Soo I did some investigation and found #45186 but this was never implemented following #47071 (review).

Problem is, sessions handlers are wrapped into a SessionHandlerProxy, so your custom handler would need to extend AbstractProxy and implement SessionIdInterface.

EDIT: opened #50626

Thanks for your reply, i will give it a try

Not sure how it helps setting the a custom session id using setId($some_id) function of Symfony\Component\HttpFoundation\Session\Session ?? Shouldn't we change

        if ($sessionId && $this->saveHandler instanceof AbstractProxy && 'files' === $this->saveHandler->getSaveHandlerName() && !preg_match('/^[a-zA-Z0-9,-]{22,250}$/', $sessionId)) {
            // the session ID in the header is invalid, create a new one
            session_id(session_create_id());
        }

to

if ($sessionId && $this->saveHandler instanceof AbstractProxy && 'files' === $this->saveHandler->getSaveHandlerName() && !preg_match('/^[a-zA-Z0-9,-]{22,250}$/', $sessionId)) {
            if(!empty($sessionId = $this->getId())) {
            session_id($sessionId);
            } else {
            // the session ID in the header is invalid, create a new one
            session_id(session_create_id());
            }
        }

in NativeSessionStorage class

You cannot use Session::setId yourself because Symfony forces the session.use_strict_mode (which is a good thing), which rejects preset ids if they don’t match an initialized session.

From what I understood you need to create a class extending SessionHandlerProxy and implementing SessionIdInterface:

<?php

namespace App\Session;

use Symfony\Component\HttpFoundation\Session\Storage\Proxy\SessionHandlerProxy;

class CustomIdSessionHandler extends SessionHandlerProxy implements \SessionIdInterface
{
    public function create_sid(): string
    {
        return 'a-valid-session-identifier';
    }
}

Then you must register it as a service and wire its $handler argument to the service corresponding to your framework.session.handler_id (this is session.handler.native by default):

services:
    App\Session\CustomIdSessionHandler:
        arguments:
            - '@session.handler.native'

At last, you must replace framework.session.handler_id accordingly to your service’s id:

framework:
    session:
        handler_id: App\Session\CustomIdSessionHandler

I’m not sure this is the way to go yet, but it works.