Thread race on HWIFileDownloader.activeDownloadsDictionary

[rhcpfan]

I'm getting thread race warnings for HWIFileDownloader.activeDownloadsDictionary (of type NSMutableDictionary<NSNumber *, HWIFileDownloadItem *>).

When I open my application, I verify if a certain number of files are downloaded on the device storage (±80 files). For the ones that are missing, I'm starting the download process (in parallel).

The race condition that I get is related to

- (void)startDownloadWithDownloadToken:(nonnull NSString *)aDownloadToken
                         fromRemoteURL:(nullable NSURL *)aRemoteURL
                       usingResumeData:(nullable NSData *)aResumeData

At line 313, we add the aDownloadItem in the activeDownloadsDictionary ([self.activeDownloadsDictionary setObject:aDownloadItem forKey:@(aDownloadID)];)

At the same time, on another thread, a download has finished and

- (void)handleSuccessfulDownloadToLocalFileURL:(nonnull NSURL *)aLocalFileURL
                                  downloadItem:(nonnull HWIFileDownloadItem *)aDownloadItem
                                    downloadID:(NSUInteger)aDownloadID

is called. Inside this method, we remove the download item from the activeDownloadsDictionary ([self.activeDownloadsDictionary removeObjectForKey:@(aDownloadID)];).

Here's the full Thread Sanitizer output

==================
WARNING: ThreadSanitizer: race on NSMutableDictionary (pid=18849)
  Modifying access of NSMutableDictionary at 0x7b08002e6180 by thread T10:
    #0 -[__NSDictionaryM setObject:forKey:] <null> (CoreFoundation:x86_64+0x19160b)
    #1 -[HWIFileDownloader startDownloadWithDownloadToken:fromRemoteURL:usingResumeData:] <null> (HWIFileDownload:x86_64+0x63ba)
    #2 -[HWIFileDownloader startDownloadWithIdentifier:fromRemoteURL:] <null> (HWIFileDownload:x86_64+0x4848)
    #3 -[DVTemplateDownloadStore startDownloadForTemplateFileModel:] <null>
    #4 -[DVTemplateDownloadStore startDownloadingIncompleteFilesInTemplateArray:] <null>
    #5 __52-[DVMainViewController verifyUserTemplatesIntegrity]_block_invoke <null>
    #6 __tsan::invoke_and_release_block(void*) <null> (libclang_rt.tsan_iossim_dynamic.dylib:x86_64+0x676ab)
    #7 _dispatch_client_callout <null> (libdispatch.dylib:x86_64+0x3db4)

  Previous modifying access of NSMutableDictionary at 0x7b08002e6180 by main thread:
    #0 -[__NSDictionaryM removeObjectForKey:] <null> (CoreFoundation:x86_64+0x1912ef)
    #1 -[HWIFileDownloader handleSuccessfulDownloadToLocalFileURL:downloadItem:downloadID:] <null> (HWIFileDownload:x86_64+0x1a6fd)
    #2 -[HWIFileDownloader URLSession:task:didCompleteWithError:] <null> (HWIFileDownload:x86_64+0x105f9)
    #3 __51-[NSURLSession delegate_task:didCompleteWithError:]_block_invoke.182 <null> (CFNetwork:x86_64+0x1bf001)
    #4 _dispatch_client_callout <null> (libdispatch.dylib:x86_64+0x3db4)
    #5 start <null> (libdyld.dylib:x86_64+0x1540)

  Location is heap block of size 32 at 0x7b08002e6180 allocated by main thread:
    #0 calloc <null> (libclang_rt.tsan_iossim_dynamic.dylib:x86_64+0x4a6b2)
    #1 class_createInstance <null> (libobjc.A.dylib:x86_64+0xf34f)
    #2 -[HWIFileDownloader initWithDelegate:maxConcurrentDownloads:] <null> (HWIFileDownload:x86_64+0x1b4b)
    #3 -[HWIFileDownloader initWithDelegate:] <null> (HWIFileDownload:x86_64+0x179f)
    #4 __40+[DVTemplateDownloadStore sharedManager]_block_invoke <null>
    #5 dispatch_once <null> (libclang_rt.tsan_iossim_dynamic.dylib:x86_64+0x682d4)
    #6 +[DVTemplateDownloadStore sharedManager] <null>
    #7 -[TemplateModel isDownloaded] <null>
    #8 -[DVMainViewController initTemplateView] <null> 
    #9 -[DVMainViewController viewWillAppear:] <null>
    #10 -[UIViewController _setViewAppearState:isAnimating:] <null> (UIKitCore:x86_64+0x351436)
    #11 start <null> (libdyld.dylib:x86_64+0x1540)

  Thread T10 (tid=1095123, running) is a GCD worker thread

SUMMARY: ThreadSanitizer: race on NSMutableDictionary (/Applications/Xcode-10.2.1.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/Library/CoreSimulator/Profiles/Runtimes/iOS.simruntime/Contents/Resources/RuntimeRoot/System/Library/Frameworks/CoreFoundation.framework/CoreFoundation:x86_64+0x19160b) in -[__NSDictionaryM setObject:forKey:]
==================