Intermittent crash on CFRelease() called with NULL for some JPG images - ios

Question

In my iOS app, I download game assets from s3. My artist updated one of the assets, and we immediately started seeing crashes with the new asset.
Here's the code that processes them:
+ (UIImage *)imageAtPath:(NSString *)imagePath scaledToSize:(CGSize)size
{
// Create the image source (from path)
CGImageSourceRef imageSource = CGImageSourceCreateWithURL((__bridge CFURLRef) [NSURL fileURLWithPath:imagePath], NULL);
NSParameterAssert(imageSource);
// Get the image dimensions (without loading the image into memory)
CGFloat width = 512.0f, height = 384.0f;
CFDictionaryRef imageProperties = CGImageSourceCopyPropertiesAtIndex(imageSource, 0, NULL);
NSParameterAssert(imageProperties);
if (imageProperties) {
CFNumberRef widthNumRef = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelWidth);
if (widthNumRef != NULL) {
CFNumberGetValue(widthNumRef, kCFNumberCGFloatType, &width);
}
CFNumberRef heightNumRef = CFDictionaryGetValue(imageProperties, kCGImagePropertyPixelHeight);
if (heightNumRef != NULL) {
CFNumberGetValue(heightNumRef, kCFNumberCGFloatType, &height);
}
CFRelease(imageProperties);
} else {
// If the image info is somehow missing, make up some numbers so we don't divide by zero
width = 512;
height = 384;
}
// Create thumbnail options
CGFloat maxDimension = size.height;
if (useDeviceNativeScale) {
maxDimension *= [UIScreen mainScreen].scale;
}
NSParameterAssert(maxDimension);
// If we have a really wide image, scaling it to the screen height will make it too blurry.
// Here we calculate the maximum dimension we want (probably width) to make the image be the full height of the device
CGFloat imageAspectRatio = width / height;
if (width > height) {
maxDimension *= imageAspectRatio;
}
CFDictionaryRef options = (__bridge CFDictionaryRef) #{
(id) kCGImageSourceCreateThumbnailWithTransform : #YES,
(id) kCGImageSourceCreateThumbnailFromImageAlways : #YES,
(id) kCGImageSourceShouldCache : #YES,
(id) kCGImageSourceThumbnailMaxPixelSize : #(maxDimension)
};
// Generate the thumbnail
CGImageRef thumbnail = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options);
CFRelease(imageSource);
// 🔻 Crashlytics says the crash is on this line, even though the line above is the one that uses `CFRelease()`
UIImage *image = [UIImage imageWithCGImage:thumbnail];
CGImageRelease(thumbnail);
NSAssert(image.size.height - SCREEN_MIN_LENGTH * [UIScreen mainScreen].scale < 1, #"Image should be the height of the view");
return image;
}
And here's a stack trace from Fabric's Crashlytics:
0. Crashed: com.apple.main-thread
0 CoreFoundation 0x1820072a0 CFRelease + 120
1 Homer 0x10014e7f0 +[UIImage(ResizeBeforeLoading) imageAtPath:scaledToSize:] (UIImage+ResizeBeforeLoading.m:98)
With this crash info key:
CRASH_INFO_ENTRY_0
* CFRelease() called with NULL *
The odd thing is that this crash is not 100%, it's happening to a very low percentage of users. I can't reproduce it on any of my own devices. There is also no pattern to what version of iOS it happens on, nor what iOS hardware it happens on.
Here is a file that does not cause a crash:
And here is a file that does cause a crash:
And here are links to them in my production image host (s3 with cloudfront):
(good)
http://d3iq9oupxk0b1m.cloudfront.net/PirateDinosaur/2x/dinoBack._k91G0.jpg
(crashy)
http://d3iq9oupxk0b1m.cloudfront.net/PirateDinosaur/2x/PirateDino.3LHRmc.jpg
I can't see a meaningful difference between them, even when I look at them using Imagemagick's identify -verbose <filename>. Can any jpeg experts weigh in?
Note: In my next app version, to be released, I have added guards around releasing NULL references, to protect against this crash:
if (imageSource) { CFRelease(imageSource), imageSource = nil; }
...
if (thumbnail) { CGImageRelease(thumbnail), thumbnail = nil; }
However, I would still like to know what in the world is wrong with this jpeg that is causing this crash, so my artist can avoid it in the future.