Fix: Raise RuntimeError when ClientSession is used without context manager to prevent hangs#1849
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…nager This change ensures ClientSession is always used within an 'async with' block by checking the context manager state before allowing operations that depend on the background receive loop. Without this check, calling initialize() outside of a context manager causes the program to hang indefinitely because the _receive_loop task is only started in __aenter__. Changes: - Add _entered flag to track context manager state - Override __aenter__ and __aexit__ to manage the flag - Add _check_is_active() method to enforce proper usage - Call _check_is_active() in initialize() to prevent hangs - Add test case for invalid usage Fixes modelcontextprotocol#1564 Co-authored-by: Cursor <cursoragent@cursor.com>
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Fixes #1564
Summary
This PR adds a runtime check to
ClientSessionto ensure it is being used within an asynchronous context manager (async with). Ifinitialize()is called without entering the context, it now raises a clearRuntimeErrorinstead of hanging indefinitely.Motivation and Context
Currently, if a user attempts to call
await session.initialize()without wrapping the session in anasync withblock, the program hangs indefinitely. This happens because the background receive loop (_receive_loop) is only started inside__aenter__.This behavior leads to a frustrating debugging experience (deadlock) for developers. This change provides immediate feedback by enforcing correct usage of the session, preventing the deadlock.
How Has This Been Tested?
I reproduced the issue using the script provided in #1564.
await session.initialize().RuntimeError: ClientSession must be used within an 'async with' block., as expected.async with) continues to work correctly.Breaking Changes
No. This change is backward compatible for all correct usages. It only affects code that was previously incorrect and hanging.
Types of changes
Checklist
Additional context
I implemented this using a simple state flag (
_entered) to maintain backward compatibility without requiring significant architectural refactoring of the session class.