Add DB::sync_wal to support syncing the WAL without a flush (#2)

src/db.rs

@@ -886,6 +886,22 @@ impl<T: ThreadMode, D: DBInner> DBCommon<T, D> {
         Ok(())
     }
 
+    /// Ensure all WAL writes have been synced to storage, so that (assuming OS
+    /// and hardware support) data will survive power loss. This function does
+    /// not imply flush_wal, so flush_wal(true) is recommended if using
+    /// manual_wal_flush=true. Currently only works if allow_mmap_writes = false
+    /// in Options.
+    ///
+    /// Note that write() followed by sync_wal() is not exactly the same as write()
+    /// with sync=true: in the latter case the changes won't be visible until the
+    /// sync is done.
+    pub fn sync_wal(&self) -> Result<(), Error> {
+        unsafe {
+            ffi_try!(ffi::rocksdb_sync_wal(self.inner.inner()));
+        }
+        Ok(())
+    }
+
     /// Flushes database memtables to SST files on the disk.
     pub fn flush_opt(&self, flushopts: &FlushOptions) -> Result<(), Error> {
         unsafe {