Deterministic LuaJIT 2.0.4
--------------------------
This modification of LuaJIT aims to ensure deterministic execution of lua scripts.
The motivation for a deterministic execution is that running a script multiple times
with the same inputs always yields the same results. This can be used for fault-
tolerance using state machine replication or, as intended here, to simplify debugging
by looking at the problem over and over again.
This requires recording and replaying all inputs that are observable by the program.
These inputs comprise of every input directly passed to the program, everything read
by the program, but also of indirect inputs like memory layout. Another way to
avoid recording too much is to hide inputs by introducing certain conventions that
replace an undefined behavior of an implementation by a deterministic one.
In LUA memory addresses can't be observed directly (ignoring the FFI-library),
however some information is leaked by tostring and pairs/next. The first returns
a string containing the objects memory address, whereas the latter may produce
a different ordering during iteration. This is caused by deriving the hash codes
from the memory address. The problem is solved by ensuring that the iteration
always proceeds in insertion order.
LuaJIT modifications:
Implemented:
- pairs/next -> iteration in insertion order
Todo:
- Prevent Memory address leaks (tostring, lua_tostring)
- "Deterministic" Garbage collection? (collectgarbage, lua_gc, weak tables)
- Input recording (either input data or result of function call)
-- luaL_loadfile
-- file:*, io.*
-- package.{loaders, loadlib, searchpath}
-- os.*
-- debug.debug
Possibly problematic (must be reviewed):
- Random numbers (ensure that math.randomseed input is recorded)
- Garbage collection with __gc metafunction
- Debugging hook (debug.sethook, lua_sethook, triggering is dependent on JIT activation)
- Environment variables (LUA_CPATH, LUA_INIT, LUA_PATH)
- ffi.*
- jit.*
- Own C functions (ensure determinism or remember results)
- External modules
LuaJIT is a Just-In-Time (JIT) compiler for the Lua programming language.
Project Homepage: http://luajit.org/
LuaJIT is Copyright (C) 2005-2016 Mike Pall.
LuaJIT is free software, released under the MIT license.
See full Copyright Notice in the COPYRIGHT file or in luajit.h.
Documentation for LuaJIT is available in HTML format.
Please point your favorite browser to:
doc/luajit.html