PSBT: Fix compare to not mutate memory

PSBT changeset routines were using linearize_output which mutated the memory of the objects it was comparing.

This commit fixes that and also cleans up the memory usage to be more clear and more guarentee there is no memory corruption.

Changelog-None

Author: ddustin

common/psbt_open.c

@@ -61,14 +61,16 @@ static int compare_outputs_at(const struct output_set *a,
 }
 
 static const u8 *linearize_input(const tal_t *ctx,
-				 const struct wally_psbt_input *in)
+				 const struct wally_psbt *parent,
+				 size_t index)
 {
+	struct wally_psbt *copy = clone_psbt(NULL, parent);
 	struct wally_psbt *psbt = create_psbt(NULL, 1, 0, 0);
+	struct wally_psbt_input dummy_in;
 	size_t byte_len;
 
-	psbt->inputs[0] = *in;
-	psbt->num_inputs++;
-
+	dummy_in = psbt->inputs[0];
+	psbt->inputs[0] = copy->inputs[index];
 
 	/* Sort the inputs, so serializing them is ok */
 	wally_map_sort(&psbt->inputs[0].unknowns, 0);
@@ -88,25 +90,31 @@ static const u8 *linearize_input(const tal_t *ctx,
 
 	const u8 *bytes = psbt_get_bytes(ctx, psbt, &byte_len);
 
-	/* Hide the inputs we added, so it doesn't get freed */
-	psbt->num_inputs--;
+	psbt->inputs[0] = dummy_in;
+
 	tal_free(psbt);
+	tal_free(copy);
+
 	return bytes;
 }
 
 static const u8 *linearize_output(const tal_t *ctx,
-				  const struct wally_psbt_output *out)
+				  const struct wally_psbt *parent,
+				  size_t index)
 {
-	struct wally_psbt *psbt = create_psbt(NULL, 1, 1, 0);
+	struct wally_psbt *copy = clone_psbt(NULL, parent);
+	struct wally_psbt *psbt = create_psbt(NULL, 0, 1, 0);
+	struct wally_psbt_output dummy_out;
 	size_t byte_len;
 	struct bitcoin_outpoint outpoint;
 
 	/* Add a 'fake' non-zero input so libwally will agree to linearize the tx */
 	memset(&outpoint, 1, sizeof(outpoint));
 	psbt_append_input(psbt, &outpoint, 0, NULL, NULL, NULL);
 
-	psbt->outputs[0] = *out;
-	psbt->num_outputs++;
+	dummy_out = psbt->outputs[0];
+	psbt->outputs[0] = copy->outputs[index];
+
 	/* Sort the outputs, so serializing them is ok */
 	wally_map_sort(&psbt->outputs[0].unknowns, 0);
 
@@ -120,9 +128,10 @@ static const u8 *linearize_output(const tal_t *ctx,
 
 	const u8 *bytes = psbt_get_bytes(ctx, psbt, &byte_len);
 
-	/* Hide the outputs we added, so it doesn't get freed */
-	psbt->num_outputs--;
+	psbt->outputs[0] = dummy_out;
 	tal_free(psbt);
+	tal_free(copy);
+
 	return bytes;
 }
 
@@ -131,10 +140,8 @@ static bool input_identical(const struct wally_psbt *a,
 			    const struct wally_psbt *b,
 			    size_t b_index)
 {
-	const u8 *a_in = linearize_input(tmpctx,
-					 &a->inputs[a_index]);
-	const u8 *b_in = linearize_input(tmpctx,
-					 &b->inputs[b_index]);
+	const u8 *a_in = linearize_input(tmpctx, a, a_index);
+	const u8 *b_in = linearize_input(tmpctx, b, b_index);
 
 	return tal_arr_eq(a_in, b_in);
 }
@@ -144,10 +151,8 @@ static bool output_identical(const struct wally_psbt *a,
 			     const struct wally_psbt *b,
 			     size_t b_index)
 {
-	const u8 *a_out = linearize_output(tmpctx,
-					   &a->outputs[a_index]);
-	const u8 *b_out = linearize_output(tmpctx,
-					   &b->outputs[b_index]);
+	const u8 *a_out = linearize_output(tmpctx, a, a_index);
+	const u8 *b_out = linearize_output(tmpctx, b, b_index);
 	return tal_arr_eq(a_out, b_out);
 }