Sentry: Fixing 'Token' Error With Async Generators

Hey everyone! Today, we're diving deep into a tricky issue in Sentry that can pop up when you're using isolation scopes with async generators. Specifically, we're tackling the dreaded "Token was created in a different Context" error. This can be a real head-scratcher, so let's break it down and see how to avoid it.

Understanding the Problem

This error typically occurs when you're using sentry_sdk.isolation_scope() as a context manager around an async generator that yields values. The problem arises when the generator exits early – whether it's due to a break statement, an exception, or even garbage collection. Let's look at a simplified example to illustrate this:

import asyncio
import sentry_sdk
from sentry_sdk.integrations.asyncio import AsyncioIntegration


async def inner_generator():
    """Simple async generator that yields values"""
    for i in range(3):
        print(f"  Inner generator yielding: {i}")
        yield i


async def problematic_async_generator():
    """
    This pattern causes the context error.
    The isolation_scope wraps yield statements in an async generator.
    """
    with sentry_sdk.isolation_scope() as scope:
        scope.set_user({"id": "test-user-123"})
        scope.set_tag("example", "value")

        async for value in inner_generator():
            # THIS YIELD INSIDE ISOLATION_SCOPE IS THE PROBLEM
            yield value


async def main():
    print("Reproducing Sentry isolation_scope + async generator bug\n")
    print("=" * 60)

    # Initialize Sentry
    sentry_sdk.init(
        dsn=None,  # No DSN needed to reproduce
        integrations=[AsyncioIntegration()],
        debug=True,
    )

    print("Test 1: Normal completion (works fine)")
    print("-" * 40)
    async for val in problematic_async_generator():
        print(f"Received: {val}")
    print("✅ No error when generator completes normally\n")

    print("Test 2: Early exit with break (causes error)")
    print("-" * 40)
    async for val in problematic_async_generator():
        print(f"Received: {val}")
        if val == 1:
            print("Breaking early...")
            break  # This causes the context error

    # Give time for error to appear in output
    await asyncio.sleep(0.1)
    print("\n❌ Error appears above: 'Token was created in a different Context'")

    print("\n" + "=" * 60)
    print("The error happens because:")
    print("1. isolation_scope() saves the current context when entering")
    print("2. The async generator suspends/resumes across context boundaries")
    print("3. When exiting early, cleanup happens in a different context")
    print("4. Python's contextvars raises ValueError")


if __name__ == "__main__":
    asyncio.run(main())

Deep Dive into the Code

Let's walk through the code. We have two async generators: inner_generator and problematic_async_generator. The inner_generator is straightforward – it just yields values. The problematic_async_generator, however, is where the magic (and the bug) happens.

Inside problematic_async_generator, we use sentry_sdk.isolation_scope() to create an isolated scope. This is great for setting user context and tags that should only apply to a specific part of your code. We set a user ID and an example tag within this scope. Then, we iterate through the inner_generator and yield its values.

The crucial part is that we're yielding inside the isolation_scope. This is what triggers the error when the generator exits early. When the generator completes normally, everything is fine. But if we break out of the loop (as shown in "Test 2" in the example), we run into trouble.

Why Does This Happen?

To really understand this, we need to grasp what isolation_scope does and how async generators work:

1. isolation_scope() saves the current context: When you enter an isolation_scope, Sentry saves the current context using Python's contextvars. This allows Sentry to isolate changes made within the scope.
2. Async generators suspend/resume across context boundaries: Async generators, by their nature, suspend and resume execution. This means they can cross context boundaries, potentially switching between different contexts during their lifecycle.
3. Early exit means cleanup in a different context: When you exit the generator early (e.g., via break), the cleanup operations for the isolation_scope happen in a potentially different context than where the scope was entered.
4. contextvars raises ValueError: Python's contextvars raises a ValueError when you try to reset a context variable with a token that was created in a different context. This is exactly what happens when Sentry tries to clean up the isolation_scope after an early exit.

In essence, the isolation_scope sets up a context, but the async generator's behavior can lead to a mismatch between the context where the scope was entered and the context where it's being exited. This mismatch triggers the ValueError.

The Expected Result vs. The Actual Result

Ideally, we'd want our code to handle early exits from generators gracefully without crashing. We expect Sentry to maintain its context isolation without throwing errors. However, the actual result is a ValueError and a crash, which can be quite frustrating.

