nondeterministic failure of allmydata.test.test_introducer.SystemTest.test_system on Windows: Address already in use

[davidsarah]

​https://tahoe-lafs.org/buildbot-tahoe-lafs/builders/Marcus%20Cygwin%20WinXP/builds/8/steps/test/logs/stdio :

[ERROR]
Traceback (most recent call last):
  File "/home/marcus/bbtahoe/Marcus_Cygwin_WinXP/build/src/allmydata/test/test_introducer.py", line 217, in _restart_introducer_tub
    self.create_tub(self.central_portnum)
  File "/home/marcus/bbtahoe/Marcus_Cygwin_WinXP/build/src/allmydata/test/test_introducer.py", line 85, in create_tub
    l = tub.listenOn("tcp:%d" % portnum)
  File "/home/marcus/bbtahoe/Marcus_Cygwin_WinXP/build/support/lib/python2.6/site-packages/foolscap-0.6.2-py2.6.egg/foolscap/pb.py", line 573, in listenOn
    l.addTub(self)
  File "/home/marcus/bbtahoe/Marcus_Cygwin_WinXP/build/support/lib/python2.6/site-packages/foolscap-0.6.2-py2.6.egg/foolscap/pb.py", line 111, in addTub
    self.s.setServiceParent(self.parentTub)
  File "/usr/lib/python2.6/site-packages/Twisted-11.1.0-py2.6-cygwin-1.7.9-i686.egg/twisted/application/service.py", line 186, in setServiceParent
    self.parent.addService(self)
  File "/usr/lib/python2.6/site-packages/Twisted-11.1.0-py2.6-cygwin-1.7.9-i686.egg/twisted/application/service.py", line 308, in addService
    service.privilegedStartService()
  File "/usr/lib/python2.6/site-packages/Twisted-11.1.0-py2.6-cygwin-1.7.9-i686.egg/twisted/application/internet.py", line 105, in privilegedStartService
    self._port = self._getPort()
  File "/usr/lib/python2.6/site-packages/Twisted-11.1.0-py2.6-cygwin-1.7.9-i686.egg/twisted/application/internet.py", line 133, in _getPort
    'listen%s' % (self.method,))(*self.args, **self.kwargs)
  File "/usr/lib/python2.6/site-packages/Twisted-11.1.0-py2.6-cygwin-1.7.9-i686.egg/twisted/internet/posixbase.py", line 431, in listenTCP
    p.startListening()
  File "/usr/lib/python2.6/site-packages/Twisted-11.1.0-py2.6-cygwin-1.7.9-i686.egg/twisted/internet/tcp.py", line 622, in startListening
    raise CannotListenError, (self.interface, self.port, le)
twisted.internet.error.CannotListenError: Couldn't listen on any:2795: [Errno 112] Address already in use.

allmydata.test.test_introducer.SystemTest.test_system

This is on Windows cygwin. I don't know whether it is specific to cygwin; I seem to remember that Windows in general will give an error if you try to reuse a port just after it has been released.