chore: un-allow clippy's "readonly_write_lock" (#5862)

src/log-store/src/raft_engine/backend.rs

@@ -114,7 +114,13 @@ impl TxnService for RaftEngineBackend {
         } = txn.into();
 
         let mut succeeded = true;
+
+        // Here we are using the write lock to guard against parallel access inside "txn", and
+        // outside "get" or "put" etc. It doesn't serve the purpose of mutating some Rust data, so
+        // the variable is not "mut". Suppress the clippy warning because of this.
+        #[allow(clippy::readonly_write_lock)]
         let engine = self.engine.write().unwrap();
+
         for cmp in compare {
             let existing_value = engine_get(&engine, &cmp.key)?.map(|kv| kv.value);
             if !cmp.compare_value(existing_value.as_ref()) {