -50,6 +50,9 @@ struct amount_sat amount_tx_fee(u32 fee_per_kw UNNEEDED, size_t weight UNNEEDED)
 void fromwire_sha256_double(const u8 **cursor UNNEEDED, size_t *max UNNEEDED,
 			    struct sha256_double *sha256d UNNEEDED)
 { fprintf(stderr, "fromwire_sha256_double 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_to_remote_anchored_witness_script */
 bool is_to_remote_anchored_witness_script(const u8 *script UNNEEDED, size_t script_len UNNEEDED)
 { fprintf(stderr, "is_to_remote_anchored_witness_script called!\n"); abort(); }

bitcoin/test/run-tx-encode.c

@@ -70,6 +70,9 @@ u8 fromwire_u8(const u8 **cursor UNNEEDED, size_t *max UNNEEDED)
 /* Generated stub for fromwire_u8_array */
 void fromwire_u8_array(const u8 **cursor UNNEEDED, size_t *max UNNEEDED, u8 *arr UNNEEDED, size_t num UNNEEDED)
 { fprintf(stderr, "fromwire_u8_array 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_to_remote_anchored_witness_script */
 bool is_to_remote_anchored_witness_script(const u8 *script UNNEEDED, size_t script_len UNNEEDED)
 { fprintf(stderr, "is_to_remote_anchored_witness_script called!\n"); abort(); }