fix(lua): fix vim.deepcopy for metatables & cycled tables (#16435)

vim.deepcopy previously didn't retain metatables in copies
and caused stackoverflow on recursive tables/cycled tables this
fixes these issues

2 files changed, 23 insertions, 10 deletions

diff --git a/runtime/lua/vim/shared.lua b/runtime/lua/vim/shared.lua
index 6e40b6ca52..bf77f0c776 100644
--- a/runtime/lua/vim/shared.lua
+++ b/runtime/lua/vim/shared.lua
@@ -12,7 +12,7 @@ local vim = vim or {}
 --- same functions as those in the input table. Userdata and threads are not
 --- copied and will throw an error.
 ---
----@param orig Table to copy
+---@param orig table Table to copy
 ---@returns New table of copied keys and (nested) values.
 function vim.deepcopy(orig) end  -- luacheck: no unused
 vim.deepcopy = (function()
@@ -21,17 +21,16 @@ vim.deepcopy = (function()
   end
 
   local deepcopy_funcs = {
-    table = function(orig)
+    table = function(orig, cache)
+      if cache[orig] then return cache[orig] end
       local copy = {}
 
-      if vim._empty_dict_mt ~= nil and getmetatable(orig) == vim._empty_dict_mt then
-        copy = vim.empty_dict()
-      end
-
+      cache[orig] = copy
+      local mt = getmetatable(orig)
       for k, v in pairs(orig) do
-        copy[vim.deepcopy(k)] = vim.deepcopy(v)
+        copy[vim.deepcopy(k, cache)] = vim.deepcopy(v, cache)
       end
-      return copy
+      return setmetatable(copy, mt)
     end,
     number = _id,
     string = _id,
@@ -40,10 +39,10 @@ vim.deepcopy = (function()
     ['function'] = _id,
   }
 
-  return function(orig)
+  return function(orig, cache)
     local f = deepcopy_funcs[type(orig)]
     if f then
-      return f(orig)
+      return f(orig, cache or {})
     else
       error("Cannot deepcopy object of type "..type(orig))
     end
diff --git a/test/functional/lua/vim_spec.lua b/test/functional/lua/vim_spec.lua
index 3832d27a22..28471bdd46 100644
--- a/test/functional/lua/vim_spec.lua
+++ b/test/functional/lua/vim_spec.lua
@@ -396,6 +396,20 @@ describe('lua stdlib', function()
       return t1.f() ~= t2.f()
     ]]))
 
+    ok(exec_lua([[
+      local t1 = {a = 5}
+      t1.self = t1
+      local t2 = vim.deepcopy(t1)
+      return t2.self == t2 and t2.self ~= t1
+    ]]))
+
+    ok(exec_lua([[
+      local mt = {mt=true}
+      local t1 = setmetatable({a = 5}, mt)
+      local t2 = vim.deepcopy(t1)
+      return getmetatable(t2) == mt
+    ]]))
+
     eq('Error executing lua: vim/shared.lua:0: Cannot deepcopy object of type thread',
       pcall_err(exec_lua, [[
         local thread = coroutine.create(function () return 0 end)