feat(server): add async context manager support to EventQueue

[cchinchilla-dev]

Description

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Fixes #720 🦕

Problem

EventQueue has a sophisticated close() with graceful/immediate modes, child propagation, and cross-version handling — but doesn't support async with. Server-side code must use explicit try/finally or risk leaking resources on exceptions:

queue = EventQueue()
try:
    await queue.enqueue_event(event)
    ...
finally:
    await queue.close()

Fix

Add __aenter__ and __aexit__ as concrete methods on EventQueue:

• __aenter__ returns Self (via typing_extensions).
• __aexit__ delegates to close() with default immediate=False (graceful). Code needing immediate shutdown can still call await queue.close(immediate=True) explicitly.

This enables the idiomatic pattern:

async with EventQueue() as queue:
    await queue.enqueue_event(event)
    ...
# close() called automatically, even on exceptions

Unlike the client-side hierarchy where __aenter__/__aexit__ were lifted to the abstract Client (#719), EventQueue is a concrete class with no abstract base above it — QueueManager manages queue lifecycles by task ID but does not wrap or extend EventQueue. This is the correct and only place for these methods.

Non-breaking, additive change. Manual close() and try/finally continue to work as before.

Follows the pattern established in ClientTransport (#682), BaseClient (#688), and Client (#719).

Tests

Two tests added to tests/server/events/test_event_queue.py, following the same approach as ClientTransport and BaseClient.

[gemini-code-assist]

Summary of Changes

Hello @cchinchilla-dev, 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 asynchronous context manager support to the EventQueue class, enabling developers to manage event queue lifecycles more robustly and idiomatically. By implementing __aenter__ and __aexit__, the EventQueue can now be used with async with statements, ensuring that its close() method is automatically invoked upon exiting the context, even in the presence of exceptions. This enhancement simplifies resource management and promotes cleaner, more reliable asynchronous code.

Highlights

• Async Context Manager Support: The EventQueue class now supports the async with statement by implementing __aenter__ and __aexit__ methods.
• Improved Resource Management: This change ensures that EventQueue instances are automatically and gracefully closed upon exiting an async with block, even if exceptions occur, thereby preventing resource leaks and simplifying cleanup logic.
• Idiomatic Asynchronous Code: The addition of async context manager capabilities aligns EventQueue with modern asynchronous Python patterns, promoting cleaner and more robust code, consistent with other client-side components.
• New Test Coverage: Two dedicated asynchronous tests were added to verify the correct functionality of the EventQueue context manager, covering both normal execution and scenarios with exceptions.

Changelog

• src/a2a/server/events/event_queue.py
  • Imported TracebackType from types and Self from typing_extensions for type hinting.
  • Implemented async def __aenter__(self) -> Self: to return the EventQueue instance itself upon entering the context.
  • Implemented async def __aexit__(self, exc_type, exc_val, exc_tb) -> None: to ensure await self.close() is called when exiting the context.
• tests/server/events/test_event_queue.py
  • Added test_event_queue_async_context_manager to verify that EventQueue correctly enters and exits an async with block and closes itself.
  • Added test_event_queue_async_context_manager_on_exception to confirm that close() is invoked even when an exception is raised within the async with block.

Activity

• No human activity has been recorded on this pull request yet.

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[gemini-code-assist]

Code Review

This pull request adds async context manager support to the EventQueue class by implementing __aenter__ and __aexit__. This is a great improvement, as it allows for more idiomatic and safer resource management using async with. The implementation is clean, and the accompanying tests correctly verify the new functionality, including the happy path and exception handling. I've left a few minor stylistic suggestions in the test code to align with common Python practices for boolean assertions.

[gemini-code-assist]

For boolean comparisons, it's more idiomatic and generally preferred to use assert not ... instead of assert ... is False.

Suggested change

	assert event_queue.is_closed() is False
	assert not event_queue.is_closed()

[gemini-code-assist]

For boolean comparisons, it's more idiomatic and generally preferred to use assert ... directly instead of assert ... is True. This is also flagged by ruff under rule SIM103.

Suggested change

	assert event_queue.is_closed() is True
	assert event_queue.is_closed()

[gemini-code-assist]

For boolean comparisons, it's more idiomatic and generally preferred to use assert ... directly instead of assert ... is True. This is also flagged by ruff under rule SIM103.

Suggested change

	assert event_queue.is_closed() is True
	assert event_queue.is_closed()

[cchinchilla-dev]

Thanks for the suggestion! I kept is True/is False for consistency with the existing assertions in this file (e.g., test_close_idempotent, test_is_closed_reflects_state, etc.), which all use the same style. Happy to change if maintainers prefer updating.