--- Deep copies a table into a new table.
-- Tables used as keys are also deep copied, as are metatables
-- @param orig The table to copy
-- @return Returns a copy of the input table
local function deep_copy(orig)
  local copy
  if type(orig) == "table" then
    copy = {}
    for orig_key, orig_value in next, orig, nil do
      copy[deep_copy(orig_key)] = deep_copy(orig_value)
    end
    setmetatable(copy, deep_copy(getmetatable(orig)))
  else
    copy = orig
  end
  return copy
end

--- Copies a table into a new table.
-- neither sub tables nor metatables will be copied.
-- @param orig The table to copy
-- @return Returns a copy of the input table
local function shallow_copy(orig)
  local copy
  if type(orig) == "table" then
    copy = {}
    for orig_key, orig_value in pairs(orig) do
      copy[orig_key] = orig_value
    end
  else -- number, string, boolean, etc
    copy = orig
  end
  return copy
end

例子如下：

local a = { aa = 1, bb = 2, cc = { dd = { ee = 3 } } }
--local b = deep_copy(a)
local b = shallow_copy(a)
b.cc.dd.ee = 111
b.bb = 111
ngx.say(cjson.encode(a))
ngx.say(cjson.encode(b))

结果：{"aa":1,"bb":2,"cc":{"dd":{"ee":111}}}
{"aa":1,"bb":111,"cc":{"dd":{"ee":111}}}

• 浅拷贝修改拷贝的某个键对应的值并不影响原始的表的键对应值（只能作用于第一层，如果多层嵌套就会导致原始表被修改）

local men = { mkey = 555 }
local a = { aa = 1, bb = 2, cc = { dd = { ee = { ff = 3 } } } }
setmetatable(a, { __index = men })
local b = deep_copy(a)

ngx.say(a.cc.dd.ee.ff)
ngx.say(a.mkey)
ngx.say(b.mkey)

3
555
555

• 这个深拷贝可以同时复制原始表的元表。如果不许要可以将setmetatable(copy, deep_copy(getmetatable(orig)))去掉。