One way to disable logging in a configuration file is to use /dev/null as the filename for logs. It is fine to use that with FileHandler.

The problem is when TimedRotatingFileHandler is used. When Python decides to rotate the "file", it calls os.rename() on /dev/null *device* and create a new /dev/null *file* which will be quickly filled by all applications on the system writing into /dev/null.

The problem is only possible if the process is allowed to rename the device /dev/null. For example, a regular user gets a permission error.

I see different options:

* TimedRotatingFileHandler should disable rotation if it detects that the file is a device (not a regular file)
* logging should switch to NullHandler if filename is equal to os.path.devnull.

I guess that the problem is the same if the filename points to a socket or a named pipe.


RotatingFileHandler may also be affected, I didn't check.


... I'm quite sure that I had the same issue with Twisted years ago, but I don't recall the details :-) Twisted didn't use the logging module if I recall correctly.


RHEL downstream issue: https://bugzilla.redhat.com/show_bug.cgi?id=2009200

RotatingFileHandler is also affected by the same issue.

Both RotatingFileHandler and TimedRotatingFileHandler have a shouldRollover method which, if it returns False, should prevent rollover.

I would think that putting something like

    if not os.path.isfile(self.baseFilename):
        return False

at the top of each of these methods should avoid rolling over any thing that isn't a regular file - devices, pipes etc. Would you agree? Can you see any drawbacks?

I'm not sure this would cover sockets, but there are separate handlers for using sockets so there would be no reason for people to use sockets with file handlers. And of course sockets are a completely different sort of beast on Windows.

New changeset 62a667784ba7b84611ebd50fa8a1a464cde32235 by Vinay Sajip in branch 'main':
bpo-45401: Change shouldRollover() methods to only rollover regular f… (GH-28822)
https://github.com/python/cpython/commit/62a667784ba7b84611ebd50fa8a1a464cde32235

Seems it introduced a resource warning in tests.

Where did you see the warnings? I didn't spot anything in the GitHub Actions logs.

test_should_not_rollover (test.test_logging.TimedRotatingFileHandlerTest) ... /home/serhiy/py/cpython/Lib/unittest/case.py:547: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='a' encoding='utf-8'>
  if method() is not None:
ResourceWarning: Enable tracemalloc to get the object allocation traceback
ok

With tracemalloc enabled:

$ ./python -X tracemalloc -m test -v test_logging -m test_should_not_rollover
...
test_should_not_rollover (test.test_logging.TimedRotatingFileHandlerTest) ... /home/serhiy/py/cpython/Lib/unittest/case.py:547: ResourceWarning: unclosed file <_io.TextIOWrapper name='/dev/null' mode='a' encoding='utf-8'>
  if method() is not None:
Object allocated at (most recent call last):
  File "/home/serhiy/py/cpython/Lib/logging/__init__.py", lineno 1205
    return open_func(self.baseFilename, self.mode,
ok

Vinay: Oh, thanks for the fix! Using os.path.isfile() is a reasonable fix to detect if the file is a device.

> $ ./python -X tracemalloc -m test -v test_logging -m test_should_not_rollover

FYI you can use -X tracemalloc=10 to get a more complete traceback.

New changeset 15188b115a2da815556053372c912a81a74be43b by Serhiy Storchaka in branch 'main':
bpo-45401: Fix a resource warning in test_logging (GH-28864)
https://github.com/python/cpython/commit/15188b115a2da815556053372c912a81a74be43b

New changeset ac421c348bf422f9a0d85fe0a1de3fa3f4886650 by Miss Islington (bot) in branch '3.9':
bpo-45401: Change shouldRollover() methods to only rollover regular f… (GH-28822) (#28866)
https://github.com/python/cpython/commit/ac421c348bf422f9a0d85fe0a1de3fa3f4886650

New changeset 5aca34f17c4baf8e4882a7e8a827cff06ac6ef25 by Miss Islington (bot) in branch '3.10':
bpo-45401: Change shouldRollover() methods to only rollover regular f… (GH-28822) (#28867)
https://github.com/python/cpython/commit/5aca34f17c4baf8e4882a7e8a827cff06ac6ef25

New changeset faa87f7f3b60f79b9018aaef0efa5e00d82b817b by Miss Islington (bot) in branch '3.9':
bpo-45401: Fix a resource warning in test_logging (GH-28864) (GH-28873)
https://github.com/python/cpython/commit/faa87f7f3b60f79b9018aaef0efa5e00d82b817b

New changeset 47a50fe16f9f074daaaa3e6aa85e76502955ed40 by Miss Islington (bot) in branch '3.10':
bpo-45401: Fix a resource warning in test_logging (GH-28864) (GH-28872)
https://github.com/python/cpython/commit/47a50fe16f9f074daaaa3e6aa85e76502955ed40

Thanks for the fixes Vinay and Serhiy!