wire: add fromwire_tal_arrn() helper.

Does the allocation and copying; this is useful because we can
avoid being fooled into doing giant allocations.

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

common/onionreply.c

@@ -12,8 +12,8 @@ struct onionreply *fromwire_onionreply(const tal_t *ctx,
 				       const u8 **cursor, size_t *max)
 {
 	struct onionreply *r = tal(ctx, struct onionreply);
-	r->contents = tal_arr(r, u8, fromwire_u16(cursor, max));
-	fromwire_u8_array(cursor, max, r->contents, tal_count(r->contents));
+	r->contents = fromwire_tal_arrn(r, cursor, max,
+					fromwire_u16(cursor, max));
 	if (!*cursor)
 		return tal_free(r);
 	return r;

devtools/print_wire.c

@@ -104,10 +104,9 @@ static void printwire_addresses(const u8 **cursor, size_t *plen, size_t len)
 static void printwire_encoded_short_ids(const u8 **cursor, size_t *plen, size_t len)
 {
 	struct short_channel_id *scids;
-	u8 *arr = tal_arr(tmpctx, u8, len);
+	u8 *arr = fromwire_tal_arrn(tmpctx, cursor, plen, len);
 
-	fromwire_u8_array(cursor, plen, arr, len);
-	if (!*cursor)
+	if (!arr)
 		return;
 
 	printf("[");

lightningd/gossip_msg.c

@@ -21,8 +21,7 @@ struct gossip_getnodes_entry *fromwire_gossip_getnodes_entry(const tal_t *ctx,
 	}
 
 	flen = fromwire_u16(pptr, max);
-	entry->features = tal_arr(entry, u8, flen);
-	fromwire_u8_array(pptr, max, entry->features, flen);
+	entry->features = fromwire_tal_arrn(entry, pptr, max, flen);
 
 	numaddresses = fromwire_u8(pptr, max);
 

wire/fromwire.c

@@ -308,6 +308,19 @@ void fromwire_u8_array(const u8 **cursor, size_t *max, u8 *arr, size_t num)
 	fromwire(cursor, max, arr, num);
 }
 
+u8 *fromwire_tal_arrn(const tal_t *ctx,
+		      const u8 **cursor, size_t *max, size_t num)
+{
+	u8 *arr;
+	if (num > *max) {
+		fromwire_fail(cursor, max);
+		return NULL;
+	}
+	arr = tal_arr(ctx, u8, num);
+	fromwire_u8_array(cursor, max, arr, num);
+	return arr;
+}
+
 void fromwire_pad(const u8 **cursor, size_t *max, size_t num)
 {
 	fromwire(cursor, max, NULL, num);
@@ -393,20 +406,19 @@ struct bitcoin_tx_output *fromwire_bitcoin_tx_output(const tal_t *ctx,
 	struct bitcoin_tx_output *output = tal(ctx, struct bitcoin_tx_output);
 	output->amount = fromwire_amount_sat(cursor, max);
 	u16 script_len = fromwire_u16(cursor, max);
-	output->script = tal_arr(output, u8, script_len);
-	fromwire_u8_array(cursor, max, output->script, script_len);
+	output->script = fromwire_tal_arrn(output, cursor, max, script_len);
+	if (!*cursor)
+		return tal_free(output);
 	return output;
 }
 
 struct witscript *fromwire_witscript(const tal_t *ctx, const u8 **cursor, size_t *max)
 {
-	struct witscript *retval;
+	struct witscript *retval = tal(ctx, struct witscript);
 	u16 len = fromwire_u16(cursor, max);
-	if (!len)
-		return NULL;
-	retval = tal(ctx, struct witscript);
-	retval->ptr = tal_arr(retval, u8, len);
-	fromwire_u8_array(cursor, max, retval->ptr, len);
+	retval->ptr = fromwire_tal_arrn(retval, cursor, max, len);
+	if (!*cursor)
+		return tal_free(retval);
 	return retval;
 }
 

wire/wire.h

@@ -136,6 +136,8 @@ struct amount_sat fromwire_amount_sat(const u8 **cursor, size_t *max);
 void fromwire_pad(const u8 **cursor, size_t *max, size_t num);
 
 void fromwire_u8_array(const u8 **cursor, size_t *max, u8 *arr, size_t num);
+u8 *fromwire_tal_arrn(const tal_t *ctx,
+		       const u8 **cursor, size_t *max, size_t num);
 char *fromwire_wirestring(const tal_t *ctx, const u8 **cursor, size_t *max);
 struct bitcoin_tx *fromwire_bitcoin_tx(const tal_t *ctx,
 				       const u8 **cursor, size_t *max);