Support selector `finally` function.

[izaakschroeder]

There are sometimes resources attached to a connection that need cleaning up – e.g. database connection that's been checked out from a pool.

createSelector(getA, getB, getC, async (a, b, c) => {
  return await dbPool.checkout(); // how to clean up this connection?
});

Possible option:

createSelector(getA, getB, getC, async (a, b, c) => {
  return await dbPool.checkout();
}).withDisposer(async (a, b, c, con) => {
  return await dbPool.release(con);
});

[izaakschroeder]

NOTE: I think this could be solved with Observable. See: https://github.com/tc39/proposal-observable.

Something like

createSelector(() => {
  return new Observable(observer => {
    const connection = await dbPool.checkout();
    next(connection);
    return () => {
      connection.release();
    };
});

Something similar for creating the dbPool object itself as well.

const dbPool = new Observable(observer => {
  const dbPool = ...;
  next(dbPool);
  return () => {
    await dbPool.end();
  };
});

Except the difference here being the observe would trigger on server creation and shutdown instead of on request.