I'm working on a user-pairing algorithm using socket.io, and it is running on Node.js. I want to make sure that no repeated pairings will happen when people connect in.

The steps of the algorithm as follows:

1. Divide the players into two rooms, one for retailers and the other for suppliers.
2. When a player enters one of the rooms, I will fetch the socketlist from the other room and pair the player with the first socket of it.
   • If the pairing succeeds, remove both players from the rooms.
   • If it fails (i.e. no one in the other room), retry every five seconds.

I'm not sure about whether there would be any race-condition problems during the process of fetching the socketlist of the room. In short, is it possible that a socket in the socketlist would be paired twice, and both pairings succeed?

Here is the code:

const wait = ms => new Promise(resolve => setTimeout(resolve, ms))

io.on('connection', (socket) => {
    console.log('New websocket connection', socket.id)

    socket.on('match', async (userId, role, classroomId) => {
        let roles = { 'retailer': 'supplier', 'supplier': 'retailer' }
        room = role + classroomId
        otherroom = roles[role] + classroomId

        socket.userId = userId
        socket.join(room)
        
        for (let i = 0; i < 60; ++i) {
            if (socket.rooms.has(room)) {
                const sockets = await io.in(otherroom).fetchSockets()
                
                if (sockets.length > 0
                    && io.sockets.sockets.get(socket[0].id).rooms.has(otherroom)) {
                    const teammate = io.sockets.sockets.get(sockets[0].id)
                    console.log(teammate.userId)
                    console.log(room, otherroom)
                    socket.leave(room)
                    teammate.leave(room)

                    const pair = await models.Pair.create({
                        supplierId: role == 'supplier' ? userId : teammate.userId,
                        retailerId: role == 'supplier' ? teammate.userId : userId,
                        currentTime: new Date().toISOString().slice(0, 10)
                    })
                    io.to(teammate.id).to(socket.id).emit('match-success', pair)
                    console.log((role == 'supplier' ? 's' : 'r'), pair)
                    break
                }
                
                console.log(socket.id, i)
                // if no match, rematch every 5 seconds.
                await wait(5000)
            } else {
                break
            }
        }

        socket.on('join', (room) => {
            console.log('join' + room)
            socket.join(room)
        })

        socket.on('leave', (room, callback) => {
            socket.leave(room)
            callback()
        })
    })
})