Problem/Motivation
The semantics of Request::get() have changed in Symfony 3. Request attributes are now examined first before request query parameters. If you have an "op" request attribute AND an "op" query parameter at the same time as for example in update.php then the Batch API breaks.
Proposed resolution
Explicitly call $request->query->get() in the Batch API to only access URL query parameters and never request attributes.
Remaining tasks
Patch.
User interface changes
None.
API changes
None.
Data model changes
None.