Using vim.emtpy_dict or empty table in combination with deep_extend is breaking user configs

[asmodeus812]

Description

This is quite an obscure problem but, but here is the issue. We have the following static default_config map which is initialized when the util file is sources like that

M.default_config = {
  log_level = lsp.protocol.MessageType.Warning,
  message_level = lsp.protocol.MessageType.Warning,
  settings = vim.empty_dict(),
  init_options = vim.empty_dict(),
  handlers = {},
  autostart = true,
  capabilities = lsp.protocol.make_client_capabilities(),
}

Then in several places we deep_extend from user's config like that

  local default_config = tbl_deep_extend('keep', config_def.default_config, util.default_config)

This seems like a good idea, until you realize that if the config_def.default_config config has no settings or init_options, the deep extend will simply copy by reference of the vim.empty_dict and not 'deep' copy it (same is applicable for {}). This means that the result of the deep extend will now carry not a copy of the default init_options or settings but the reference created from vim.emtpy_dict itself.

Now every subsequent deep_extend executed on the result of the above deep_extend (default_config) will extend the initial reference and not a copy / clone of it for settings or init_options.

This is something i noticed when i started passing around default configs without init_options, and having init_options be dynamically extending the current config after on_setup, where some configs would be coming in with properties from another unrelated server.

Here is a reproduction steps on what is going on

local default_config = {
  settings = vim.empty_dict(),
  init_options = vim.empty_dict(),
  handlers = {},
}

local emtpy1 = {
    root_dir = "e1",
    cmd = { "test1" }
}

local empty2 = {
    root_dir = "e2",
    cmd = { "test1" }
}

local extend1 = vim.tbl_deep_extend('keep', emtpy1, default_config)
extend1.init_options.from_extend_one = "extend1"
extend1.handlers.from_extend_one = "extend1"
vim.print(extend1)

local extend2 = vim.tbl_deep_extend('keep', empty2, default_config)
extend2.init_options.from_extend_two = "extend2"
extend2.handlers.from_extend_two = "extend2"
vim.print(extend2)

You would notice that extend2 has the property _extend_one on top of the newly attached _extend_two. Same goes for handlers table too. It would seem deep extend would not deep copy empty tables, which might cause unexpected side effects.

The solution i recommend is vim.fn.deepcopy or equivalent the default_config provided by nvim-lspconfig, to create a proper deep copy of all referenced tables.

lua local default_config = tbl_deep_extend('keep', config_def.default_config, vim.fn.deepcopy(util.default_config))