[TSan][Test-Only] Account for race in cxa_guard_acquire.cpp test

It is possible for "Enter potentially blocking region" to come before "Enter constructor" in this test - if the thread that acquires the guard fails to reach its printf before the other thread that enters the blocking region reaches its own printf. Note that for the exit logs this inversion is not possible.

This patch addresses this by allowing those two log lines to come in either order.

Author: DanBlackwell

compiler-rt/test/tsan/cxa_guard_acquire.cpp

@@ -66,10 +66,17 @@ int main(int argc, char **argv) {
   printf("Enter main\n");
 
   // If initialization is contended, the blocked thread should enter a
-  // potentially blocking region.
+  // potentially blocking region. Note that we use a DAG check because it is
+  // possible for Thread 1 to acquire the guard, then Thread 2 fail to acquire
+  // the guard then call `OnPotentiallyBlockingRegionBegin` and print "Enter
+  // potentially blocking region\n", before Thread 1 manages to reach "Enter
+  // constructor\n". This is exceptionally rare, but can be replicated by
+  // inserting a `sleep(1)` between `LazyInit() {` and `printf("Enter
+  // constructor\n");`. Due to the barrier it is not possible for the exit logs
+  // to be inverted.
   //
-  // CHECK-NEXT: Enter constructor
-  // CHECK-NEXT: Enter potentially blocking region
+  // CHECK-DAG: Enter constructor
+  // CHECK-DAG: Enter potentially blocking region
   // CHECK-NEXT: Exit constructor
   // CHECK-NEXT: Exit potentially blocking region
   barrier_init(&barrier, 2);