tests: fix race condition in timestamp checking

Linux kernel caches a "coarse" time that is only updated by timer
ticks in a periodic way (see CONFIG_HZ).
This introduces an artifical delay to ensure the coarse cached time
has been updated between timestamp reads.
Sleeping for 100ms should be enough to ensure several timer interrrupts
elapsed, for common kernel configurations.

Author: lucab

src/lib.rs

@@ -908,21 +908,16 @@ mod tests {
         let td = tempfile::tempdir().unwrap();
         let d = openat::Dir::open(td.path()).unwrap();
         d.ensure_dir("foo", 0o755).unwrap();
-        d.syncfs().unwrap();
         let before = d.metadata("foo").unwrap();
+        // File timestamps can not be updated faster than kernel ticking granularity,
+        // so this artificially sleeps through several timer interrupts.
+        std::thread::sleep(std::time::Duration::from_millis(100));
 
         d.update_timestamps("foo").unwrap();
-        d.syncfs().unwrap();
         let after = d.metadata("foo").unwrap();
-
-        assert!(
-            before.stat().st_atime != after.stat().st_atime
-                || before.stat().st_atime_nsec != after.stat().st_atime_nsec
-        );
-        assert!(
-            before.stat().st_mtime != after.stat().st_mtime
-                || before.stat().st_mtime_nsec != after.stat().st_mtime_nsec
-        );
+        if before.stat().st_mtime == after.stat().st_mtime {
+            assert_ne!(before.stat().st_mtime_nsec, after.stat().st_mtime_nsec);
+        }
     }
 
     #[test]