Motivation for this PR (comment from @vstinner in bpo issue):
```
Warning seen o AMD64 Ubuntu Shared 3.x buildbot:
https://buildbot.python.org/all/#/builders/141/builds/2593
test_devnull_output (test.test_a=syncio.test_subprocess.SubprocessThreadedWatcherTests) ...
Warning -- threading_cleanup() failed to cleanup 1 threads (count: 1, dangling: 2)
```
The following implementation details for the new method are TBD:
1) Public vs private
2) Inclusion in `close()`
3) Name
4) Coroutine vs subroutine method
5) *timeout* parameter
If it's a private method, 3, 4, and 5 are significantly less important.
I started with the most minimal implementation that fixes the dangling threads without modifying the regression tests, which I think is particularly important. I typically try to avoid directly modifying existing tests as much as possible unless it's necessary to do so. However, I am open to changing any part of this.
https://bugs.python.org/issue38356
Although the underlying libffi issue remains open, adding these
checks have caused problems in third-party projects which are in
widespread use. See the issue for examples.
The corresponding tests have also been skipped.
nntplib.NNTP and nntplib.NNTP_SSL now raise a ValueError
if the given timeout for their constructor is zero to
prevent the creation of a non-blocking socket.
poplib.POP3 and poplib.POP3_SSL now raise a ValueError
if the given timeout for their constructor is zero to
prevent the creation of a non-blocking socket.
The fix changes copy_location() to require an extra node from which to extract the end location, and fixing all 5 call sites.
https://bugs.python.org/issue39235
* Add test cases for dataclasses.
* Add test for repr output of field.
* Add test for ValueError to be raised when both default and default_factory are passed.
Filename and line numbers are not needed when Py_REF_DEBUG are not
defined.
The static inline _Py_DECREF() function was introduced by
commit 2aaf0c1204.
imaplib.IMAP4 and imaplib.IMAP4_SSL now have an
optional *timeout* parameter for their constructors.
Also, the imaplib.IMAP4.open() method now has an optional *timeout* parameter
with this change. The overridden methods of imaplib.IMAP4_SSL and
imaplib.IMAP4_stream were applied to this change.
To be consistent with document layout, it should say when the feature was added.
Although it's mentioned few other places in the doc but it's not explicitly say that at that place.
https://bugs.python.org/issue39130
Kyle Stanley
Zac Hatfield-Dodds
Vinay Sajip
Victor Stinner
Dong-hee Na
Jason R. Coombs
Karthikeyan Singaravelan
Pablo Galindo
Serhiy Storchaka
Guido van Rossum
Daniel Hahler
Steve Dower
Petr Viktorin
Alex Henrie
An Long
Derek Brown
Andrew Svetlov
Inada Naoki
YoSTEALTH
Anthony Wee
Chandan Singh
Tal Einat
Khalid Mammadov
HongWeipeng
Oleg Höfling