examples/webrtc/signalling: Fix compatibility with Python 3.10

- ssl module requires an explicit TLS_SERVER role
- asyncio throws a deprecation warning when using
  asyncio.get_event_loop().  Remove custom event loop handling entirely
- No need to keep the websocket server in a member variable, can use
  a future to signal exit case along with the async with context manager
  of websockets.serve()

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2711>

subprojects/gst-examples/webrtc/signalling/simple_server.py

@@ -20,7 +20,7 @@ import concurrent
 
 class WebRTCSimpleServer(object):
 
-    def __init__(self, loop, options):
+    def __init__(self, options):
         ############### Global data ###############
 
         # Format: {uid: (Peer WebSocketServerProtocol,
@@ -35,11 +35,6 @@ class WebRTCSimpleServer(object):
         # Room dict with a set of peers in each room
         self.rooms = dict()
 
-        # Event loop
-        self.loop = loop
-        # Websocket Server Instance
-        self.server = None
-
         # Options
         self.addr = options.addr
         self.port = options.port
@@ -112,7 +107,6 @@ class WebRTCSimpleServer(object):
 
     ############### Handler functions ###############
 
-    
     async def connection_handler(self, ws, uid):
         raddr = ws.remote_address
         peer_status = None
@@ -241,7 +235,7 @@ class WebRTCSimpleServer(object):
         # Create an SSL context to be used by the websocket server
         print('Using TLS with keys in {!r}'.format(self.cert_path))
         chain_pem, key_pem = self.get_ssl_certs()
-        sslctx = ssl.create_default_context()
+        sslctx = ssl.SSLContext(ssl.PROTOCOL_TLS_SERVER)
         try:
             sslctx.load_cert_chain(chain_pem, keyfile=key_pem)
         except FileNotFoundError:
@@ -252,7 +246,7 @@ class WebRTCSimpleServer(object):
         sslctx.verify_mode = ssl.CERT_NONE
         return sslctx
 
-    def run(self):
+    async def run(self):
         async def handler(ws, path):
             '''
             All incoming messages are handled here. @path is unused.
@@ -277,22 +271,30 @@ class WebRTCSimpleServer(object):
                                # https://websockets.readthedocs.io/en/stable/api.html#websockets.protocol.WebSocketCommonProtocol
                                max_queue=16)
 
-        # Setup logging
         logger = logging.getLogger('websockets')
         logger.setLevel(logging.INFO)
-        logger.addHandler(logging.StreamHandler())
+        handler = logging.StreamHandler()
+        logger.addHandler(handler)
+
+        try:
+            self.exit_future = asyncio.Future()
+            task = asyncio.create_task(self.check_server_needs_restart())
 
             # Run the server
-        self.server = self.loop.run_until_complete(wsd)
-        # Stop the server if certificate changes
-        self.loop.run_until_complete(self.check_server_needs_restart())
-
-    async def stop(self):
-        print('Stopping server... ', end='')
-        self.server.close()
-        await self.server.wait_closed()
-        self.loop.stop()
+            async with wsd:
+                await self.exit_future
+                self.exit_future = None
             print('Stopped.')
+        finally:
+            logger.removeHandler(handler)
+            self.peers = dict()
+            self.sessions = dict()
+            self.rooms = dict()
+
+    def stop(self):
+        if self.exit_future:
+            print('Stopping server... ', end='')
+            self.exit_future.set_result(None)
 
     def check_cert_changed(self):
         chain_pem, key_pem = self.get_ssl_certs()
@@ -313,7 +315,7 @@ class WebRTCSimpleServer(object):
             await asyncio.sleep(10)
             if self.check_cert_changed():
                 print('Certificate changed, stopping server...')
-                await self.stop()
+                self.stop()
                 return
 
 
@@ -330,16 +332,14 @@ def main():
 
     options = parser.parse_args(sys.argv[1:])
 
-    loop = asyncio.get_event_loop()
-
-    r = WebRTCSimpleServer(loop, options)
-
     print('Starting server...')
     while True:
-        r.run()
-        loop.run_forever()
+        r = WebRTCSimpleServer(options)
+        asyncio.run(r.run())
         print('Restarting server...')
+
     print("Goodbye!")
 
+
 if __name__ == "__main__":
     main()