PYTHON-2956 Drivers should check out an implicit session only after checking out a connection

[juliusgeo]

No description provided.

[ShaneHarvey]

Looks like this test is failing on pypy 3.8:

FAIL: test_implicit_sessions_checkout (test_session.TestSession)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/mongo-python-driver/mongo-python-driver/test/test_session.py", line 220, in test_implicit_sessions_checkout
    self.assertTrue(succeeded)

Could be related to delayed garbage collection in pypy? Does it also fail locally for you?

[blink1073]

I think we should only do the manual gc collect if we detect we're on PyPy: import platform; "PyPy" in platform.python_implementation(). https://stackoverflow.com/q/45685549

[blink1073]

LGTM!

[ShaneHarvey]

Still waiting for an explanation for why changing from $listLocalSessions to client.admin.aggregate, [[{"$currentOp": {}}]] fixes the test failure on pypy. Is there a bug in our CommandCursor implementation? Does $listLocalSessions require multiple batches whereas $currentOp is a single batch? A multi-batch cursor would cause this test to almost always create one extra session since the session needs to be kept alive until all batches complete. In this case I believe we should update the test to impose a limit=1 for all find/aggregate cursors.

[ShaneHarvey]

LGTM