rpc: add check command

The check command allows us to check the parameters of a command
without running it. Example:

	lightning-cli check invoice 234 foo desc

We do this by removing the "command_to_check" parameter and then using the
remaining parameters as-is.

I chose the parameter name "command_to_check" instead of just "command" because
it must be unique to all other parameter names for all other commands. Why?
Because it may be ambiguous in the case of a json object, where the parameters are
not necessary ordered.  We don't know which one is the command to check and
which one is a parameter.

Signed-off-by: Mark Beckwith <wythe@intrig.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 
+- JSON API: New command `check` checks the validity of a JSON API call without running it.
 - JSON API: `getinfo` now returns `num_peers` `num_pending_channels`,
   `num_active_channels` and `num_inactive_channels` fields.
 - JSON API: use `\n\n` to terminate responses, for simplified parsing (pylightning now relies on this)

lightningd/jsonrpc.c

@@ -937,3 +937,71 @@ bool json_tok_wtx(struct wallet_tx * tx, const char * buffer,
 	}
         return true;
 }
+
+static bool json_tok_command(struct command *cmd, const char *name,
+			     const char *buffer, const jsmntok_t *tok,
+			     const jsmntok_t **out)
+{
+	cmd->json_cmd = find_cmd(cmd->jcon->ld->jsonrpc, buffer, tok);
+	if (cmd->json_cmd)
+		return (*out = tok);
+
+	command_fail(cmd, JSONRPC2_INVALID_PARAMS,
+		     "'%s' of '%.*s' is invalid",
+		     name, tok->end - tok->start, buffer + tok->start);
+	return false;
+}
+
+static void json_check(struct command *cmd,
+		       const char *buffer, const jsmntok_t *params)
+{
+	jsmntok_t *mod_params;
+	const jsmntok_t *name_tok;
+	bool ok;
+	struct json_stream *response;
+
+	if (cmd->mode == CMD_USAGE) {
+		mod_params = NULL;
+	} else {
+		mod_params = json_tok_copy(cmd, params);
+		cmd->allow_unused = true;
+	}
+
+	if (!param(cmd, buffer, mod_params,
+		   p_req("command_to_check", json_tok_command, &name_tok),
+		   NULL))
+		return;
+
+	/* Point name_tok to the name, not the value */
+	if (params->type == JSMN_OBJECT)
+		name_tok--;
+
+	json_tok_remove(&mod_params, (jsmntok_t *)name_tok, 1);
+
+	cmd->mode = CMD_CHECK;
+	cmd->allow_unused = false;
+	/* FIXME(wythe): Maybe change "ok" to "failed" since that's really what
+	 * we're after and would be more clear. */
+	ok = true;
+	cmd->ok = &ok;
+	cmd->json_cmd->dispatch(cmd, buffer, mod_params);
+
+	if (!ok)
+		return;
+
+	response = json_stream_success(cmd);
+	json_object_start(response, NULL);
+	json_add_string(response, "command", cmd->json_cmd->name);
+	json_add_string(response, "parameters", "ok");
+	json_object_end(response);
+	command_success(cmd, response);
+}
+
+static const struct json_command check_command = {
+	"check",
+	json_check,
+	"Don't run {command_to_check}, just verify parameters.",
+	.verbose = "check command_to_check [parameters...]\n"
+};
+
+AUTODATA(json_command, &check_command);

lightningd/jsonrpc.h

@@ -13,7 +13,9 @@ enum command_mode {
 	/* Normal command processing */
 	CMD_NORMAL,
 	/* Create command usage string, nothing else. */
-	CMD_USAGE
+	CMD_USAGE,
+	/* Check parameters, nothing else. */
+	CMD_CHECK
 };
 
 /* Context for a command (from JSON, but might outlive the connection!). */

lightningd/param.c

@@ -283,5 +283,9 @@ bool param(struct command *cmd, const char *buffer,
 		return false;
 	}
 
-	return param_arr(cmd, buffer, tokens, params);
+	/* Always return false for CMD_USAGE and CMD_CHECK, signaling the caller
+	 * to return immediately.  For CMD_NORMAL, return true if all parameters
+	 * are valid.
+	 */
+	return param_arr(cmd, buffer, tokens, params) && cmd->mode == CMD_NORMAL;
 }