Troubleshooting when success is not success

[eimermusic]

I never see this in my own testing but Sentry crash reports show that some real-world users are getting the following issue. I wonder how I continue to diagnose it.

TL;DR

Fetch (NetworkFetcher) calls the success closure but the data is apparently not ok.

           myCache.fetch(URL: url, failure: {error in
                print("Image fetch error \(String(describing: error)) setting image \(url)")
            }, success: {data in
                print("loaded image: \(url)")
                if let image = UIImage(data: data) {
                    self.scrollView.display(image: image)
                } else {
                    // This is where I want to figure out why data is not actually the image data.
                    // Would it make sense to remove this item from the cache here and have it re-download?
                }
            })

Questions / Ideas

• Is Haneke caching the http response for 4XX or 5XX results? Then I might occasionally get a bad result from a sad server into the cache.

• Some users could have a lot of images. Could this be a timing issue where the app gets a memory warning and the data is cleared before my code uses it to make a UIImage?