/** Serve the game state of any valid game id, but block until
 * the client is no longer blocked (long-polling). */
app.get('/:gameid/longpoll', (req, res) => {
    const userid = getUserId(req, res);
    const game = games[parseInt(req.params['gameid'])]
    if (game == undefined) {
        res.status(404).json("no such game");
    } else {
        if (isWaiting(game, userid)) {
            console.log(`Game ${game.id} directly joined by ${userid}`)
            join(game, userid)
            res.json(toJson(game, userid))
            res.end()
            return
        }
        // Check if userid needs to wait, then either:
        // - stash the [response, userid] pair for later
        // - instantly return actionable state
        if (shouldBlockRequest(game, userid)) {
            console.log(`Stash long-poll request for ${game.id} by ${userid}`)
            longpolls[game.id] ??= []
            longpolls[game.id].push([res, userid])
            // do not end here but keep request hanging.
        } else {
            // User can act on the state, no point in waiting.
            res.json(toJson(game, userid))
            res.end()
        }
    }
})