Lua curl-backed JSON GET with hard timeout

local function http_json(url, opts)
  opts = opts or {}
  local timeout = opts.timeout or 5
  local cmd = ('curl -fsS --max-time %d -H %q %q'):format(timeout, 'Accept: application/json', url)
  local pipe = assert(io.popen(cmd, 'r'))
  local body = pipe:read('a')
  local ok, why, code = pipe:close()
  if not ok then return nil, ('request failed: %s %s'):format(why, tostring(code)) end
  local decode = opts.decode
  if not decode then return nil, 'opts.decode JSON decoder is required' end
  local decoded, parsed = pcall(decode, body)
  if not decoded then return nil, 'invalid JSON: ' .. tostring(parsed) end
  return parsed
end

return http_json