`va_arg()` causes `EXC_BAD_ACCESS` in mocked objects

[karolszafranski]

Executing a variadic method on a mocked object results in an "EXC_BAD_ACCESS" crash.

The variadic method does not have to be executed with OCMock (https://github.com/erikdoe/ocmock/issues/191). It occurs even if called directly.

#import <XCTest/XCTest.h>
#import <OCMock.h>

@interface BaseClass: NSObject
@end

@implementation BaseClass

- (void)logLevel:(NSUInteger)messageLogLevel message:(NSString *)format, ... {
    va_list args;
    va_start(args, format);
    NSString* message = [[NSString alloc] initWithFormat:format arguments:args];
    va_end(args);
    NSLog(@"%li: %@", messageLogLevel, message);
}

@end

@interface OCMockFailureTestTests : XCTestCase
@end

@implementation OCMockFailureTestTests

- (void)testExample {
    BaseClass* obj = [BaseClass new];
    id mock = [OCMockObject partialMockForObject:obj];

    [obj logLevel:3 message:@"%@", @"abc"]; // crash
}

@end

Executed on an iOS Simulator running on a Macbook Pro with M1, not reproducible with i5.

OCMock v3.9.1

[JeromeTonnelierOgury]

Same here! Any news ?

[erikdoe]

Still don't have access to a Mac with Apple Silicon.

[marchv]

I've looked a little at this and it seems like NSInvocation is used. I then skimmed the documentation and found

I have no experience with NSInvocation but I am wondering if those if those two sections are fulfilled for OCMock's implementation or if OCMock just doesn't support variable arguments for the time being?

[smorr]

The best workaround I have is to get OCPartialMock to ignore (or "demock") specific selectors.

When mocking an object, OCM adds all the methods of the real object to call objc_msgForward. This allows OCM to handle the stubs. but because _objc_msgForward wraps the message into an NSInvocation, it will fail for variadic args.

The thing I do is redirect specific methods on the mockObject back to the original implementation of the real object. so that any calls to the variadic argument method is calling the actual method, rather than forwarding using an NSInvocation. While it means there is no test mocking for that method 😔, it also means that things won't crash 😃

I have written a category on OCPartialMockObject to do this and a macro "OCMIgnore" for convenience:

and added it to the top of my XCTTest Class.

#define OCMIgnore(object,method) do { if (object_getClass(object) == OCPartialMockObject.class) [(OCPartialMockObject*)object _ocm_ignoreMethod:method];} while (0)

@interface  OCPartialMockObject : NSProxy  @end // unimplemented declaration so the category implementation doesn't fail

@implementation OCPartialMockObject(SC)
-(void)_ocm_ignoreMethod:(SEL)methodSelector{
    Ivar ivar = class_getInstanceVariable(OCPartialMockObject.class,"realObject");
    if (ivar){
        id mockObject = object_getIvar(self, ivar); // despite ivar name, the value is the created Mock object
        if (mockObject){
            Method mockMethod = class_getInstanceMethod(object_getClass(mockObject), methodSelector);
            Method realMethod = class_getInstanceMethod(self.class, methodSelector);
            if (realMethod && mockMethod){
                IMP realIMP = method_getImplementation(realMethod);
                IMP mockIMP = method_getImplementation(mockMethod);
                if (mockIMP && realIMP && mockIMP == _objc_msgForward){
                    method_setImplementation(mockMethod, realIMP);
                }
            }
        }
    }
}
@end

Usage:

@interface MyObject : NSObject
-(void)log:(NSString*)format , ...;
@end

// Partial mock a new object
MYObject * mockObject = OCMPartialMock([MyObject.alloc init]);

// ignore (undo the mock) of the following selector:
OCMIgnore(mockObject,@selector(log:));

BTW this can be used for any method, not just ones that have variadic arguments.

[smorr]

Actually in further use it is more complicated than this because stubbing new methods will rewire implementation pointers after an ignore :(

Wince my comment yesterday, I reworked things so that you can ignore mocking a specific selector for a class (and its descendent classes).

OCMIgnore([MyClass class],@selector(log:));

now registers the log: selector for the class as to be ignored. When mocking, stubbing and message forwarding, OCM mock will lookup the selector to by forwarded and ensure it is not forwarded in the mocking process. an that the real object gets the message:

The changes are multifold but can be found in pull request https://github.com/erikdoe/ocmock/pull/539