Race condition in savepoint ID generation causes duplicate identifiers

[Mijo Kristo]

Problem

The BaseDatabaseWrapper.savepoint() method contains a race condition where the operation self.savepoint_state += 1 is not atomic. This can lead to duplicate savepoint IDs in multithreaded environments.

Location

django/db/backends/base/base.py, line ~608

Problematic Code

thread_ident = _thread.get_ident()
tid = str(thread_ident).replace("-", "")

self.savepoint_state += 1  # Race condition here
sid = "s%s_x%d" % (tid, self.savepoint_state)

Impact

• Multiple threads can read the same savepoint_state value
• Results in duplicate savepoint IDs (e.g., "s123_x5", "s123_x5")
• Causes database errors when rolling back to savepoints
• Can lead to data corruption in high-concurrency applications
• Affects applications using nested transaction.atomic() blocks

Reproduction

The race condition can be reproduced with this simple test:

import threading, time

counter = 0

def buggy_increment():
    global counter
    temp = counter
    time.sleep(0.001)
    counter = temp + 1
    print(f'Thread got: {counter}')

threads = [threading.Thread(target=buggy_increment) for _ in range(5)]
for t in threads: t.start()
for t in threads: t.join()

print(f'Final: {counter} (should be 5)')

Expected output: Final: 5
Actual output: Final: 1 (lost updates due to race condition)

Real-world scenarios

This bug can manifest in production applications as:

• "savepoint does not exist" database errors
• Transaction rollback failures
• Silent data corruption in e-commerce/banking applications
• Inventory overselling in high-traffic scenarios

Proposed Fix

Wrap the increment operation in the existing _thread_sharing_lock:

thread_ident = _thread.get_ident()
tid = str(thread_ident).replace("-", "")

with self._thread_sharing_lock:
    self.savepoint_state += 1
    sid = "s%s_x%d" % (tid, self.savepoint_state)

This solution:

• Uses Django's existing thread safety infrastructure
• Has minimal performance impact
• Maintains backward compatibility
• Fixes the race condition completely

Testing

The fix can be verified by running the reproduction case above - with the fix, all threads will get unique values.

Django's existing test suite should continue to pass as this change only affects the thread safety of savepoint generation without changing the API.

Additional Notes

• This is an ideal "easy pickings" issue for new contributors
• The bug is timing-dependent and may not reproduce consistently
• High-concurrency production environments are most affected
• The fix leverages existing Django patterns for thread safety

[Natalia Bidart]

Hello Mijo Kristo, thank you for your ticket report. Can you please confirm:

• If any part of this report was generated or assisted by an AI/LLM?
• Does this come from a real-world use case, or is it primarily a theoretical scenario?

This will help us understand and prioritize the issue better.