DRIVERS-3218 Avoid clearing the connection pool when the server connection rate limiter triggers

[blink1073]

Currently testing with this script:

import asyncio
from pymongo import AsyncMongoClient

client = AsyncMongoClient(maxConnecting=100)

async def target():
    return await client.admin.command("ping")

async def main():
    # Warm the pool so there are existing connections.
    tasks = []
    for i in range(10):
        tasks.append(asyncio.create_task(target()))
    await asyncio.wait(tasks)

    tasks = []
    for i in range(200):
        tasks.append(asyncio.create_task(target()))

    await asyncio.wait(tasks)

asyncio.run(main())

[ShaneHarvey]

2 comments:

1. I believe this logic needs to move to connection checkout. Here in connection check in we already know the connection is useable because we're checking it back in after a successful command.
2. Instead of a 1ms read can we reuse the existing _perished() + conn_closed() methods?

[blink1073]

Done

[ShaneHarvey]

Nice work!

[blink1073]

Current status:

• I added an EVG project for this branch so patch builds will run
• The test suite is now passing except for one encryption test

Next steps:

• Work on a PERF repro
• Add new unified tests to verify behavior
• Work on specifications changes

[blink1073]

Encryption failure is unrelated: https://jira.mongodb.org/browse/PYTHON-5521