Lua - Download file asynchronously via HTTP - asynchronous

Question

I just finish reading copas core code. And I want to write code to download file from website asynchronously, but copas seems to only support socket IO.
Since Lua does not provide async syntax, and other packages will surely have their own event loop that, I think, can not run along side copas' loop.
So to async download file via http, do I have to find a package that suppprt async http and async file IO at the same time? Or any other ideas?

Answer 1

After reading bunches of code, I can finally answer my own question.
As I mention in my comment to the question, one can make use of the step function exported by async IO library, and merge multiple stepping into a bigger loop.
In the case of luv, it uses external thread pool in C to manage file IO, and use a single-threaded loop to call pending callbacks and manage IO polling (polling is not needed in my use case).
One can simply call file operation function provided by luv to make async file IO. But still need to step luv's loop to call callbacks bind to IO operations.
The integerated main loop looks goes like this:
local function main_loop()
copas.running = true
while not copas.finished() or uv.loop_alive() do
if not copas.finished() then
copas.step()
end
if uv.loop_alive() then
uv.run("nowait")
end
end
end
copas.step() is the stepping function of copas. And uv.run("nowait") make luv run just one pass of event loop and don't block if there is no ready IO when polling.
A working solution looks like this:
local copas = require "copas"
local http = require "copas.http"
local uv = require "luv"
local urls = {
"http://example.com",
"http://example.com"
}
local function main_loop()
copas.running = true
while not copas.finished() or uv.loop_alive() do
if not copas.finished() then
copas.step()
end
if uv.loop_alive() then
uv.run("nowait")
end
end
end
local function write_file(file_path, data)
-- ** call to luv async file IO **
uv.fs_open(file_path, "w+", 438, function(err, fd)
assert(not err, err)
uv.fs_write(fd, data, nil, function(err_o, _)
assert(not err_o, err_o)
uv.fs_close(fd, function(err_c)
assert(not err_c, err_c)
print("finished:", file_path)
end)
end)
end)
end
local function dl_url(url)
local content, _, _, _ = http.request(url)
write_file("foo.txt", content)
end
-- adding task to copas' loop
for _, url in ipairs(urls) do
copas.addthread(dl_url, url)
end
main_loop()