[compiler-rt] [TSan] Add lock_during_write flag on Apple platforms to avoid deadlock (PR #157928)

[Dan Blackwell via llvm-commits]

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@@ -1649,6 +1652,13 @@ TSAN_INTERCEPTOR(int, pthread_barrier_wait, void *b) {
 
 TSAN_INTERCEPTOR(int, pthread_once, void *o, void (*f)()) {
   SCOPED_INTERCEPTOR_RAW(pthread_once, o, f);
+#if SANITIZER_APPLE
+  if (flags()->lock_during_write != kLockDuringAllWrites &&
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DanBlackwell wrote:

TODO: Add a comment to explain _why_ this is necessary.

https://github.com/llvm/llvm-project/pull/157928