[New-bugs-announce] [issue36784] __import__ with empty folder after importlib.invalidate_caches causes reference leak

[Karthikeyan Singaravelan]

New submission from Karthikeyan Singaravelan <tir.karthi at gmail.com>:

I originally hit upon this with issue36777 where I have used support.script_helper.make_script which calls importlib.invalidate_caches and then trying to use __import__ on an empty folder causes reference leak. I tried 3.8 to 3.5 and it exists on each version. A sample script as below. I tried using try finally instead of DirsOnSysPath in doubt and it still causes leak. I couldn't find any issues on search and let me know if I am using something in an incorrect manner.

import importlib
import unittest
import os, sys
import os.path
from test import support

def test_importlib_cache():

    with support.temp_dir() as path:
        dirname, basename = os.path.split(path)
        os.mkdir(os.path.join(path, 'test2'))
        importlib.invalidate_caches()

        with support.DirsOnSysPath(dirname):
            __import__("{basename}.test2".format(basename=basename))


class Tests(unittest.TestCase):

    def test_bug(self):
        for _ in range(10):
            test_importlib_cache()

➜  cpython git:(master) ✗ ./python.exe -m test -R 3:3 test_import_bug
Run tests sequentially
0:00:00 load avg: 1.56 [1/1] test_import_bug
beginning 6 repetitions
123456
......
test_import_bug leaked [980, 980, 980] references, sum=2940
test_import_bug leaked [370, 370, 370] memory blocks, sum=1110
test_import_bug failed

== Tests result: FAILURE ==

1 test failed:
    test_import_bug

Total duration: 1 sec 529 ms
Tests result: FAILURE


I also tried __import__('test1.test2') instead of __import__("{basename}.test2".format(basename=basename)) and the program doesn't cause reference leak. Moving importlib.invalidate_caches() above support.temp_dir() also causes leak so I guess it's not something to do with temporary directories that are cleaned up after tests.

➜  cpython git:(master) ✗ mkdir -p test1/test2
➜  cpython git:(master) ✗ ./python.exe -m test -R 3:3 test_import_bug
Run tests sequentially
0:00:00 load avg: 1.97 [1/1] test_import_bug
beginning 6 repetitions
123456
......
test_import_bug passed

== Tests result: SUCCESS ==

1 test OK.

Total duration: 557 ms
Tests result: SUCCESS

----------
components: Library (Lib)
messages: 341338
nosy: brett.cannon, eric.snow, ncoghlan, xtreak
priority: normal
severity: normal
status: open
title: __import__ with empty folder after importlib.invalidate_caches causes reference leak
type: behavior
versions: Python 3.7, Python 3.8

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue36784>
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