Linux: reuse ports faster (#4036)

* [WIP] Reuse ports on linux

* Check socket

* platform dependent behavior

* Added a comment

* Update ml-agents-envs/mlagents_envs/rpc_communicator.py

Co-authored-by: Jonathan Harper <jharper+moar@unity3d.com>

Co-authored-by: Jonathan Harper <jharper+moar@unity3d.com>

ml-agents-envs/mlagents_envs/rpc_communicator.py

 import grpc
 from typing import Optional
 
+from sys import platform
 import socket
 from multiprocessing import Pipe
 from concurrent.futures import ThreadPoolExecutor
         Attempts to bind to the requested communicator port, checking if it is already in use.
         """
         s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        if platform == "linux" or platform == "linux2":
+            # On linux, the port remains unusable for TIME_WAIT=60 seconds after closing
+            # SO_REUSEADDR frees the port right after closing the environment
+            s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
         try:
             s.bind(("localhost", port))
         except socket.error: