json: add json_add_txid.

I prefer the typesafety of specific functions, rather than having the
caller know that txids are traditionally reversed in bitcoin.

And we already have a bitcoin_txid_to_hex() function for this.

Closes: #411
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

lightningd/jsonrpc.c

@@ -358,6 +358,15 @@ void json_add_pubkey(struct json_result *response,
 	json_add_hex(response, fieldname, der, sizeof(der));
 }
 
+void json_add_txid(struct json_result *result, const char *fieldname,
+		   const struct sha256_double *txid)
+{
+	char hex[hex_str_size(sizeof(*txid))];
+
+	bitcoin_txid_to_hex(txid, hex, sizeof(hex));
+	json_add_string(result, fieldname, hex);
+}
+
 bool json_tok_pubkey(const char *buffer, const jsmntok_t *tok,
 		     struct pubkey *pubkey)
 {

lightningd/jsonrpc.h

@@ -5,6 +5,7 @@
 #include <ccan/list/list.h>
 #include <common/json.h>
 
+struct sha256_double;
 struct wireaddr;
 
 /* Context for a command (from JSON, but might outlive the connection!)
@@ -66,6 +67,10 @@ void json_add_pubkey(struct json_result *response,
 		     const char *fieldname,
 		     const struct pubkey *key);
 
+/* '"fieldname" : <hexrev>' or "<hexrev>" if fieldname is NULL */
+void json_add_txid(struct json_result *result, const char *fieldname,
+		   const struct sha256_double *txid);
+
 /* Extract a pubkey from this */
 bool json_tok_pubkey(const char *buffer, const jsmntok_t *tok,
 		     struct pubkey *pubkey);

wallet/walletrpc.c

@@ -456,8 +456,7 @@ static void json_listfunds(struct command *cmd, const char *buffer,
 	json_array_start(response, "outputs");
 	for (int i = 0; i < tal_count(utxos); i++) {
 		json_object_start(response, NULL);
-		json_add_hex_reversed(response, "txid", &utxos[i]->txid,
-			     sizeof(struct sha256_double));
+		json_add_txid(response, "txid", &utxos[i]->txid);
 		json_add_num(response, "output", utxos[i]->outnum);
 		json_add_u64(response, "value", utxos[i]->amount);
 		json_object_end(response);