All re-randomization has to be bucketed to get a reasonable cache hit rate.

common/lib/capa/capa/capa_problem.py

@@ -17,9 +17,8 @@ from datetime import datetime
 import logging
 import math
 import numpy
-import os, os.path
+import os.path
 import re
-import struct
 import sys
 
 from lxml import etree
@@ -73,7 +72,7 @@ class LoncapaProblem(object):
 
          - problem_text (string): xml defining the problem
          - id           (string): identifier for this problem; often a filename (no spaces)
-         - seed         (int): random number generator seed (int) 
+         - seed         (int): random number generator seed (int)
          - state        (dict): containing the following keys:
                                 - 'seed' - (int) random number generator seed
                                 - 'student_answers' - (dict) maps input id to the stored answer for that input
@@ -92,23 +91,20 @@ class LoncapaProblem(object):
         if self.system is None:
             raise Exception()
 
-        state = state if state else {}
+        state = state or {}
 
         # Set seed according to the following priority:
         #       1. Contained in problem's state
         #       2. Passed into capa_problem via constructor
-        #       3. Assign from the OS's random number generator
         self.seed = state.get('seed', seed)
-        if self.seed is None:
-            self.seed = struct.unpack('i', os.urandom(4))[0]
+        assert self.seed is not None, "Seed must be provided for LoncapaProblem."
+
         self.student_answers = state.get('student_answers', {})
         if 'correct_map' in state:
             self.correct_map.set_dict(state['correct_map'])
         self.done = state.get('done', False)
         self.input_state = state.get('input_state', {})
 
-
-
         # Convert startouttext and endouttext to proper <text></text>
         problem_text = re.sub("startouttext\s*/", "text", problem_text)
         problem_text = re.sub("endouttext\s*/", "/text", problem_text)

common/lib/capa/capa/tests/__init__.py

-import fs
 import fs.osfs
-import os
+import os, os.path
 
+from capa.capa_problem import LoncapaProblem
 from mock import Mock, MagicMock
 
 import xml.sax.saxutils as saxutils
@@ -36,3 +36,7 @@ test_system = Mock(
     anonymous_student_id='student',
     cache=None,
 )
+
+def new_loncapa_problem(xml):
+    """Construct a `LoncapaProblem` suitable for unit tests."""
+    return LoncapaProblem(xml, id='1', seed=723, system=test_system)

common/lib/capa/capa/tests/test_html_render.py

@@ -6,9 +6,8 @@ import json
 
 import mock
 
-from capa.capa_problem import LoncapaProblem
 from .response_xml_factory import StringResponseXMLFactory, CustomResponseXMLFactory
-from . import test_system
+from . import test_system, new_loncapa_problem
 
 class CapaHtmlRenderTest(unittest.TestCase):
 
@@ -20,7 +19,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         xml_str = "<problem> </problem>"
 
         # Create the problem
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
 
         # Render the HTML
         rendered_html = etree.XML(problem.get_html())
@@ -39,7 +38,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         """)
 
         # Create the problem
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
 
         # Render the HTML
         rendered_html = etree.XML(problem.get_html())
@@ -61,7 +60,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         """)
 
         # Create the problem
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
 
         # Render the HTML
         rendered_html = etree.XML(problem.get_html())
@@ -80,7 +79,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         """)
 
         # Create the problem
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
 
         # Render the HTML
         rendered_html = etree.XML(problem.get_html())
@@ -98,7 +97,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         """)
 
         # Create the problem
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
 
         # Render the HTML
         rendered_html = etree.XML(problem.get_html())
@@ -121,7 +120,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         test_system.render_template.return_value = "<div>Input Template Render</div>"
 
         # Create the problem and render the HTML
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
         rendered_html = etree.XML(problem.get_html())
 
         # Expect problem has been turned into a <div>
@@ -184,7 +183,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         xml_str = CustomResponseXMLFactory().build_xml(**kwargs)
 
         # Create the problem and render the html
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
 
         # Grade the problem
         correctmap = problem.grade_answers({'1_2_1': 'test'})
@@ -219,7 +218,7 @@ class CapaHtmlRenderTest(unittest.TestCase):
         """)
 
         # Create the problem and render the HTML
-        problem = LoncapaProblem(xml_str, '1', system=test_system)
+        problem = new_loncapa_problem(xml_str)
         rendered_html = etree.XML(problem.get_html())
 
