Thread-Safe Deque Implementation with Multi-thread Synchronization

meta Design a system to safely operate a deque in a multi-threading environment. Define the data structure for a deque. Describe in detail how to safely operate the deque in a multi-threading environment without causing race conditions. Assume two threads call the same operation simultaneously and design a solution to handle it. Requirements: Provide a complete code implementation and add test cases to verify thread safety. Example Input append_left 10 append_right 20 pop_right pop_left