[BUG] nio.fn.rpcrequest hangs indefinitely

[olafurkarl]

NeoVim Version

NVIM v0.10.3
Build type: RelWithDebInfo
LuaJIT 2.1.1731601260

Describe the bug
When using neotest, my nvim process would completely freeze up.
I bisected the issue to the first and any following rpc requests made by the plugin via nvim-nio, e.g.:

local mode = nio.fn.rpcrequest(child_chan, "nvim_get_mode")

To Reproduce
Please provide a minimal init.lua to reproduce which can be run as the following:

local lazypath = vim.fn.stdpath("data") .. "/lazy/lazy.nvim"
if not (vim.uv or vim.loop).fs_stat(lazypath) then
	local lazyrepo = "https://github.com/folke/lazy.nvim.git"
	local out = vim.fn.system({ "git", "clone", "--filter=blob:none", "--branch=stable", lazyrepo, lazypath })
	if vim.v.shell_error ~= 0 then
		vim.api.nvim_echo({
			{ "Failed to clone lazy.nvim:\n", "ErrorMsg" },
			{ out, "WarningMsg" },
			{ "\nPress any key to exit..." },
		}, true, {})
		vim.fn.getchar()
		os.exit(1)
	end
end
vim.opt.rtp:prepend(lazypath)

-- Setup lazy.nvim
require("lazy").setup({
	spec = {
		{
			{
				"nvim-neotest/neotest",
				dependencies = {
					"nvim-neotest/nvim-nio",
					"nvim-lua/plenary.nvim",
					"antoinemadec/FixCursorHold.nvim",
					"nvim-treesitter/nvim-treesitter",
					"marilari88/neotest-vitest",
				},
				config = function()
					local neotest = require("neotest")
					neotest.setup({
						log_level = vim.log.levels.info,
						adapters = {
							require("neotest-vitest"),
						},
					})
				end,
			},
		},
	},
})

Steps to reproduce the behavior:

1. Open typescript test file
2. Hover test
3. Run lua require("neotest").run.run(). (either command line or using a keybind)
4. NeoVim will completely hang while waiting for rpc response

Expected behavior
I expect it to run my test and give the appropriate updates to vim.

Logs
The preceding logs look like this:

INFO | 2025-01-24T21:17:38Z+0000 | ...l/share/nvim/lazy/neotest/lua/neotest/lib/subprocess.lua:33 | Starting child process
INFO | 2025-01-24T21:17:38Z+0000 | ...l/share/nvim/lazy/neotest/lua/neotest/lib/subprocess.lua:36 | Parent address: localhost:40351
INFO | 2025-01-24T21:17:38Z+0000 | ...l/share/nvim/lazy/neotest/lua/neotest/lib/subprocess.lua:42 | Starting child process with command: /usr/bin/nvim --embed --headless -n

There does not seem to be any exception that occurs, it simply waits indefinitely for a response.

Additional context
Not sure what could have caused this, but I had seen this working before on my machine.

I wasn't sure if this should be posted to nvim-neotest or nvim-nio, but I figured since I had traced the call to nvim-nio, I'd start here.

Any hints on how to solve it would be appreciated, I'm a big fan of this plugin and love running my tests through the editor! :)

[olafurkarl]

I found that disabling flatten.nvim solves the issue, per nvim-neotest/neotest#468 (comment)