async vim.loop calls with coroutines block tests

Start by running make test with the reproduction in test/test_spec.lua and test/minimal_init.lua. Trace how plenary.test_harness handles the coroutine yield and vim.loop.fs_stat callback, using the Makefile command as the entry point. Done means the reproduced test exits successfully instead of hanging, while synchronous vim.loop calls remain working.

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, https://github.com/nvim-lua/plenary.nvim/issues/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.