Reset RNG seed with current time and pid for each test started

This will hopefully fix fullstack failures where different process
fixtures running in parallel test processes and relying on the same
random.choice() generator seeded by the same initial value could pick up
the same value as a service free port, and spawn their respective
resources using the same port.

Which made one of those unlucky services to fail.

Change-Id: I13cfa9392fd138c5e1b1b7d397b9ea91b2a47ed2
Closes-Bug: #1551288

neutron/tests/base.py

@@ -154,6 +154,8 @@ class DietTestCase(base.BaseTestCase):
         self.addOnException(self.check_for_systemexit)
         self.orig_pid = os.getpid()
 
+        tools.reset_random_seed()
+
     def addOnException(self, handler):
 
         def safe_handler(*args, **kwargs):

neutron/tests/fullstack/base.py

@@ -18,6 +18,7 @@ from oslo_db.sqlalchemy import test_base
 from neutron.db.migration import cli as migration
 from neutron.tests.common import base
 from neutron.tests.fullstack.resources import client as client_resource
+from neutron.tests import tools
 
 
 class BaseFullStackTestCase(base.MySQLTestCase):
@@ -25,6 +26,10 @@ class BaseFullStackTestCase(base.MySQLTestCase):
 
     def setUp(self, environment):
         super(BaseFullStackTestCase, self).setUp()
+        # NOTE(ihrachys): seed should be reset before environment fixture below
+        # since the latter starts services that may rely on generated port
+        # numbers
+        tools.reset_random_seed()
         self.create_db_tables()
         self.environment = environment
         self.environment.test_name = self.get_name()

neutron/tests/tools.py

@@ -13,9 +13,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import os
 import platform
 import random
 import string
+import time
 import warnings
 
 import fixtures
@@ -204,3 +206,13 @@ def is_bsd():
     if 'bsd' in system.lower():
         return True
     return False
+
+
+def reset_random_seed():
+    # reset random seed to make sure other processes extracting values from RNG
+    # don't get the same results (useful especially when you then use the
+    # random values to allocate system resources from global pool, like ports
+    # to listen). Use both current time and pid to make sure no tests started
+    # at the same time get the same values from RNG
+    seed = time.time() + os.getpid()
+    random.seed(seed)