feat(code): finish part 4

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm/src/mem_table.rs

@@ -1,4 +1,5 @@
 use std::ops::Bound;
+use std::sync::atomic::AtomicBool;
 use std::sync::Arc;
 
 use anyhow::Result;
@@ -13,6 +14,7 @@ use crate::table::SsTableBuilder;
 /// A basic mem-table based on crossbeam-skiplist
 pub struct MemTable {
     map: Arc<SkipMap<Bytes, Bytes>>,
+    sealed: AtomicBool,
 }
 
 pub(crate) fn map_bound(bound: Bound<&[u8]>) -> Bound<Bytes> {
@@ -28,20 +30,24 @@ impl MemTable {
     pub fn create() -> Self {
         Self {
             map: Arc::new(SkipMap::new()),
+            sealed: AtomicBool::new(false),
         }
     }
 
     /// Get a value by key.
-    pub fn get(&self, key: &[u8]) -> Result<Option<Bytes>> {
-        let entry = self.map.get(key).map(|e| e.value().clone());
-        Ok(entry)
+    pub fn get(&self, key: &[u8]) -> Option<Bytes> {
+        self.map.get(key).map(|e| e.value().clone())
     }
 
-    /// Put a key-value pair into the mem-table.
-    pub fn put(&self, key: &[u8], value: &[u8]) -> Result<()> {
+    /// Put a key-value pair into the mem-table. If the current mem-table is sealed, return false.
+    pub fn put(&self, key: &[u8], value: &[u8]) -> bool {
+        use std::sync::atomic::Ordering;
+        if self.sealed.load(Ordering::Acquire) {
+            return false;
+        }
         self.map
             .insert(Bytes::copy_from_slice(key), Bytes::copy_from_slice(value));
-        Ok(())
+        true
     }
 
     /// Get an iterator over a range of keys.
@@ -65,6 +71,12 @@ impl MemTable {
         }
         Ok(())
     }
+
+    /// Disable writes to this memtable.
+    pub(crate) fn seal(&self) {
+        use std::sync::atomic::Ordering;
+        self.sealed.store(true, Ordering::Release);
+    }
 }
 
 type SkipMapRangeIter<'a> =