Race condition with dynamic namespaces

[sebamarynissen]

I have noticed that a race condition exists with dynamic namespaces. Consider the following setup

// Server side
import { Server } from 'socket.io';
const io = new Server(3000, {});

io.of(async (nsp, auth, next) => {
  let exists = !!await queryDatabaseForResource({});
  next(null, exists);
});

If the query to the database takes some time and a lot of clients are trying to connect to the same namespace in a short period of time, the namespace will be created multiple times. This will result in sockets being connected to a namespace that is overridden by another one, and hence they will no longer be receiving events.

Looking at Server.prototype._checkNamespace, I think this can be solved as follows:

Server.prototype._checkNamespace = function(name, auth, fn) {
  if (this.parentNsps.size === 0) return fn(false);
  const keysIterator = this.parentNsps.keys();
  const run = () => {
    const nextFn = keysIterator.next();
    if (nextFn.done) return fn(false);
    nextFn.value(name, auth, (err, allow) => {
      if (err || !allow) {
        run();
      } else if (this._nsps.has(name)) {

        // If the namespace has been created in the meantime, do not create it again.
        return fn(this._nsps.get(name));

      } else {

        const namespace = this.parentNsps
          .get(nextFn.value)
          .createChild(name);

        this.sockets.emitReserved('new_namespace', namespace);
        fn(namespace);

      }
    });
  };
  run();
};

I will file a PR to solve this.

Context

The odds of this race condition happening are obviously rather low, but I experienced them on a website of mine where I organize tournaments. If the tournament starts, people are all sent to a socket.io namespace where their match is hosted. These namespaces are created dynamically, and so it happens that a large amount of people tries to connect at the same moment.

[sebamarynissen]

PR filed (#4137).

[darrachequesne]

Good catch! Thanks for the detailed analysis 👍

This should be fixed by 9d86397