Don't use a buffer we've extended as a cache key

Author: DRMacIver

hypothesis-python/RELEASE.rst

@@ -0,0 +1,5 @@
+RELEASE_TYPE: patch
+
+This release fixes a small caching bug in Hypothesis internals that may under
+some circumstances have resulted in a less diverse set of test cases being
+generated than was intended.

hypothesis-python/src/hypothesis/internal/conjecture/engine.py

@@ -1033,7 +1033,8 @@ def kill_branch(self):
  )
  self.test_function(data)
  result = check_result(data.as_result())
- self.__data_cache[buffer] = result
+ if extend == 0 or len(result.buffer) <= len(buffer):
+ self.__data_cache[buffer] = result
  return result
 
  def event_to_string(self, event):

hypothesis-python/tests/conjecture/test_engine.py

@@ -1465,3 +1465,34 @@ def test(data):
  except RunIsComplete:
  pass
  assert runner.optimise_targets.call_count == 0
+
+
+def test_does_not_cache_extended_prefix():
+ def test(data):
+ data.draw_bits(64)
+
+ with deterministic_PRNG():
+ runner = ConjectureRunner(test, settings=TEST_SETTINGS)
+
+ d1 = runner.cached_test_function(b"", extend=8)
+ d2 = runner.cached_test_function(b"", extend=8)
+ assert d1.status == d2.status == Status.VALID
+
+ assert d1.buffer != d2.buffer
+
+
+def test_does_cache_if_extend_is_not_used():
+ calls = [0]
+
+ def test(data):
+ calls[0] += 1
+ data.draw_bits(8)
+
+ with deterministic_PRNG():
+ runner = ConjectureRunner(test, settings=TEST_SETTINGS)
+
+ d1 = runner.cached_test_function(b"\0", extend=8)
+ d2 = runner.cached_test_function(b"\0", extend=8)
+ assert d1.status == d2.status == Status.VALID
+ assert d1.buffer == d2.buffer
+ assert calls[0] == 1