Allow retrieving WriteBatch serialized size

src/db.rs

@@ -1106,6 +1106,15 @@ impl WriteBatch {
         unsafe { ffi::rocksdb_writebatch_count(self.inner) as usize }
     }
 
+    /// Return WriteBatch serialized size (in bytes).
+    pub fn size_in_bytes(&self) -> usize {
+        unsafe {
+            let mut batch_size: size_t = 0;
+            ffi::rocksdb_writebatch_data(self.inner, &mut batch_size);
+            batch_size as usize
+        }
+    }
+
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
@@ -1403,6 +1412,14 @@ fn writebatch_works() {
             assert!(p.is_ok());
             assert!(db.get(b"k1").unwrap().is_none());
         }
+        {
+            // test size_in_bytes
+            let mut batch = WriteBatch::default();
+            let before = batch.size_in_bytes();
+            let _ = batch.put(b"k1", b"v1234567890");
+            let after = batch.size_in_bytes();
+            assert!(before + 10 <= after);
+        }
     }
     let opts = Options::default();
     assert!(DB::destroy(&opts, path).is_ok());