Lua 5.2/5.3 hotfix. Hot update functions and keep old data.

If we have a test.lua

local M = {}

M.count = 0

function M.func()
    M.count = M.count + 1
    return "v1"
end

return M

Require test and call func(), then count will be 1.

> test = require("test")                              
> test.func()                                         
v1                                                    
> test.count                                          
1                                                     

Change "v1" to "v2" in test.lua, then hotfix module test and call func() again. The result shows that func() has been updated, but the count is kept.

> hotfix = require("hotfix.hotfix")                          
> hotfix.hotfix_module("test")                        
table: 0000000002752060                               
> test.func()                                         
v2                                                    
> test.count                                          
2                                                     

Using LuaRocks:

luarocks install hotfix

Or manually copy lua/hotfix directory into your Lua module path.

local hotfix = require("hotfix.hotfix")
hotfix.hotfix_module("mymodule.sub_module")

helper/hotfix_helper.lua is an example to hotfix modified modules using lfs. Please see helper/README.md.

hotfix_module() uses package.searchpath(module_name, package.path) to search the path of module. The module is reloaded and the returned value is updated to package.loaded[module_name]. If the returned value is nil, then package.loaded[module_name] is assigned to true. hotfix_module() returns the final value of package.loaded[module_name].

hotfix_module() will skip unloaded module to avoid unexpected loading, and also to work around the issue of "Three dots module name will be nil".

Functons are updated to new ones but old upvalues are kept. Old tables are kept and new fields are inserted. All references to old functions are replaced to new ones.

The module may change any global variables if it wants to. See "Why not protect the global variables" below.

Local variable which is not referenced by _G is not updated.

-- test.lua: return { function func() return "old" end }
local test = require("test")  -- referenced by _G.package.loaded["test"]
local func = test.func        -- is not upvalue nor is referenced by _G
-- test.lua: return { function func() return "new" end }
require("hotfix.hotfix").hotfix_module("test")
test.func()  -- "new"  
func()       -- "old"

We can protect the global variables on loading in some ways, but there are other problems.

• [1] uses a read only ENV to load.

    local env = {}
    setmetatable(env, { __index = _G })
    load(chunk, check_name, 't', env)

But it can not stop indirect write. Global variables may be changed. In the following example, t is OK but math.sin is changed.

Lua 5.3.2  Copyright (C) 1994-2015 Lua.org, PUC-Rio
> math.sin(123)
-0.45990349068959
> do
>> local _ENV = setmetatable({}, {__index = _G})
>> t = 123
>> math.sin = print
>> end
> t
nil
> math.sin(123)
123

• [2] uses a fake ENV to load and ignores all operations. In this case, we can not init new local variables.

local M = {}
+ local log = require("log")  -- Can not require!
function M.foo()
+    log.info("test")
end
return M

Another problem is the new function's _ENV is not the real ENV. Following test will fail because set_global() has a protected ENV.

log("New upvalue which is a function set global...")
run_test([[
        local M = {}
        function M.foo() return 12345 end
        return M
    ]],
    function() assert(nil == global_test) end,
    [[
        local M = {}
        local function set_global() global_test = 11111 end
        function M.foo()
            set_global()
        end
        return M
    ]],
    function()
        assert(nil == test.foo())
        assert(11111 == global_test)  -- FAIL!
        global_test = nil
    end)

Run main.lua in test dir. main.lua will write a test.lua file and hotfix it. main.lua will write log to log.txt.

D:\Jinq\Git\hotfix\test>..\..\..\tools\lua-5.3.2_Win64_bin\lua53
Lua 5.3.2  Copyright (C) 1994-2015 Lua.org, PUC-Rio
> require("main").run()
main.lua:80: assertion failed!

log function is changed from print to an empty function. The hotfix will replace all print to an empty function which is totally unexpected.

local M = {}
local log = print
function M.foo() log("Old") end
return M

local M = {}
local log = function() end
function M.foo() log("Old") end
return M

hotfix.add_protect{print} can protect print function from being replaced. But it also means that log can not be updated.

hotfix.lua:210: file.lua:1: unexpected symbol near '<\239>'

--- test.lua.
-- @module test
local module_name = ...
print(module_name)

require("test") will print "test", but hotfix which uses load() will print "nil".

• [1] hotfix by tickbh
  https://github.com/tickbh/td_rlua/blob/11523931b0dd271ad4c5e9c532a9d3bae252a264/td_rlua/src/hotfix.rs
  http://www.cnblogs.com/tickbh/articles/5459120.html (In Chinese)
  Lua 5.2/5.3.

  Can only update global functions.

local M = {}
+ function M.foo() end  -- Can not add M.foo().
return M

• [2] lua_hotupdate
  https://github.com/asqbtcupid/lua_hotupdate
  Lua 5.1.

  Using a fake ENV, the module's init statements result in noop.