Validate rule outputs when loading rules.

Validate rule outputs when loading rules by attempting to create a
formatter based on the rule's output field. If there's an error, it will
propagate up through load_rules and cause falco to exit rather than
discover the problem only when trying to format the event and the rule's
output field.

This required moving formats.{cpp,h} into the falco engine directory
from the falco general directory. Note that these functions are loaded
twice in the two lua states used by falco (engine and outputs).

There's also a couple of minor cleanups:

 - falco_formats had a private instance variable that was unused, remove
   it.
 - rename the package for the falco_formats functions to formats instead
   of falco so it's more standalone.
 - don't throw a c++ exception in falco_formats::formatter. Instead
   generate a lua error, which is handled more cleanly.
 - free_formatter doesn't return any values, so set the return value of
   the function to 0.

userspace/engine/formats.cpp

@@ -0,0 +1,130 @@
+/*
+Copyright (C) 2016 Draios inc.
+
+This file is part of falco.
+
+falco is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License version 2 as
+published by the Free Software Foundation.
+
+falco is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with falco.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#include <json/json.h>
+
+#include "formats.h"
+#include "logger.h"
+#include "falco_engine.h"
+
+
+sinsp* falco_formats::s_inspector = NULL;
+bool s_json_output = false;
+
+const static struct luaL_reg ll_falco [] =
+{
+	{"formatter", &falco_formats::formatter},
+	{"free_formatter", &falco_formats::free_formatter},
+	{"format_event", &falco_formats::format_event},
+	{NULL,NULL}
+};
+
+void falco_formats::init(sinsp* inspector, lua_State *ls, bool json_output)
+{
+	s_inspector = inspector;
+	s_json_output = json_output;
+
+	luaL_openlib(ls, "formats", ll_falco, 0);
+}
+
+int falco_formats::formatter(lua_State *ls)
+{
+	string format = luaL_checkstring(ls, 1);
+	sinsp_evt_formatter* formatter;
+	try
+	{
+		formatter = new sinsp_evt_formatter(s_inspector, format);
+	}
+	catch(sinsp_exception& e)
+	{
+		luaL_error(ls, "Invalid output format '%s': '%s'", format.c_str(), e.what());
+	}
+
+	lua_pushlightuserdata(ls, formatter);
+
+	return 1;
+}
+
+int falco_formats::free_formatter(lua_State *ls)
+{
+	if (!lua_islightuserdata(ls, -1))
+	{
+		luaL_error(ls, "Invalid argument passed to free_formatter");
+	}
+
+	sinsp_evt_formatter *formatter = (sinsp_evt_formatter *) lua_topointer(ls, 1);
+
+	delete(formatter);
+
+	return 0;
+}
+
+int falco_formats::format_event (lua_State *ls)
+{
+	string line;
+
+	if (!lua_islightuserdata(ls, -1) ||
+	    !lua_isstring(ls, -2) ||
+	    !lua_isstring(ls, -3) ||
+	    !lua_islightuserdata(ls, -4)) {
+		throw falco_exception("Invalid arguments passed to format_event()\n");
+	}
+	sinsp_evt* evt = (sinsp_evt*)lua_topointer(ls, 1);
+	const char *rule = (char *) lua_tostring(ls, 2);
+	const char *level = (char *) lua_tostring(ls, 3);
+	sinsp_evt_formatter* formatter = (sinsp_evt_formatter*)lua_topointer(ls, 4);
+
+	formatter->tostring(evt, &line);
+
+	// For JSON output, the formatter returned just the output
+	// string containing the format text and values. Use this to
+	// build a more detailed object containing the event time,
+	// rule, severity, full output, and fields.
+	if (s_json_output) {
+		Json::Value event;
+		Json::FastWriter writer;
+
+		// Convert the time-as-nanoseconds to a more json-friendly ISO8601.
+		time_t evttime = evt->get_ts()/1000000000;
+		char time_sec[20]; // sizeof "YYYY-MM-DDTHH:MM:SS"
+		char time_ns[12]; // sizeof ".sssssssssZ"
+		string iso8601evttime;
+
+		strftime(time_sec, sizeof(time_sec), "%FT%T", gmtime(&evttime));
+		snprintf(time_ns, sizeof(time_ns), ".%09luZ", evt->get_ts() % 1000000000);
+		iso8601evttime = time_sec;
+		iso8601evttime += time_ns;
+		event["time"] = iso8601evttime;
+		event["rule"] = rule;
+		event["priority"] = level;
+		event["output"] = line;
+
+		line = writer.write(event);
+
+		// Json::FastWriter may add a trailing newline. If it
+		// does, remove it.
+		if (line[line.length()-1] == '\n')
+		{
+			line.resize(line.length()-1);
+		}
+	}
+
+	lua_pushstring(ls, line.c_str());
+	return 1;
+}
+