Fetch work-alike for CC, implementing async HTTP requests

fetch.lua

-- Made by JackMacWindows, licensed under CC0

local expect = require "cc.expect".expect

local activeRequests = {}
local kPromiseSuccess, kPromiseFailure = {}, {}
local openHandlers = 0

-- To use, call this function with each URL and optionally a table with options.
-- Then call it with no arguments to process the requests in the queue.
-- The function returns once the request queue is depleted.
return function(resource, init)
    if resource == nil then
        while next(activeRequests) do
            for k,v in pairs(activeRequests) do
                if not v.sent and openHandlers < 16 then
                    http.request(v.options)
                    openHandlers = openHandlers + 1
                    v.sent = true
                end
            end
            local ev, url, res, errh = os.pullEvent()
            if (ev == "http_success" or (ev == "http_failure" and errh)) and activeRequests[url] then
                openHandlers = openHandlers - 1
                if ev == "http_failure" then
                    res = errh
                    res.ok = false
                else res.ok = true end
                local req = activeRequests[url]
                activeRequests[url] = nil
                if req.resolve == 0 then res.close() else
                    res.array = function()
                        local retval = {}
                        local c, n = res.read(1), 1
                        while c ~= nil do
                            retval[n] = c:byte()
                            c, n = res.read(1), n + 1
                        end
                        res.close()
                        return kPromiseSuccess, retval
                    end
                    res.formData = function()
                        return kPromiseFailure, "Not implemented"
                    end
                    res.json = function()
                        local text = res.readAll()
                        res.close()
                        if textutils.unserializeJSON == nil then return kPromiseFailure, "Unserializing JSON requires CC:T 1.87+" end
                        local json, err = textutils.unserializeJSON(text)
                        if json then return kPromiseSuccess, json
                        else return kPromiseFailure, err end
                    end
                    res.text = function()
                        local text = res.readAll()
                        res.close()
                        return kPromiseSuccess, text
                    end
                    local i = 1
                    local data, key = res
                    repeat
                        key, data = req.resolve[i](data)
                        if key == kPromiseFailure then req.reject(data) end
                        i=i+1
                    until key ~= kPromiseSuccess or not req.resolve[i]
                end
                if req.finish then req.finish() end
            elseif ev == "http_failure" and activeRequests[url] then
                openHandlers = openHandlers - 1
                local req = activeRequests[url]
                activeRequests[url] = nil
                if req.reject then req.reject(res) end
            end
        end
    else
        expect(1, resource, "string")
        expect(2, init, "table", "nil")
        if not init then init = {} end
        init.url = resource
        local req = {options = init, resolve = {}}
        activeRequests[resource] = req
        local promise
        promise = {
            Then = function(fn)
                req.resolve[#req.resolve+1] = fn
                return promise
            end,
            catch = function(fn)
                req.reject = fn
                return promise
            end,
            finally = function(fn)
                req.finish = fn
                return promise
            end
        }
        promise["then"] = promise.Then -- for compatibility, but you'll have to do ["then"]
        return promise
    end
end