Thread-safe Stock Buy/Sell API Preventing Oversell

Implement two functions buy(symbol, qty) and sell(symbol, qty) to maintain stock positions, and ensure that under concurrent multi-threaded calls the system never oversells. Requirements Maintain the current position (share count) for each symbol (initially 0). buy(symbol, qty): increase the position by qty. sell(symbol, qty): decrease the position by qty, but must guarantee: The position can never go below 0 at any time (no overselling). If the current position is less than qty, the call must fail (e.g., return False, throw an exception, or return an error code—choose one and keep it consistent). Concurrency buy/sell may be called by multiple threads concurrently. Use locks (mutex) or an equivalent synchronization mechanism so that the check-and-update is atomic. I/O contract (you may choose and fix one) symbol is a string. qty is a positive integer. Return value should indicate success or failure. Scale & edge cases Multiple symbols may exist. Correctness under high contention is required. Example (logical) Start: AAPL = 0 After buy("AAPL", 10): AAPL = 10 If two threads call sell("AAPL", 7) concurrently, only one can succeed (or at most one such that the position never becomes negative), and the position must never drop below 0. Example Input N/A (API design question; consider two threads both calling sell('AAPL', 7) after buy('AAPL', 10)) Output At most one sell succeeds; final AAPL position is either 3 (one success) or 10 (zero success), never negative.