Fix async connection pool lock contention during connection establishment

[Copilot]

• Understand the issue and explore the codebase
• Reduce lock scope in ConnectionPool.get_connection()
• Add test for lock not held during connection establishment
• Add test for concurrent connection acquisition performance
• Fix linter formatting errors in test file

Original prompt

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This section details on the original issue you should resolve

<issue_title>Async performance issue since version 5.3.0</issue_title>
<issue_description>The unannounced introduction of asyncio.Lock when acquiring a connection from the pool in #3445 is causing noticeable performance degradation:

redis-py/redis/asyncio/connection.py

Lines 1095 to 1105 in 653d9ef

	async def get_connection(self, command_name=None, *keys, **options):
	async with self._lock:
	"""Get a connected connection from the pool"""
	connection = self.get_available_connection()
	try:
	await self.ensure_connection(connection)
	except BaseException:
	await self.release(connection)
	raise
	return connection

This becomes especially evident under high load, where lock contention becomes a bottleneck. The issue affects version 5.3.0 and later. Others have also reported performance impacts, as noted in #3624.

I kind of understand the rationale behind this change, but personally, I would acquire the lock only to pop a connection and then performs ensure_connection outside the lock. Alternatively, if it didn’t appear to cause issues in earlier versions, may be no lock at all?</issue_description>

<agent_instructions>Fix the lock range - it should not include the call to establishing a socket connection and handshake execution. Add proper unit tests.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@petyaslavova Hi @d33kayyy, thanks for bringing this to our attention. We will take a look at it soon.

• Fixes Async performance issue since version 5.3.0 #3692

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[petyaslavova]

@copilot fix linter errors

[Copilot]

@copilot fix linter errors

Fixed in commit 7104055. The issue was a formatting preference for assert statements - ruff wanted the error message on the same line as the condition when it can fit.

[petyaslavova]

LGTM.