Merge pull request #2147 from Zac-HD/fix-prng-1919

Check for PRNG state leaks

hypothesis-python/tests/conftest.py

@@ -18,11 +18,13 @@
 from __future__ import absolute_import, division, print_function
 
 import gc
+import random
 import sys
 import time as time_module
 
 import pytest
 
+from hypothesis.internal.detection import is_hypothesis_test
 from tests.common import TIME_INCREMENT
 from tests.common.setup import run
 
@@ -76,3 +78,31 @@ def freeze():
         assert sys.version_info[0] == 2
     monkeypatch.setattr(time_module, "sleep", sleep)
     monkeypatch.setattr(time_module, "freeze", freeze, raising=False)
+
+
+random_states_after_tests = {}
+independent_random = random.Random()
+
+
+@pytest.hookimpl(hookwrapper=True)
+def pytest_runtest_call(item):
+    # This hookwrapper checks for PRNG state leaks from Hypothesis tests.
+    # See: https://github.com/HypothesisWorks/hypothesis/issues/1919
+    if not (hasattr(item, "obj") and is_hypothesis_test(item.obj)):
+        yield
+    else:
+        # We start by peturbing the state of the PRNG, because repeatedly
+        # leaking PRNG state resets state_after to the (previously leaked)
+        # state_before, and that just shows as "no use of random".
+        random.seed(independent_random.randrange(2 ** 32))
+        before = random.getstate()
+        yield
+        after = random.getstate()
+        if before != after:
+            if after in random_states_after_tests:
+                raise Exception(
+                    "%r and %r both used the `random` module, and finished with the "
+                    "same global `random.getstate()`; this is probably a nasty bug!"
+                    % (item.nodeid, random_states_after_tests[after])
+                )
+            random_states_after_tests[after] = item.nodeid