Merge pull request #4525 from Liam-DeVoe/threading-wrapped-strategy

Fix a race condition in `DeferredStrategy.wrapped_strategy`

Author: Zac-HD

hypothesis-python/RELEASE.rst

@@ -0,0 +1,3 @@
+RELEASE_TYPE: patch
+
+Fixes a race condition under threading when using |st.deferred|.

hypothesis-python/src/hypothesis/strategies/_internal/deferred.py

@@ -35,15 +35,18 @@ def __init__(self, definition: Callable[[], SearchStrategy[Ex]]):
 
     @property
     def wrapped_strategy(self) -> SearchStrategy[Ex]:
+        # we assign this before entering the condition to avoid a race condition
+        # under threading. See issue #4523.
+        definition = self.__definition
         if self.__wrapped_strategy is None:
             check_sideeffect_during_initialization("deferred evaluation of {!r}", self)
 
-            if not inspect.isfunction(self.__definition):
+            if not inspect.isfunction(definition):
                 raise InvalidArgument(
-                    f"Expected definition to be a function but got {self.__definition!r} "
-                    f"of type {type(self.__definition).__name__} instead."
+                    f"Expected definition to be a function but got {definition!r} "
+                    f"of type {type(definition).__name__} instead."
                 )
-            result = self.__definition()
+            result = definition()
             if result is self:
                 raise InvalidArgument("Cannot define a deferred strategy to be itself")
             check_strategy(result, "definition()")

hypothesis-python/tests/conjecture/test_shrinker.py

@@ -625,7 +625,7 @@ def shrinker(data: ConjectureData):
     )
     assert (
         # pytest.approx for differences in floating-point summations
-        pytest.approx(shrinker.choices[0] + shrinker.choices[1])
+        pytest.approx(shrinker.choices[0] + shrinker.choices[1], rel=0.001)
         == node1.value + node2.value
     )