json: Add two param parsers for secrets and hex-encoded binary data

These are useful for the `createonion` JSON-RPC we're going to build next. The
secret is used for the optional `session_key` while the hex-encoded binary is
used for the `assocdata` field to which the onion commits. The latter does not
have a constant size, hence the raw binary conversion.

common/json_tok.c

@@ -228,3 +228,31 @@ struct command_result *param_channel_id(struct command *cmd, const char *name,
 			    name, json_tok_full_len(tok),
 			    json_tok_full(buffer, tok));
 }
+
+struct command_result *param_secret(struct command *cmd, const char *name,
+				    const char *buffer, const jsmntok_t *tok,
+				    struct secret **secret)
+{
+	*secret = tal(cmd, struct secret);
+	if (hex_decode(buffer + tok->start,
+		       tok->end - tok->start,
+		       *secret, sizeof(**secret)))
+		return NULL;
+
+	return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+			    "'%s' should be a 32 byte hex value, not '%.*s'",
+			    name, tok->end - tok->start, buffer + tok->start);
+}
+
+struct command_result *param_bin_from_hex(struct command *cmd, const char *name,
+				    const char *buffer, const jsmntok_t *tok,
+				    u8 **bin)
+{
+	*bin = json_tok_bin_from_hex(cmd, buffer, tok);
+	if (bin != NULL)
+		return NULL;
+	else
+		return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+				    "'%s' should be a hex value, not '%.*s'",
+				    name, tok->end - tok->start, buffer + tok->start);
+}

common/json_tok.h

@@ -111,4 +111,14 @@ struct command_result *param_ignore(struct command *cmd, const char *name,
 				    const char *buffer, const jsmntok_t *tok,
 				    const void *unused);
 
+/* Extract a secret from this string */
+struct command_result *param_secret(struct command *cmd, const char *name,
+				    const char *buffer, const jsmntok_t *tok,
+				    struct secret **secret);
+
+/* Extract a binary value from the param and unhexlify it. */
+struct command_result *param_bin_from_hex(struct command *cmd, const char *name,
+					  const char *buffer, const jsmntok_t *tok,
+					  u8 **bin);
+
 #endif /* LIGHTNING_COMMON_JSON_TOK_H */

lightningd/pay.c

@@ -842,23 +842,6 @@ static struct command_result *param_route_hop_style(struct command *cmd,
 			    json_tok_full(buffer, tok));
 }
 
-static struct command_result *param_secret(struct command *cmd,
-					   const char *name,
-					   const char *buffer,
-					   const jsmntok_t *tok,
-					   struct secret **secret)
-{
-	*secret = tal(cmd, struct secret);
-	if (hex_decode(buffer + tok->start,
-		       tok->end - tok->start,
-		       *secret, sizeof(**secret)))
-		return NULL;
-
-	return command_fail(cmd, JSONRPC2_INVALID_PARAMS,
-			    "'%s' should be a 32 byte hex value, not '%.*s'",
-			    name, tok->end - tok->start, buffer + tok->start);
-}
-
 static struct command_result *json_sendpay(struct command *cmd,
 					   const char *buffer,
 					   const jsmntok_t *obj UNNEEDED,