fix: safely close all ClientSessions with ContextVar

[garrett4wade]

Description

#589 introduced a bug that ClientSessions are not properly closed when workflow.arun_episode exits. We should use a ContextVar instead of a threading.local() to reuse the ClientSession object.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not
  work as expected)
• Documentation update
• Code refactoring (no functional changes)
• Performance improvement
• Test coverage improvement

Checklist

• I have read the Contributing Guide
• I have run formatting tools (pre-commit or manual)
• I have run relevant unit tests and they pass
• I have added tests for new functionality
• I have updated documentation if needed
• My branch is up to date with main
• This PR introduces breaking changes (if yes, fill out details below)
• If this PR changes documentation, I have built and previewed it locally with
  jb build docs
• No critical issues raised by AI reviewers (/gemini review)

[gemini-code-assist]

Summary of Changes

Hello @garrett4wade, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a critical fix for improper aiohttp.ClientSession closure within asynchronous workflows. By migrating the session management mechanism from threading.local() to contextvars.ContextVar, the changes ensure that each ClientSession is correctly scoped to its specific asynchronous task and is reliably closed upon the task's completion. This enhancement prevents resource leaks and contributes to the overall stability and efficiency of the application.

Highlights

• Session Management Refactor: Replaced threading.local() with contextvars.ContextVar for managing aiohttp.ClientSession objects, providing more robust and context-aware session handling in asynchronous environments.
• ClientSession Lifecycle: Implemented explicit creation and closing of ClientSession instances within the scope of workflow.arun_episode using ContextVar, ensuring sessions are properly isolated and reliably closed upon task completion.
• Bug Fix: Addressed a bug (related to fix: reuse aiohttp.ClientSession in agenerate #589) where ClientSessions were not consistently closed after workflow.arun_episode exits, preventing potential resource leaks and improving application stability.

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[gemini-code-assist]

Code Review

This pull request effectively addresses a bug concerning the improper closure of ClientSession objects by transitioning from threading.local() to contextvars.ContextVar. This change correctly scopes sessions to asynchronous tasks, ensuring they are properly managed and closed. My review includes a few suggestions to enhance code quality and maintainability, such as correcting a typo, updating a docstring, and improving the encapsulation of session management logic.

[garrett4wade]

/gemini review

[gemini-code-assist]

Code Review

This pull request correctly addresses a bug causing ClientSession leaks by replacing threading.local with contextvars.ContextVar. The introduction of the managed_session async context manager is an excellent pattern for ensuring robust session lifecycle management within asyncio tasks. I've identified a critical resource leak in the fallback logic within agenerate where a locally created session might not be closed upon an exception. I've provided a fix using a try...finally block. Additionally, I've suggested a minor docstring update for better clarity. Overall, this is a strong fix, and with the suggested improvements, it will be even more reliable.

[nuzant]

LGTM.