cardstories server race condition fixed

The cardstories server is based on twisted and each server operation that is not CPU is run in a fiber using a combination of deferred and yield. This makes for an efficient code that reads almost like a sequential program. However, after each yield the state of the program may have changed and this creates race conditions.

The Crash: CardstoriesGame instance has no attribute ‘service’ on results publication bug is an example of such a race condition. It can be reproduced by artificially blocking a fiber as demonstrated in the following test case:

diff --git a/tests/test_game.py b/tests/test_game.py
index 395723c..ee31fd6 100644
--- a/tests/test_game.py
+++ b/tests/test_game.py
@@ -623,9 +623,100 @@ class CardstoriesGameTest(unittest.TestCase):
             pass
         self.assertTrue(raises_UserWarning)

+    @defer.inlineCallbacks
+    def test18_complete_and_game_race(self):
+        winner_card = 5
+        sentence = 'SENTENCE'
+        owner_id = 15
+        game_id = yield self.game.create(winner_card, sentence, owner_id)
+        voting_players = [ 16, 17 ]
+        players = voting_players + [ 18 ]
+        for player_id in players:
+            yield self.game.participate(player_id)
+            player = yield self.game.player2game(player_id)
+            card = player['cards'][0]
+            yield self.game.pick(player_id,card)
+
+        yield self.game.voting(owner_id)
+
+        c = self.db.cursor()
+        c.execute("SELECT board FROM games WHERE id = %d" % game_id)
+        board = c.fetchone()[0]
+        winner_id = 16
+        yield self.game.vote(winner_id, winner_card)
+        loser_id = 17
+        yield self.game.vote(loser_id, 120)
+        self.assertEquals(self.game.get_players(), [owner_id] + players)
+        #
+        # the game is about to be completed.
+        # Create the race condition by:
+        # a) calling game() and block it on the first runQuery
+        # b) calling complete() and unblock the game() when it returns
+        # c) resume game() which then needs to cope with the fact
+        #    that it is now using a game that has been destroyed
+        #
+        #
+        # Replace the runQuery function by a wrapper that blocks the query.
+        # It does it by creating a deferred that will run the original
+        # query when it fires. This deferred will need to be fired manually
+        # by the enclosing test, when it needs it. This effectively creates
+        # a condition that simulates a lag because the database is not
+        # available.
+        #
+        original_runQuery = self.game.service.db.runQuery
+        query_deferred = defer.Deferred()
+        def fake_runQuery(*args):
+            def f(result):
+                r = original_runQuery(*args)
+                return r
+            query_deferred.addCallback(f)
+            #
+            # Now that the query has been wrapped in the deferred and the
+            # calling function has been interrupted, restore the original
+            # runQuery function so that the code keeps running as expected.
+            #
+            self.game.service.db.runQuery = original_runQuery
+            return query_deferred
+        #
+        # Because the runQuery is a fake, the deferred returned by self.game.game()
+        # will actually be the deferred returned by the fake_runQuery function.
+        # As a consequence the game.game() function will be blocked in the middle
+        # of its execution, waiting for the deferred returned by fake_runQuery to
+        # be fired.
+        #
+        self.game.service.db.runQuery = fake_runQuery
+        game_deferred = self.game.game(owner_id)
+        #
+        # The complete() function returns a deferred. The triggerGame() function is
+        # added to the callback list of this deferred so that it resumes the interrupted
+        # game.game() call by firing the deferred returned by fake_runQuery.
+        # As a consequence, game.game() will complete its execution after the game has
+        # been destroyed. It must cope with this because such concurrency will happen,
+        # even if only rarely.
+        #
+        complete_deferred = self.game.complete(owner_id)
+        def triggerGame(result):
+            #
+            # call destroy to reproduce the conditions of the service.py
+            # complete() function.
+            #
+            self.game.destroy()
+            query_deferred.callback(True)
+            return result
+        complete_deferred.addCallback(triggerGame)
+        result = yield complete_deferred
+        game_result = yield game_deferred
+        self.assertEquals(result['type'], 'complete')
+        self.assertEquals(self.game.get_players(), [owner_id] + voting_players)
+        c.execute("SELECT win FROM player2game WHERE game_id = %d AND player_id != %d" % ( game_id, owner_id ))
+        self.assertEqual(u'y', c.fetchone()[0])
+        self.assertEqual(u'n', c.fetchone()[0])
+        self.assertEqual(c.fetchone(), None)
+        c.close()
+
 def Run():
     loader = runner.TestLoader()
-#    loader.methodPrefix = "test08_"
+    loader.methodPrefix = "test18_"
     suite = loader.suiteFactory()
     suite.addTest(loader.loadClass(CardstoriesGameTest))

This test fails with the following code:

    @defer.inlineCallbacks
    def game(self, player_id):
        rows = yield self.service.db.runQuery("SELECT owner_id, sentence, cards, board, state FROM games WHERE id = ?", [self.get_id()])
... some instructions ...
        rows = yield self.service.db.runQuery("SELECT player_id, cards, picked, vote, win FROM player2game WHERE game_id = ? ORDER BY serial", [ self.get_id() ])
        picked_count = 0

because the first yield blocks before some instructions are run and the second runs only after self.service has been deleted by the destroy method. The proposed patch is to cache the db value so that the execution of the function no longer depends on the availability of the self.service data member.

    @defer.inlineCallbacks
    def game(self, player_id):
        db = self.service.db
        rows = yield db.runQuery("SELECT owner_id, sentence, cards, board, state FROM games WHERE id = ?", [self.get_id()])
... some instructions ...
        rows = yield db.runQuery("SELECT player_id, cards, picked, vote, win FROM player2game WHERE game_id = ? ORDER BY serial", [ self.get_id() ])
        picked_count = 0
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