Fix race condition in concurrent deposit
Company: xAI
Role: Software Engineer
Category: Software Engineering Fundamentals
Difficulty: medium
Interview Round: Technical Screen
## Concurrent bank account debugging (thread safety)
You are given a simple `BankAccount` object that is used concurrently from multiple threads. Two deposits run at the same time:
```python
account = BankAccount(0)
with ThreadPoolExecutor(max_workers=2) as executor:
futures = [
executor.submit(account.deposit, 500),
executor.submit(account.deposit, 700),
]
```
A simplified implementation of `deposit()` looks like this:
```python
class BankAccount:
def __init__(self, balance=0):
self.balance = balance
def deposit(self, amount):
new_balance = self.balance + amount # Read
time.sleep(0.1) # Delay
self.balance = new_balance # Write
```
### Tasks
1. Explain why this code can produce an incorrect final balance (e.g., `700` instead of the expected `1200`).
2. Identify the underlying concurrency bug(s).
3. Propose and describe a correct fix that ensures thread safety for `deposit()`.
4. Mention any important caveats or alternatives (e.g., performance trade-offs, other synchronization approaches).
Quick Answer: This question evaluates understanding of concurrency and thread safety, specifically recognizing race conditions and reasoning about synchronization for shared mutable state in multithreaded programs.