sync: synchronize the methods used in the w3d4 test to mini-lsm-starter (#141)

Co-authored-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-starter/src/lsm_storage.rs

@@ -248,6 +248,10 @@ impl LsmStorageInner {
             .fetch_add(1, std::sync::atomic::Ordering::SeqCst)
     }
 
+    pub(crate) fn mvcc(&self) -> &LsmMvccInner {
+        self.mvcc.as_ref().unwrap()
+    }
+
     /// Start the storage engine by either loading an existing directory or creating a new one if the directory does
     /// not exist.
     pub(crate) fn open(path: impl AsRef<Path>, options: LsmStorageOptions) -> Result<Self> {

mini-lsm-starter/src/mvcc/watermark.rs

@@ -33,7 +33,7 @@ impl Watermark {
     pub fn remove_reader(&mut self, ts: u64) {}
 
     pub fn num_retained_snapshots(&self) -> usize {
-        0
+        self.readers.len()
     }
 
     pub fn watermark(&self) -> Option<u64> {