Try to avoid port collisions

On our CI environment it seems that every now and then these tests fail
because the port it tries to bind to is already in use,
see #16

plone/cachepurging/tests/test_purger.py

@@ -14,11 +14,6 @@
 import unittest
 
 
-# Define a test HTTP server that returns canned responses
-
-SERVER_PORT = int(os.environ.get("ZSERVER_PORT", 8765))
-
-
 class TestHandler(BaseHTTPRequestHandler):
     def do_PURGE(self):
         # Get the pre-defined response from the server's queue.
@@ -75,7 +70,7 @@ def queue_response(self, **kw):
 class TestCase(unittest.TestCase):
     def setUp(self):
         self.purger = DefaultPurger()
-        self.httpd, self.httpt = self.startServer()
+        self.httpd, self.httpt, self.port = self.startServer()
 
     def tearDown(self):
         try:
@@ -101,16 +96,18 @@ def tearDown(self):
                 self.purger = None
                 self.httpd, self.httpt = None, None
 
-    def startServer(self, port=SERVER_PORT, start=True):
+    def startServer(self, start=True):
         """Start a TestHTTPServer in a separate thread, returning a tuple
         (server, thread). If start is true, the thread is started.
         """
-        server_address = ("localhost", port)
+        environment_port = int(os.environ.get("ZSERVER_PORT", 0))
+        server_address = ("localhost", environment_port)
         httpd = TestHTTPServer(server_address, TestHandler)
+        _, actual_port = httpd.socket.getsockname()
         t = threading.Thread(target=httpd.serve_forever)
         if start:
             t.start()
-        return httpd, t
+        return httpd, t, actual_port
 
 
 class TestSync(TestCase):
@@ -121,8 +118,8 @@ def setUp(self):
     def tearDown(self):
         super(TestSync, self).tearDown()
 
-    def dispatchURL(self, path, method="PURGE", port=SERVER_PORT):
-        url = "http://localhost:%s%s" % (port, path)
+    def dispatchURL(self, path, method="PURGE"):
+        url = "http://localhost:%s%s" % (self.port, path)
         return self.purger.purgeSync(url, method)
 
     def testSimpleSync(self):
@@ -145,8 +142,8 @@ def testError(self):
 
 
 class TestAsync(TestCase):
-    def dispatchURL(self, path, method="PURGE", port=SERVER_PORT):
-        url = "http://localhost:%s%s" % (port, path)
+    def dispatchURL(self, path, method="PURGE"):
+        url = "http://localhost:%s%s" % (self.port, path)
         self.purger.purgeAsync(url, method)
 
         # Item should now be in the queue!
@@ -197,10 +194,10 @@ class TestAsyncConnectionFailure(TestCase):
     def setUp(self):
         # Override setup to not start the server immediately
         self.purger = DefaultPurger()
-        self.httpd, self.httpt = self.startServer(start=False)
+        self.httpd, self.httpt, self.port = self.startServer(start=False)
 
-    def dispatchURL(self, path, method="PURGE", port=SERVER_PORT):
-        url = "http://localhost:%s%s" % (port, path)
+    def dispatchURL(self, path, method="PURGE"):
+        url = "http://localhost:%s%s" % (self.port, path)
         self.purger.purgeAsync(url, method)
 
         # Item should now be in the queue!