async vim.loop calls with coroutines block tests

[gregorias]

Using async vim.loop functions (not the Plenary async ones) block Plenary's test harness.

Reproduction

Have the following files in a directory:

-- test/test_spec.lua
describe("test", function()
	it("runs the test", function()
		local co = coroutine.running()
		assert(co, "not running inside a coroutine")

		local is_done = false
		vim.loop.fs_stat("/bin", function(err, stats)
			is_done = true
			coroutine.resume(co)
		end)
		if not is_done then
			coroutine.yield()
		end
		assert.equals(true, true)
	end)
end)

-- test/minimal_init.lua
local M = {}

function M.root(root)
    local f = debug.getinfo(1, "S").source:sub(2)
    return vim.fn.fnamemodify(f, ":p:h:h") .. "/" .. (root or "")
end

---@param plugin string
function M.load(plugin)
    local name = plugin:match(".*/(.*)")
    local package_root = M.root(".tests/site/pack/deps/start/")
    if not vim.loop.fs_stat(package_root .. name) then
        print("Installing " .. plugin)
        vim.fn.mkdir(package_root, "p")
        vim.fn.system({
            "git",
            "clone",
            "--depth=1",
            "https://github.com/" .. plugin .. ".git",
            package_root .. "/" .. name,
        })
    end
end

function M.setup()
    vim.cmd([[set runtimepath=$VIMRUNTIME]])
    vim.opt.runtimepath:append(M.root())
    vim.opt.packpath = { M.root(".tests/site") }
    M.load("nvim-lua/plenary.nvim")
    vim.env.XDG_CONFIG_HOME = M.root(".tests/config")
    vim.env.XDG_DATA_HOME = M.root(".tests/data")
    vim.env.XDG_STATE_HOME = M.root(".tests/state")
    vim.env.XDG_CACHE_HOME = M.root(".tests/cache")
end

vim.o.swapfile = false
_G.__TEST = true

M.setup()

# Makefile
.PHONY: test

test:
	nvim --headless --noplugin -u test/minimal_init.lua -c "lua require(\"plenary.test_harness\").test_directory_command('test {minimal_init = \"test/minimal_init.lua\"}')"

Run make test. This will get printed:

nvim --headless --noplugin -u test/minimal_init.lua -c "lua require(\"plenary.test_harness\").test_directory_command('test {minimal_init = \"test/minimal_init.lua\"}')"
Starting...Scheduling: test/client_spec.lua

========================================	
Testing: 	/Users/grzesiek/Code/plenary-busted-loop-async-bug/test/client_spec.lua
Success	||	test runs the test	
	
Success: 	1	
Failed : 	0	
Errors : 	0	
========================================	

and the run will hang and never finish.

Expected behavior

make test finishes successfully.

Notes

There's an issue on testing async code, #424. The solution provided there works with code that uses vim.defer_fn or vim.schedule. It doesn't work with vim.loop functions.

Synchronous vim.loop functions work just fine.