DEBUG test if this pleases TSAN

src/backend/mod.rs

@@ -674,13 +674,19 @@ extern "C" fn audiounit_input_callback(
 
 fn host_time_to_ns(host_time: u64) -> u64 {
     static HOST_TIME_TO_NS_RATIO: OnceLock<(u32, u32)> = OnceLock::new();
-    let (num, den) = HOST_TIME_TO_NS_RATIO.get_or_init(|| {
+    let mut val = HOST_TIME_TO_NS_RATIO.get();
+    if val.is_none() {
+        // Slow path with extra get() because TSAN flags OnceLock as having a data race
+        // when dereferencing `num` below.
         let mut timebase_info = mach_timebase_info { numer: 0, denom: 0 };
         unsafe {
             mach_timebase_info(&mut timebase_info);
         }
-        (timebase_info.numer, timebase_info.denom)
-    });
+        let new_val = (timebase_info.numer, timebase_info.denom);
+        HOST_TIME_TO_NS_RATIO.set(new_val);
+        val = HOST_TIME_TO_NS_RATIO.get();
+    }
+    let (num, den) = val.unwrap();
     let mut rv: f64 = host_time as f64;
     rv *= *num as f64;
     rv /= *den as f64;