filter empty batch

Signed-off-by: Little-Wallace <bupt2013211450@gmail.com>

src/rocksdb.rs

@@ -738,13 +738,30 @@ impl DB {
         &self.path
     }
 
-    pub fn multi_thread_write(&self, batches: &Vec<WriteBatch>, writeopts: &WriteOptions) -> Result<(), String> {
+    pub fn multi_thread_write(
+        &self,
+        batches: &Vec<WriteBatch>,
+        writeopts: &WriteOptions,
+    ) -> Result<(), String> {
         unsafe {
             if batches.len() == 1 {
-                ffi_try!(crocksdb_write(self.inner, writeopts.inner, batches[0].inner));
+                ffi_try!(crocksdb_write(
+                    self.inner,
+                    writeopts.inner,
+                    batches[0].inner
+                ));
             } else {
-                let b : Vec<*mut DBWriteBatch> = batches.iter().map(|w| w.inner).collect();
-                ffi_try!(crocksdb_write_multi_batch(self.inner, writeopts.inner, b.as_ptr(), b.len()));
+                let b: Vec<*mut DBWriteBatch> = batches
+                    .iter()
+                    .filter(|w| w.count() > 0)
+                    .map(|w| w.inner)
+                    .collect();
+                ffi_try!(crocksdb_write_multi_batch(
+                    self.inner,
+                    writeopts.inner,
+                    b.as_ptr(),
+                    b.len()
+                ));
             }
         }
         Ok(())