[ServiceBus] Fix async sender AttributeError race in _open (#35618)

[SAY-5]

Description

Fixes #35618.

When multiple coroutines concurrently call ServiceBusSender.send_messages on a shared async sender, a race in _servicebus_sender_async.py:_open can null out self._handler between await self._handler.open_async(...) and await self._handler.client_ready_async(), producing:

AttributeError: 'NoneType' object has no attribute 'client_ready_async'

This patch captures self._handler into a local variable immediately after _create_handler(auth) and uses that local for the subsequent awaits within _open. The except/_close_handler cleanup path still operates on self._handler, preserving existing teardown semantics.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message.

Testing Guidelines

• Pull request includes test coverage for the included changes. (The race is timing-dependent and difficult to reproduce deterministically in a unit test without restructuring the handler lifecycle; the patch is a defensive local-capture that does not change observable behavior on the happy path.)

[github-actions]

Thank you for your contribution @SAY-5! We will review the pull request and get back to you soon.

[Copilot]

Pull request overview

This PR aims to fix a concurrency bug in the async Service Bus sender by making _open() use a local handler reference, so shared ServiceBusSender instances are less likely to hit an AttributeError during concurrent sends. It fits into the azure-servicebus package's async connection lifecycle, specifically around sender startup and readiness checks.

Changes:

• Updated ServiceBusSender._open() to capture self._handler in a local variable before awaited startup calls.
• Switched handler-dependent readiness and max-message-size logic in _open() to use the captured local handler.
• Added a changelog entry describing the async sender race-condition fix.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
sdk/servicebus/azure-servicebus/azure/servicebus/aio/_servicebus_sender_async.py	Adjusts async sender open logic to use a local handler reference during awaited startup.
sdk/servicebus/azure-servicebus/CHANGELOG.md	Documents the intended bug fix for the upcoming unreleased version.

[SAY-5]

Addressed the serialization concern in f568e44: wrapped the create/open critical section in a per-sender asyncio.Lock (lazily initialised on first _open()), with a double-check on _running after acquiring so the loser of the race exits cleanly. The local-handler capture stays as a defence-in-depth on the awaited startup path inside the lock.

I left out a regression test in this commit — the package's existing test infra runs against live Service Bus (test_proxy fixture, no async sender mock harness today). Happy to add a unit test alongside if you'd like, but it would mean introducing a mock sender pattern that doesn't yet exist here, which felt like a separate change to keep this fix reviewable. Let me know your preference.

[SAY-5]

Heads-up: the Azure DevOps Build Test (Ubuntu2404_313) and Build Analyze are red on f568e44 but I can't see the pipeline log without an azure-sdk org account. Local python -m pytest sdk/servicebus/azure-servicebus/tests/async_tests -k sender is green and the file passes ast.parse. If a maintainer can share the relevant log lines (mypy/pylint/test name) I'll fix immediately.

[SAY-5]

Reproduced the Build Analyze pyright error locally — the reportIncompatibleMethodOverride on _open is pre-existing on main as well, not introduced by this PR. Happy to follow up with a separate annotation fix if desired.