Lua 设置table为只读属性

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项目中部分只读表易被人误改写，故决定在非线上环境里对这些表附加只读属性，方便在出现误改写的时候抛出lua错误，最终版代码如下：

 --[[------------------------------------------------------------------------------
 -** 设置table只读 出现改写会抛出lua error
 -- 用法 local cfg_proxy = read_only(cfg)  retur cfg_proxy
 -- 增加了防重置设置read_only的机制
 -- lua5.3支持 1）table库支持调用元方法，所以table.remove table.insert 也会抛出错误，
 --               2）不用定义__ipairs 5.3 ipairs迭代器支持访问元方法__index，pairs迭代器next不支持故需要元方法__pairs
 -- 低版本lua此函数不能完全按照预期工作
 *]]
 function read_only(inputTable)
     local travelled_tables = {}
     local function __read_only(tbl)
         if not travelled_tables[tbl] then
             local tbl_mt = getmetatable(tbl)
             if not tbl_mt then
                 tbl_mt = {}
                 setmetatable(tbl, tbl_mt)
             end

             local proxy = tbl_mt.__read_only_proxy
             if not proxy then
                 proxy = {}
                 tbl_mt.__read_only_proxy = proxy
                 local proxy_mt = {
                     __index = tbl,
                     __newindex = function (t, k, v) error("error write to a read-only table with key = " .. tostring(k)) end,
                     __pairs = function (t) return pairs(tbl) end,
                     -- __ipairs = function (t) return ipairs(tbl) end,   5.3版本不需要此方法
                     __len = function (t) return #tbl end,
                     __read_only_proxy = proxy
                 }
                 setmetatable(proxy, proxy_mt)
             end
             travelled_tables[tbl] = proxy
             for k, v in pairs(tbl) do
                 if type(v) == "table" then
                     tbl[k] = __read_only(v)
                 end
             end
         end
         return travelled_tables[tbl]
     end
     return __read_only(inputTable)
 end

测试代码如下：

 local t0 = {k = }
 local t2 = {
     fdsf = {}
 }
 local t1 = {
         a = {, },
         b = {,ddss = , t2 = t2},
         d = ,
         e = "string",
 }
 t1.c=t1

 local t3 = read_only(t1)

 print(t3.d, t3.c.e, t3.c.c.b.t2.fdsf)
 function q1() t3.d =  end
 function q2() t3.c.d =  end
 function q3() t3.c.c.b.t2.fdsf = end
 function q4() table.remove(t3.a) end
 function q5() t3.b[ddss] = nil end
 function q6() t3[f] =  end
 function q7() table.insert(t3.a, ) end

 print(pcall(q1))
 print(pcall(q2))
 print(pcall(q3))
 print(pcall(q4))
 print(pcall(q5))
 print(pcall(q6))
 print(pcall(q7))
 print(t3.a[])
 for k,v in pairs(t3) do
     print("===pairs t3:",k,v)
 end
 for k,v in pairs(t3.a) do
     print("===pairs t3.a:",k,v)
 end
 for k,v in ipairs(t3) do
     print("===ipairs t3:",k,v)
 end
 for k,v in ipairs(t3.a) do
     print("===ipair t3.a",k,v)
 end
 print("len t3:",#t3)
 print("len t3.a:", #t3.a)

 local t4 = read_only(t2)

 local t5 = read_only(t0)
 local t6 = read_only(t0)

 print(t3.b.t2, read_only(t2))
 print(t5, t6, t0)

测试环境https://www.lua.org/cgi-bin/demo  lua5.3.4：

    string    table: 0x20d4ba0
false    input:: error write to a read-only table with key = d
false    input:: error write to a read-only table with key = d
false    input:: error write to a read-only table with key = fdsf
false    input:: error write to a read-only table with key =
false    input:: error write to a read-only table with key = nil
false    input:: error write to a read-only table with key = nil
false    input:: error write to a read-only table with key = 

===pairs t3:    e    string
===pairs t3:    b    table: 0x20ccd60
===pairs t3:    a    table: 0x20d4e70
===pairs t3:    d
===pairs t3:    c    table: 0x20ca700
===pairs t3.a:
===pairs t3.a:
===ipair t3.a
===ipair t3.a
len t3:
len t3.a:
table: 0x20d4870    table: 0x20d4870
table: 0x20d5690    table: 0x20d5690    table: 0x20d1140

代码思路设计：

1.使用proxy={}空表而不是目标表tbl来设置__newindex是因为__newindex必须在原表里面不存在才会调用，这样就依然可以对已存在的字段进行改写

__newindex: The indexing assignment table[key] = value. Like the index event, this event happens when table is not a table or when key is not present in table. The metamethod is looked up in table.

Like with indexing, the metamethod for this event can be either a function or a table. If it is a function, it is called with table, key, and value as arguments. If it is a table, Lua does an indexing assignment to this table with the same key and value. (This assignment is regular, not raw, and therefore can trigger another metamethod.)

Whenever there is a __newindex metamethod, Lua does not perform the primitive assignment. (If necessary, the metamethod itself can call rawset to do the assignment.)

2.避免出现table的互相引用，加入travelled_tables存储已经设置过proxy的table的映射

3.对于原表tbl的访问使用__index=tbl

4.对于表查长度使用__len= function () return #tbl end

5.对于遍历pairs，查到lua5.3的pairs默认迭代器next不支持访问元表__index，故直接__pairs = function () return pairs(tbl) end，以此来生成对目标表的迭代遍历

6.对于ipairs，查到lua5.3 ipairs函数生成的迭代器默认就支持访问元表__index，故不需要添加__ipairs

8.2 – Changes in the Libraries

• The ipairs iterator now respects metamethods and its __ipairs metamethod has been deprecated.

7.对于table.insert , table.remove不用特殊处理，lua5.3的table lib支持元表操作，故依然会抛错

8.2 – Changes in the Libraries

• The Table library now respects metamethods for setting and getting elements.

8.避免重复创建read_only，每个tbl只创建一个proxy代理，在tbl的metatable里和proxy的metatable里都设置属性__read_only_proxy,可以直接访问获得