fix: clear leaked running loop in MCP client background thread#2111
fix: clear leaked running loop in MCP client background thread#2111mkmeral wants to merge 1 commit intostrands-agents:mainfrom
Conversation
When OpenTelemetry's ThreadingInstrumentor is active, Thread.run() is wrapped to propagate trace context, which leaks the parent thread's running event loop reference into child threads. This causes 'RuntimeError: Cannot run the event loop while another loop is running' when _background_task calls run_until_complete(). Add asyncio._set_running_loop(None) at the top of _background_task to clear any leaked loop state before creating the new event loop. This is the same pattern used by bedrock-agentcore-sdk-python (PR strands-agents#405).
Codecov Report✅ All modified and coverable lines are covered by tests. 📢 Thoughts on this report? Let us know! |
![github-actions[bot]](https://avatars.githubusercontent.com/in/15368?s=60&v=4){kind=small}
| This test simulates that scenario by setting a running loop before _background_task runs | ||
| and verifying it gets cleared. | ||
| """ | ||
| import asyncio |
There was a problem hiding this comment.
Issue: import asyncio is inside the function body, but the repository coding standards (AGENTS.md) require imports at the top of the file.
Suggestion: Move import asyncio to the top-level imports alongside contextvars and threading.
| self._log_debug_with_thread("setting up background task event loop") | ||
| # Clear any running-loop state leaked by OpenTelemetry's ThreadingInstrumentor, which wraps Thread.run() | ||
| # and can propagate the parent thread's event loop reference, causing run_until_complete() to fail. | ||
| asyncio._set_running_loop(None) |
There was a problem hiding this comment.
Important: asyncio._set_running_loop is a private/internal CPython API (prefixed with _). While it's been stable across Python 3.10–3.13 and is the correct low-level mechanism for this, it's worth noting the risk: this could change in a future CPython release without deprecation notice.
Suggestion: Consider adding a defensive wrapper or at minimum a brief inline comment noting the Python version range where this is known to work, so future maintainers know to verify if the minimum supported Python version changes. For example:
# asyncio._set_running_loop is a CPython internal (stable since 3.6, used by event loops themselves).
# Required to clear loop state leaked by OpenTelemetry's ThreadingInstrumentor.
asyncio._set_running_loop(None)This is non-blocking — the existing comment is already good context about why, just suggesting adding a note about the private API nature.
|
Assessment: Comment Well-scoped bug fix that addresses a real issue with OpenTelemetry's Review Details
Clean fix with good context in both the PR description and code comments. |
2 participants
Description
When
opentelemetry-instrumentation-threadingis active — the documented pattern for AgentCore observability — theThreadingInstrumentorwrapsThread.run()to propagate trace context. Combined withcontextvars.copy_context()inMCPClient.start(), this leaks the parent thread's running event loop reference into the background thread. When_background_taskthen callsrun_until_complete(), asyncio sees a "running" loop already set and raisesRuntimeError: Cannot run the event loop while another loop is running.This adds
asyncio._set_running_loop(None)at the top of_background_task()to clear leaked loop state before creating the new event loop. This is the same pattern the bedrock-agentcore team shipped in aws/bedrock-agentcore-sdk-python#405.Confirmed on SDK versions 1.28.0 and 1.35.0.
Related Issues
N/A
Documentation PR
N/A — no documentation changes needed.
Type of Change
Bug fix
Testing
How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli
hatch run prepareAdded a unit test that simulates the OTEL leak by setting a running loop before
_background_taskexecutes, verifying the method clears it and completes successfully.Checklist
By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.