Better handling of system constants in the file-log plugin

.luacheckrc

@@ -1,6 +1,6 @@
 redefined = false
 unused_args = false
-globals = {"ngx", "dao", "app", "configuration"}
+globals = {"ngx", "dao", "app", "configuration", "process_id"}
 
 files["kong/"] = {
   std = "luajit"

kong-0.5.4-1.rockspec

@@ -15,6 +15,7 @@ dependencies = {
   "luasec ~> 0.5-2",
 
   "lua_uuid ~> 0.1-8",
+  "lua_system_constants ~> 0.1-2",
   "luatz ~> 0.3-1",
   "yaml ~> 1.1.2-1",
   "lapis ~> 1.3.1-1",

kong/plugins/file-log/log.lua

@@ -1,25 +1,17 @@
 -- Copyright (C) Mashape, Inc.
 
 local ffi = require "ffi"
-local bit = require "bit"
 local cjson = require "cjson"
 local fd_util = require "kong.plugins.file-log.fd_util"
+local system_constants = require "lua_system_constants"
 local basic_serializer = require "kong.plugins.log-serializers.basic"
 
 ffi.cdef[[
 int open(char * filename, int flags, int mode);
 int write(int fd, void * ptr, int numbytes);
-]]
-
-local octal = function(n) return tonumber(n, 8) end
 
-local O_CREAT = octal('0100')
-local O_APPEND = octal('02000')
-local O_WRONLY = octal('0001')
-
-local S_IWUSR = octal('00200')
-local S_IRUSR = octal('00400')
-local S_IXUSR = octal('00100')
+char *strerror(int errnum);
+]]
 
 local function string_to_char(str)
   return ffi.cast("uint8_t*", str)
@@ -34,8 +26,15 @@ local function log(premature, conf, message)
 
   local fd = fd_util.get_fd(conf.path)
   if not fd then
-    fd = ffi.C.open(string_to_char(conf.path), bit.bor(O_CREAT, O_APPEND, O_WRONLY), bit.bor(S_IWUSR, S_IRUSR, S_IXUSR))
-    fd_util.set_fd(conf.path, fd)
+    fd = ffi.C.open(string_to_char(conf.path), 
+                    bit.bor(system_constants.O_WRONLY(), system_constants.O_CREAT(), system_constants.O_APPEND()), 
+                    bit.bor(system_constants.S_IWUSR(), system_constants.S_IRUSR(), system_constants.S_IXUSR()))
+    if fd < 0 then
+      local errno = ffi.errno()
+      ngx.log(ngx.ERR, "[file-log] failed to open the file: ", ffi.string(ffi.C.strerror(errno)))
+    else
+      fd_util.set_fd(conf.path, fd)
+    end
   end
 
   ffi.C.write(fd, string_to_char(message), string.len(message))