Question

I’ve had a very rarely occurring bug where a key in an NSMutableDictionary instance ends up pointing to the wrong object (another object in the same dictionary).

Could “key confusion” be the result of a race condition?

Here’s the code (I’ve put in new thread safety since):

- (NSNumber*) makeHashFromResizedImage:(UIImage*)original newSize:(CGSize)newSize {
    int retVal = original.hash + newSize.width * 2 + newSize.height * 4;
    return [NSNumber numberWithInteger:retVal];
}

- (UIImage *)resizeImage:(UIImage*)image newSize:(CGSize)newSize {
    NSNumber *key = [self makeHashFromResizedImage:image newSize:newSize];

    if ([self.resizedImages objectForKey:key]) {
        return [self.resizedImages objectForKey:key];
    }

    UIImage *newImage = {{image-resizing-code-here}};

    [self.resizedImages setObject:newImage forKey:key];

    return newImage;
}

Note: {{image-resizing-code-here}} was removed for brevity. I’m beginning to suspect my makeHashFromResizedImage as it probably doesn’t work as expected. Going to Unit Test it now.

Answer 1

This:

if ([self.resizedImages objectForKey:key]) {
    return [self.resizedImages objectForKey:key];
}

If another thread called setObject:forKey: with that key the result of the 2nd line will be different from the first line. Why not just store it in a variable? At least the thing you check and the thing you return will be the same.

UIImage* image = [[[[self.resizedImages objectForKey:key] retain] autorelease];
if (image) {
    return image;
}

(The retain + autorelease is just to retain image won’t be -dealloc-ed when this thread is running.)

If you have to ensure that the key corresponding to that image will not be overwritten before the function returns, you’ll need to use a lock, as others answered.

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Edit: Perhaps you should use an NSMutableSet of the UIImage instead. Especially, don’t use a hash value as a key! A hash value does not need to be different when the two objects are not the same. Create a new class X which contains a CGSize and UIImage property, and use a NSMutableSet of X, if the same UIImage can be added multiple times.