Task exception was never retrieved
future: <Task finished name='coroutine without __name__ (Sentry-wrapped)' exception=ValueError("<Token var=<ContextVar name='current_scope' default=None at 0x...> was created in a different Context")>
Traceback (most recent call last):
  File ".../sentry_sdk/scope.py", line 1755, in isolation_scope
    yield new_isolation_scope
  File "./sentry_bug_repro.py", line 33, in problematic_async_generator
    yield value
GeneratorExit

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File ".../sentry_sdk/integrations/asyncio.py", line 55, in _task_with_sentry_span_creation
    reraise(*_capture_exception())
  File ".../sentry_sdk/utils.py", line 1751, in reraise
    raise value
  File ".../sentry_sdk/integrations/asyncio.py", line 53, in _task_with_sentry_span_creation
    result = await coro
  File "./sentry_bug_repro.py", line 27, in problematic_async_generator
    with sentry_sdk.isolation_scope() as scope:
  File "/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/contextlib.py", line 130, in __exit__
    self.gen.throw(value)
  File ".../sentry_sdk/scope.py", line 1760, in isolation_scope
    _current_scope.reset(current_token)
ValueError: <Token var=<ContextVar name='current_scope' default=None at 0x...> was created in a different Context>

This traceback clearly shows the ValueError being raised during the cleanup of the isolation_scope.

How to Fix or Work Around the Issue

Now, let's get to the good stuff: how to actually solve this problem. There are a few approaches you can take.

1. Avoid Yielding Inside isolation_scope

The simplest solution is often the best: restructure your code to avoid yielding directly within the isolation_scope. This might involve moving the isolation_scope to wrap a smaller section of code or using a different pattern for managing context.

For example, instead of this:

async def problematic_async_generator():
    with sentry_sdk.isolation_scope() as scope:
        scope.set_user({"id": "test-user-123"})
        async for value in inner_generator():
            yield value

You could do this:

async def better_async_generator():
    async for value in inner_generator():
        with sentry_sdk.isolation_scope() as scope:
            scope.set_user({"id": "test-user-123"})
            yield value

In this revised version, the isolation_scope is applied to each individual value yielded, rather than wrapping the entire generator. This prevents the context mismatch issue.

2. Use a Different Context Management Strategy

If you need to maintain the context across the entire generator, you might consider using a different approach for managing the Sentry scope. Instead of isolation_scope, you could manually push and pop scopes using sentry_sdk.push_scope() and sentry_sdk.pop_scope().

Here's how you might implement this:

import sentry_sdk

async def safer_async_generator():
    scope = sentry_sdk.push_scope()
    try:
        sentry_sdk.set_user({"id": "test-user-123"})
        async for value in inner_generator():
            yield value
    finally:
        sentry_sdk.pop_scope()

In this example, we manually push a scope at the beginning of the generator and pop it in a finally block. This ensures that the scope is always cleaned up, even if the generator exits early.

3. Patch Sentry SDK (Use with Caution)

As a more advanced (and potentially risky) approach, you could patch the Sentry SDK to handle this specific scenario. This involves modifying the SDK's code to prevent the ValueError. However, this is generally not recommended unless you have a deep understanding of the SDK's internals and the potential side effects. Patches can break with future updates to the SDK.

4. Upgrade Sentry SDK

Keep your Sentry SDK version up to date. The Sentry team is actively working on improving the SDK and addressing bugs. A fix for this issue might already be available in a newer version, or it might be addressed in a future release. Check the Sentry SDK release notes for updates.

Real-World Implications

So, why is this bug important? In real-world applications, async generators are commonly used in scenarios like streaming data processing, handling large datasets, and building asynchronous pipelines. If you're using Sentry to monitor these applications, this bug can lead to missed errors or corrupted context data. Imagine you're processing a stream of user events, and an error occurs in the middle of the stream. If the generator exits early due to this error, the Sentry context might not be cleaned up correctly, leading to incorrect user attribution or missing tags on subsequent events.

Best Practices for Using Sentry with Async Generators

To avoid this issue and ensure your Sentry integration works smoothly with async generators, keep these best practices in mind:

• Be mindful of where you use isolation_scope: Avoid wrapping large sections of code that include yield statements. Instead, try to isolate the scope to smaller, more specific blocks.
• Consider manual scope management: If you need to maintain context across an entire generator, use push_scope and pop_scope for more control over scope lifecycle.
• Test your error handling: Make sure your code handles exceptions and early exits from generators gracefully. Test these scenarios thoroughly to catch any context-related issues.
• Stay updated: Keep your Sentry SDK up to date to benefit from bug fixes and performance improvements.

Conclusion

The "Token was created in a different Context" error in Sentry when using isolation scopes with async generators can be a tricky issue to debug. However, by understanding the underlying cause and applying the solutions discussed in this article, you can avoid this pitfall and ensure your Sentry integration works flawlessly.

Remember, the key takeaway is to be mindful of how you're using isolation_scope with async generators. By restructuring your code or using alternative context management strategies, you can keep your Sentry context clean and accurate, even in complex asynchronous scenarios. Happy coding, and may your contexts always be in the right place!

By understanding the intricacies of isolation_scope and async generators, you can avoid this error and ensure your Sentry integration remains robust and reliable. Keep experimenting, keep learning, and keep your code running smoothly!