autosave.lua

--[[ This script automatically saves the global environment periodically
     and restores it when this script is run. Recommended usage is with
     the standalone Lua interpreter like this:

        lua -i autosave.lua

     It was coded for and tested with Eris (https://github.com/fnuecke/eris).

     This approach is quite limited and could be improved in a number of ways:
     - Userdata cannot be persisted automatically, so if, for example, some
       files are open the session cannot be saved / restored.
     - There's the (unlikely) possibility of the script trying to save while
       already doing so, if the save takes longer than `autosaveInterval`
       and the garbage collector is triggered in the save logic.
     - I could not figure out a way to check whether Lua is shutting down in
       the autosave() function. It gets called when Lua closes, but it may
       decide not to save if the last save was earlier than the set interval.
]]

-- [[ Config ]]

-- The name of the file to persist the global state to.
local stateFile = ".lua.eris"

-- The interval in seconds in which to save the global state. Avoids
-- excessive saves if the garbage collector hyperventilates.
local autosaveInterval = 10

-- [[ General persistence bootstrapping ]]

-- Stuff we cannot persist directly. Basically all of the built-in
-- C functions and userdata. We need a mapping to and from 'permanent'
-- values, that act as placeholders for these values.
local perms, uperms = {}, {}
do
  local processed = {} -- list of seen tables, avoids cycles.
  local function flattenAndStore(path, t)
    -- Get the list of keys so we can sort it. This is needed because
    -- permanent values have to be unique, so we need to pick a name for
    -- them in a deterministic fashion (which pairs isn't).
    local keys = {}
    for k, v in pairs(t) do
      if type(k) ~= "string" and type(k) ~= "number" then
        io.stderr:write("Cannot generate permanent value for global with non-string key at " .. path .. ". Things may not go well.\n")
      else
        table.insert(keys, k)
      end
    end
    table.sort(keys)
    for _, k in ipairs(keys) do
      local name = path .. "." .. k
      local v = t[k]
      -- This avoids duplicate permanent value entries, see above.
      if perms[v] == nil then
        local vt = type(v)
        if "function" == vt or "userdata" == vt then
          perms[v] = name
          uperms[name] = v
        elseif "table" == vt and not processed[v] then
          processed[v] = true
          flattenAndStore(name, v)
        end
      end
    end
  end
  flattenAndStore("_G", _G)
end

-- [[ Autosave background task ]]

local lastSave = os.time()
local function autosave()
  if os.difftime(os.time(), lastSave) > autosaveInterval then
    lastSave = os.time()
    local data, reason = eris.persist(perms, _ENV)
    if not data then
      io.stderr:write("Failed to persist environment: " .. tostring(reason) .. "\n")
    else
      local f, reason = io.open(stateFile, "wb")
      if not f then
        io.stderr:write("Failed to save environment: " .. tostring(reason) .. "\n")
      else
        f:write(data)
        f:close()
      end
    end
  end
  -- Check again later. This basically uses the garbage collector as a
  -- scheduler (this method is called when the table is collected). It
  -- is a concept I first saw posted by Roberto on the list a while
  -- back, in the context of sandboxing I think.
  setmetatable({}, {__gc=autosave})
end
autosave()

-- [[ Startup / load autosave ]]

local f = io.open(stateFile, "rb")
if f then
  -- Got a previous state, try to restore it.
  local data = f:read("*a")
  f:close()
  local env, reason = eris.unpersist(uperms, data)
  if not env then
    io.stderr:write("Failed to unpersist environment: " .. tostring(reason) .. "\n")
  else
    for k, v in pairs(env) do
      _ENV[k] = env[k]
      setmetatable(_ENV, getmetatable(env))
    end
  end
end