Fix race condition in ImageDownloader
#4587
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azarovalex
commented
Jan 22, 2024
| @@ -6,7 +6,7 @@ typealias CachedResponseCompletionHandler = (CachedURLResponse?, Error?) -> Void | |||
| typealias ImageDownloadCompletionHandler = (DownloadError?) -> Void | |||
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| protocol ReentrantImageDownloader { | |||
| func download(with url: URL, completion: CachedResponseCompletionHandler?) -> Void | |||
| func download(with url: URL, completion: @escaping CachedResponseCompletionHandler) -> Void | |||
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I don't see why we would ever want completion to be nil, changed to non-nil.
Codecov ReportAttention:
@@ Coverage Diff @@
## main #4587 +/- ##
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- Coverage 59.79% 1.07% -58.72%
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Files 189 189
Lines 21218 21215 -3
==========================================
- Hits 12687 228 -12459
- Misses 8531 20987 +12456
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1 task
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Closing in favor of #4588 |
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ImageDownloader uses a custom
NSOperationsubclass to asynchronously download images.Instead of overriding
isConcurrentflag, this subclass has to overrideisAsynchronousflag.isConcurrentis a legacy flag, from the tests I did it seems it's never called by the system.It seems like right now the fact that we don't correctly set
isAsynchronouscan lead to crashes in some cases because by default all NSOperations are considered synchronous and iOS can deallocate them before the download is actually finished. This should be fixed by correctly marking our NSOperation asisAsynchronous == true.I also fixed another potential issue in
ImageDownloaderwhere we add a completion handler to an operation only after adding this operation to a queue, in theory this might lead to a race, though I think this could never cause issues in production.Another issue yet unfixed is:
It happens because our custom subclass of NSOperation doesn't handle correctly a state when it's cancelled before NSOperationQueue started it.