[BUG] Serve application at random port failed

[xavArtley]

Hello,

since Bokeh 2.1.0 (windows 10), trying to serve an application with random port (--port=0) does not work anymore and result in the following error:

(py37) D:\PythonWorkspace\divers>bokeh serve C:\Users\xa235208\Downloads\bokeh-master\examples\app\sliders.py --show --port=0
2020-06-16 09:39:45,845 Starting Bokeh server version 2.1.0 (running on Tornado 6.0.4)
2020-06-16 09:39:45,847 User authentication hooks NOT provided (default user enabled)
2020-06-16 09:39:45,850 Bokeh app running at: http://localhost:61622/sliders
2020-06-16 09:39:45,850 Starting Bokeh server with process id: 14652
2020-06-16 09:39:46,587 Refusing websocket connection from Origin 'http://localhost:61622';                       use --allow-websocket-origin=localhost:61622 or set BOKEH_ALLOW_WS_ORIGIN=localhost:61622
to permit this; currently we allow origins {'localhost:0'}

[bryevdv]

If this is local usage then --allow-websocket-origin="*" is a potential workaround for now. Not sure when/how this got introduced.

[bryevdv]

Most likely it was this PR #10070 the port added to the allowlist is the original passed-in opts.port, not the value returned from bind_sockets

https://github.com/bokeh/bokeh/pull/10070/files#diff-35e262b3fd4a971d380038e009fd69acR398

[bryevdv]

This diff fixes, but any PR needs to come with a test

diff --git a/bokeh/server/server.py b/bokeh/server/server.py
index 78f19bbe8..97eec3965 100644
--- a/bokeh/server/server.py
+++ b/bokeh/server/server.py
@@ -395,7 +395,7 @@ class Server(BaseServer):
         sockets, self._port = bind_sockets(opts.address, opts.port)
         self._address = opts.address

-        extra_websocket_origins = create_hosts_allowlist(opts.allow_websocket_origin, opts.port)
+        extra_websocket_origins = create_hosts_allowlist(opts.allow_websocket_origin, self.port)
         try:
             tornado_app = BokehTornado(applications,
                                        extra_websocket_origins=extra_websocket_origins,