TSan false positive with dispatch_apply() over an Objective-C NSArray (on macOS)

[seanm]

On macOS 12.x and 13.x on both x86_64 and Arm64 with Xcode this simple code in a toy app:

NSArray* array = @[@1, @2, @3, @4, @5, @6, @7];

dispatch_apply([array count], DISPATCH_APPLY_AUTO, ^(size_t idx) {
	NSNumber* unused = array[idx]; // TSan says race here?!
	(void)unused;
});

Causes a TSan warning:

WARNING: ThreadSanitizer: data race (pid=33556)
  Read of size 8 at 0x00010b863380 by thread T6:
    #0 __45-[AppDelegate applicationDidFinishLaunching:]_block_invoke <null> (TSanTest:arm64+0x100002d4c)
    #1 __wrap_dispatch_apply_block_invoke <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64+0x7db4c)
    #2 _dispatch_client_callout2 <null> (libdispatch.dylib:arm64e+0x6a10)

  Previous write of size 8 at 0x00010b863380 by main thread:
    #0 __copy_helper_block_e8_32s <null> (TSanTest:arm64+0x100002874)
    #1 _call_copy_helpers_excp <null> (libsystem_blocks.dylib:arm64e+0x2120)
    #2 -[AppDelegate applicationDidFinishLaunching:] <null> (TSanTest:arm64+0x100002c9c)
    #3 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ <null> (CoreFoundation:arm64e+0x7655c)
    #4 <null> <null> (0x0001b16b7e50)

  Location is heap block of size 40 at 0x00010b863360 allocated by main thread:
    #0 malloc <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64+0x54e90)
    #1 _Block_copy <null> (libsystem_blocks.dylib:arm64e+0x1b30)
    #2 -[AppDelegate applicationDidFinishLaunching:] <null> (TSanTest:arm64+0x100002c9c)
    #3 __CFNOTIFICATIONCENTER_IS_CALLING_OUT_TO_AN_OBSERVER__ <null> (CoreFoundation:arm64e+0x7655c)
    #4 <null> <null> (0x0001b16b7e50)

  Thread T6 (tid=1384939, running) is a GCD worker thread

I'm 99% sure there's no bug in my example code above.

I figure this is either:

1. A TSan false positive, hence this ticket
2. a macOS bug (I filed FB9995377 too)

(I'd appreciate if anyone knows which.)