Merge lp:~mbp/launchpad/less-randomness into lp:launchpad

[Martin Pool]

[r=wallyworld][no-qa] Test_create_token: save and restore random state, rather than making things more random

As an update to https://code.launchpad.net/~bac/launchpad/bug-891641/+merge/82544 this actually saves and restores the prng state, rather than randomizing it, when Test_create_token runs.

[Ian Booth (wallyworld)]

Looks like an acceptable change. Doesn't appear that anything will break :-)

review: Approve (code)

[Martin Pool (mbp)]

socratic dialog:

<wallyworld_> poolie: isn't the whole point of using a rng to provide different input each time?
<poolie> wallyworld_, so, generally, my experience is that using a rng in tests is a bad idea
 if there are some random values that catch a bug, which is possible
 there are problems
 one is, the particular values are likely to come up only very rarely, so the bug may be latent for a long time
 secondly, it will be hard to reproduce it, so perhaps hard to understand why it's failing
 oh, and also, it may fail at inconvenient times
 for example on buildbot but not ec2 etc
<poolie> wallyworld_, so this particular test is testing code that itself uses the rng, so i think it has a valid reason to touch it
 but it should isoltae itself
<wallyworld_> poolie: that all makes sense. thanks for explaining

=== modified file 'lib/canonical/launchpad/tests/test_token_creation.py'
--- lib/canonical/launchpad/tests/test_token_creation.py	2011-11-17 14:42:51 +0000
+++ lib/canonical/launchpad/tests/test_token_creation.py	2011-11-17 23:24:25 +0000
@@ -1,4 +1,4 @@
-# Copyright 2009 Canonical Ltd.  This software is licensed under the
+# Copyright 2009, 2011 Canonical Ltd.  This software is licensed under the
 # GNU Affero General Public License version 3 (see the file LICENSE).
 
 __metaclass__ = type
@@ -27,10 +27,8 @@
     layer = DatabaseFunctionalLayer
 
     def test_token_uniqueness(self):
-        # Since the prng will be seeded in this test it is important we clean
-        # up by calling seed with no parameters, which will use OS-provided
-        # entropy if available or use the system clock.
-        self.addCleanup(random.seed)
+        orig_state = random.getstate()
+        self.addCleanup(lambda: random.setstate(orig_state))
         # Calling create_unique_token_for_table() twice with the same
         # random.seed() will generate two identical tokens, as the token was
         # never inserted in the table.