Basic RPC to query values from the main Neovim instance within fn_transform

[elanmed]

👋 After our discussion in this thread, I started experimenting with using rpc to pass information between the main Neovim instance and the headless instances that run fn_transform so that I could enable multiprocess=true. This is a basic version I got working, any feedback is appreciated!

I noticed that if I used the default vim.v.servername in vim.fn.sockconnect, I got errors that _G.fzf_exec_with_rpc_data was nil - I'm assuming because the vim.v.servername isn't necessarily the same between headless nvim instances and the main instance. Strangely, I only got this error when returning the rpc response from fn_transform - when just printing the response in fn_transform, it worked fine. Not too sure what to make of this, but I decided to try making my own rpc server that would stay constant.

I saw that fzf-lua creates an rpc server and assigns it to vim.g.fzf_lua_server and the FZF_LUA_SERVER environment variable, which seemed like a cool way to share the server name between the main instance and the headless instances. I tried something similar, and it worked great:

-- based on https://github.com/ibhagwan/fzf-lua/blob/07326bb95827f1f4b7d7aead5e8094d844588d08/lua/fzf-lua/init.lua#L32
if not vim.g.fzf_lua_rpc_server then
  local ok, result = pcall(vim.fn.serverstart, "fzf-lua-rpc." .. os.time())
  if ok then
    vim.g.fzf_lua_rpc_server = result
  else
    error(result)
  end
end
vim.fn.setenv("FZF_LUA_RPC_SERVER", vim.g.fzf_lua_rpc_server)

local function fzf_exec_with_rpc(prefix)
  _G.fzf_exec_with_rpc_data = { prefix = prefix, }

  require "fzf-lua".fzf_exec("ls", {
    multiprocess = true,
    fn_transform = function(x)
      local ok, result = pcall(function()
        local chan = vim.fn.sockconnect("pipe", vim.fn.getenv "FZF_LUA_RPC_SERVER", { rpc = true, })
        local rpcresponse = vim.rpcrequest(chan, "nvim_exec_lua", "return _G.fzf_exec_with_rpc_data.prefix", {})
        vim.fn.chanclose(chan)
        return rpcresponse
      end)

      local entry = require "fzf-lua".utils.ansi_codes.magenta(x)
      local rpc_prefix = ok and result or "[RPC error]"
      return rpc_prefix .. " " .. entry
    end,
  })
end

I asked chatgpt for a review, and it suggested stopping the server with something like this:

vim.api.nvim_create_autocmd("VimLeavePre", {
  callback = function()
    if vim.g.fzf_lua_rpc_server then
      vim.fn.serverstop(vim.g.fzf_lua_rpc_server)
    end
  end,
})

but I couldn't find anything similar in the fzf-lua repo. Curious if you have any thoughts?

[nenahp]

no need serverstart another thing it's just expose a new socket handle that can be safely reused

i guess fn-transform dont allow you to use vim.fn since it's run in callback of uv.write which is fast event

maybe you can use fn-preprocess to do rpc and get+assign a global varible in your headless instance

[elanmed]

no need serverstart another thing it's just expose a new socket handle that can be safely reused

That's a good point, I can just expose vim.v.servername, no need to create a new server:

vim.fn.setenv("FZF_LUA_RPC_SERVER", vim.v.servername)

maybe you can use fn-preprocess to do rpc and get+assign a global varible in your headless instance

That's an interesting idea, something like this works:

    fn_preprocess = function()
      local chan = vim.fn.sockconnect("pipe", vim.fn.getenv "FZF_LUA_RPC_SERVER", { rpc = true, })
      local rpc_prefix = vim.rpcrequest(chan, "nvim_exec_lua", "return _G.fzf_exec_with_rpc_data.prefix", {})
      vim.fn.chanclose(chan)
      _G.cached_rpc_prefix = rpc_prefix
    end,
    fn_transform = function(x)
      local entry = require "fzf-lua".utils.ansi_codes.magenta(x)
      return _G.cached_rpc_prefix .. " " .. entry
    end,

Which means that fn_preprocess and fn_transform must run in the same headless Neovim instance, since they both have access to the same _G.cached_rpc_prefix variable 🤔

Thanks for the tips 🙏

[ibhagwan]

Which means that fn_preprocess and fn_transform must run in the same headless Neovim instance, since they both have access to the same _G.cached_rpc_prefix variable 🤔

They do run in the same neovim instance:

fzf-lua/lua/fzf-lua/libuv.lua

Line 457 in 07326bb

-- run the preprocessing fn

If you add debug=“v” to your opts you’ll be able to see all the args of the external process:

[elanmed]

Ah that's a good one to know, thanks! 🙏