add test for setfsuid

src/unistd.rs

@@ -2751,3 +2751,42 @@ impl Group {
         })
     }
 }
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::{fs, thread, io};
+
+    /// Tests setting the filesystem UID with `setfsuid`.
+    #[cfg(any(target_os = "linux", target_os = "android"))]
+    #[test]
+    fn test_setfsuid() {
+        use std::os::unix::fs::PermissionsExt;
+
+        // get the UID of the "nobody" user
+        let nobody = User::from_name("nobody").unwrap().unwrap();
+
+        // create a temporary file with permissions '-rw-r-----'
+        let file = tempfile::NamedTempFile::new().unwrap();
+        let temp_path = file.into_temp_path();
+        let temp_path_2 = (&temp_path).to_path_buf();
+        let mut permissions = fs::metadata(&temp_path).unwrap().permissions();
+        permissions.set_mode(640);
+
+        // spawn a new thread where to test setfsuid
+        thread::spawn(move || {
+            // set filesystem UID
+            let _ = setfsuid(nobody.uid);
+            // try to open the temporaty file should fail with EACCES
+            let res = fs::File::open(&temp_path);
+            assert!(res.is_err());
+            assert_eq!(res.err().unwrap().kind(), io::ErrorKind::PermissionDenied);
+        })
+        .join()
+        .unwrap();
+
+        // open the temporary file with the current thread filesystem UID
+        fs::File::open(temp_path_2).unwrap();
+    }
+}