Default port overrides requesting a system assigned port

[Ben-RK]

Expected Behavior

When running a flask application with app.run(port=0), flask should pass the '0' port down to werkzeug to use a port assigned by the operating system.

Actual Behavior

The logic in flask/app.py:932 sets the port to the default 5000, which can cause port conflicts, which is the exact situation that setting port=0 should avoid. This is a regression from the behaviour exhibited in prior versions.

>>> app.run(port=0)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../lib/python3.6/site-packages/flask/app.py", line 943, in run
    run_simple(host, port, self, **options)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 814, in run_simple
    inner()
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 774, in inner
    fd=fd)
  File "/.../lib/python3.6/site-packages/werkzeug/serving.py", line 660, in make_server
    passthrough_errors, ssl_context, fd=fd)
  File ".../lib/python3.6/site-packages/werkzeug/serving.py", line 577, in __init__
    self.address_family), handler)
  File "/.../lib/python3.6/socketserver.py", line 453, in __init__
    self.server_bind()
  File "/.../lib/python3.6/http/server.py", line 136, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/.../lib/python3.6/socketserver.py", line 467, in server_bind
    self.socket.bind(self.server_address)
OSError: [Errno 98] Address already in use

Environment

• Python version: 3.6
• Flask version: 1.0.2
• Werkzeug version: 0.14.1

[davidism]

Makes sense. Marking this as beginner friendly, I'd be happy to review a PR.

[vorelq]

Hi, I'd like to try and fix this one.

[patiencedaur]

I think it's closed, it shows expected behavior

[Man-Jain]

Is this one still up for grabs ?

[ThiefMaster]

No, there's a PR for it (see above)

[Man-Jain]

Yeah but it as inactive for over a month. That's why I asked.

[theodesp]

@Man-Jain you can politely ask the PR implementor to see if he can fix the PR comments otherwise you can fork his PR and try to fix it yourself if you are in a hurry.

[davidism]

The PR is fine, I just haven't got around to merging it.