Class Mocks Not Thread-Safe #328

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1. lilyball commented on Jan 9, 2018

   

   lilyball

   on Jan 9, 2018

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   I just hit this same issue, with OCMock 3.3 (which the release notes claims makes mocking thread-safe).

   I believe the root issue here is OCMock's class mocking isa-swizzles the metaclass prior to installing methods. This means that background threads may start interacting with the metaclass while it's in the process of installing methods, which is pretty bad.
2. lilyball commented on Jan 9, 2018

   

   lilyball

   on Jan 9, 2018

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   Since I'm not sure if moving the metaclass swizzle to the end is safe (I don't know if any of the intervening code relies on it already being installed), I believe a safe fix is to reorder two lines in -setupForwarderForClassMethodSelector::

   diff --git a/Pods/OCMock/Source/OCMock/OCClassMockObject.m b/Pods/OCMock/Source/OCMock/OCClassMockObject.m
   index f8a8180..39b5f22 100644
   --- a/Pods/OCMock/Source/OCMock/OCClassMockObject.m
   +++ b/Pods/OCMock/Source/OCMock/OCClassMockObject.m
   @@ -148,8 +148,8 @@ - (void)setupForwarderForClassMethodSelector:(SEL)selector
    
        Class metaClass = object_getClass(mockedClass);
        IMP forwarderIMP = [originalMetaClass instanceMethodForwarderForSelector:selector];
   -    class_replaceMethod(metaClass, selector, forwarderIMP, types);
        class_addMethod(metaClass, aliasSelector, originalIMP, types);
   +    class_replaceMethod(metaClass, selector, forwarderIMP, types);
    }
3. lilyball commented on Jan 9, 2018

   

   lilyball

   on Jan 9, 2018

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   @erikdoe I assume you're getting notifications for issue updates anyway, but just in case, I'm mentioning you as this is a nasty issue that has been causing spurious test failures for us for a long time and I'm surprised that this ticket never got a response.
4. lilyball commented on Oct 18, 2018

   

   lilyball

   on Oct 18, 2018

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   @erikdoe Can you please take a look at this? It's a real issue with a trivial fix.

   FWIW, it turns out that uninstalling class mocks isn't thread safe either, which doesn't seem to be a solvable short of simply never destroying the dynamically-created class pairs, but in my experience the uninstall issue crops up a lot less frequently than the install issue did.
5. erikdoe

   added a commit that references this issue on Oct 22, 2018

   Partial fix for #328.

   0b809a5
6. erikdoe commented on Oct 22, 2018

   

   erikdoe

   on Oct 22, 2018

   Last edited by erikdoe

   Owner

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   Sorry. I should have responded earlier. My concern is that the failing test provided in the issue tests a scenario that, as far as I can tell, cannot be supported by OCMock. It has always been clear that OCMock is only thread-safe to a certain degree. Repeatedly calling a method while setting up and removing a mock would, as far as I am aware, require most of the operations on the class to be atomic, and I don't see a way how to achieve this.

   The fix provided by @kballard looks benign, but it doesn't fix the issue reported. In fact, there is no test that I can think of that would fail without the change but pass with it. That said, the change is small and it feels like it brings the two operations into a "more correct" sequence. So I have accepted it now.

   👍React with 👍1Reacted by Lily Ballard
7. erikdoe

   added

   enhancement

   on Oct 22, 2018
8. erikdoe

   closed this as completedon Feb 16, 2020