         # Expect that the variable $test has been replaced with its value

common/lib/capa/capa/tests/test_responsetypes.py

@@ -12,9 +12,8 @@ import textwrap
 import mock
 import textwrap
 
-from . import test_system
+from . import new_loncapa_problem
 
-import capa.capa_problem as lcp
 from capa.responsetypes import LoncapaProblemError, \
     StudentInputError, ResponseError
 from capa.correctmap import CorrectMap
@@ -33,7 +32,7 @@ class ResponseTest(unittest.TestCase):
 
     def build_problem(self, **kwargs):
         xml = self.xml_factory.build_xml(**kwargs)
-        return lcp.LoncapaProblem(xml, '1', system=test_system)
+        return new_loncapa_problem(xml)
 
     def assert_grade(self, problem, submission, expected_correctness, msg=None):
         input_dict = {'1_2_1': submission}

common/lib/xmodule/xmodule/capa_module.py

@@ -3,7 +3,9 @@ import datetime
 import hashlib
 import json
 import logging
+import os
 import traceback
+import struct
 import sys
 
 from pkg_resources import resource_string
@@ -23,8 +25,10 @@ from xmodule.util.date_utils import time_to_datetime
 log = logging.getLogger("mitx.courseware")
 
 
-# Generated this many different variants of problems with rerandomize=per_student
+# Generate this many different variants of problems with rerandomize=per_student
 NUM_RANDOMIZATION_BINS = 20
+# Never produce more than this many different seeds, no matter what.
+MAX_RANDOMIZATION_BINS = 1000
 
 
 def randomization_bin(seed, problem_id):
@@ -108,11 +112,7 @@ class CapaModule(CapaFields, XModule):
             self.close_date = due_date
 
         if self.seed is None:
-            if self.rerandomize == 'never':
-                self.seed = 1
-            elif self.rerandomize == "per_student" and hasattr(self.system, 'seed'):
-                # see comment on randomization_bin
-                self.seed = randomization_bin(system.seed, self.location.url)
+            self.choose_new_seed()
 
         # Need the problem location in openendedresponse to send out.  Adding
         # it to the system here seems like the least clunky way to get it
@@ -156,6 +156,22 @@ class CapaModule(CapaFields, XModule):
 
             self.set_state_from_lcp()
 
+        assert self.seed is not None
+
+    def choose_new_seed(self):
+        """Choose a new seed."""
+        if self.rerandomize == 'never':
+            self.seed = 1
+        elif self.rerandomize == "per_student" and hasattr(self.system, 'seed'):
+            # see comment on randomization_bin
+            self.seed = randomization_bin(self.system.seed, self.location.url)
+        else:
+            self.seed = struct.unpack('i', os.urandom(4))[0]
+
+            # So that sandboxed code execution can be cached, but still have an interesting
+            # number of possibilities, cap the number of different random seeds.
+            self.seed %= MAX_RANDOMIZATION_BINS
+
     def new_lcp(self, state, text=None):
         if text is None:
             text = self.data
@@ -164,6 +180,7 @@ class CapaModule(CapaFields, XModule):
             problem_text=text,
             id=self.location.html_id(),
             state=state,
+            seed=self.seed,
             system=self.system,
         )
 
@@ -831,14 +848,11 @@ class CapaModule(CapaFields, XModule):
                     'error': "Refresh the page and make an attempt before resetting."}
 
         if self.rerandomize in ["always", "onreset"]:
-            # reset random number generator seed (note the self.lcp.get_state()
-            # in next line)
-            seed = None
-        else:
-            seed = self.lcp.seed
+            # Reset random number generator seed.
+            self.choose_new_seed()
 
         # Generate a new problem with either the previous seed or a new seed
-        self.lcp = self.new_lcp({'seed': seed})
+        self.lcp = self.new_lcp(None)
 
         # Pull in the new problem seed
         self.set_state_from_lcp()

common/lib/xmodule/xmodule/tests/test_capa_module.py

@@ -550,6 +550,7 @@ class CapaModuleTest(unittest.TestCase):
     def test_reset_problem(self):
         module = CapaFactory.create(done=True)
         module.new_lcp = Mock(wraps=module.new_lcp)
+        module.choose_new_seed = Mock(wraps=module.choose_new_seed)
 
         # Stub out HTML rendering
         with patch('xmodule.capa_module.CapaModule.get_problem_html') as mock_html:
@@ -567,7 +568,8 @@ class CapaModuleTest(unittest.TestCase):
         self.assertEqual(result['html'], "<div>Test HTML</div>")
 
         # Expect that the problem was reset
-        module.new_lcp.assert_called_once_with({'seed': None})
+        module.new_lcp.assert_called_once_with(None)
+        module.choose_new_seed.assert_called_once_with()
 
     def test_reset_problem_closed(self):
         module = CapaFactory.create()
@@ -1033,3 +1035,13 @@ class CapaModuleTest(unittest.TestCase):
                 self.assertTrue(module.seed is not None)
                 msg = 'Could not get a new seed from reset after 5 tries'
                 self.assertTrue(success, msg)
+
+    def test_random_seed_bins(self):
+        # Assert that we are limiting the number of possible seeds.
+
+        # Check the conditions that generate random seeds
+        for rerandomize in ['always', 'per_student', 'true', 'onreset']:
+            # Get a bunch of seeds, they should all be in 0-999.
+            for i in range(200):
+                module = CapaFactory.create(rerandomize=rerandomize)
+                assert 0 <= module.seed < 1000