[Feature] Support multiple concurrent sessions

[nvborisenko]

This is continuation of #1946. It would be nice if geckodriver supports multiple concurrent sessions.

Motivation:

I have a lot of tests, executed in parallel. Each test spawns own Firefox instance. Now each test also spawns own geckodriver instance. It would be nice, if I can run geckodriver once, and use this single instance to spawn many Firefox browsers.

According WebDriver Spec geckodriver can do it:

If the intermediary node is a multiplexer that manages multiple endpoint nodes, this might indicate its ability to purvey more sessions, for example if it has hit its maximum capacity.

Chrome and Edge drivers support it.

System

• Version: 0.35
• Platform: Linux/macOS/Windows
• Firefox: 134
• Selenium: Dev in progress

Testcase

1. Start geckodriver
2. Request new session
3. Request one more new session

Stacktrace

System.InvalidOperationException : Session is already started (SessionNotCreated)

13:17:24.293 TRACE HttpCommandExecutor: << StatusCode: 500, ReasonPhrase: Internal Server Error, Content: System.Net.Http.HttpConnectionResponseContent, Headers: 2
{"value":{"error":"session not created","message":"Session is already started","stacktrace":""}}