bpo-6721: When os.fork() was called while another thread holds a logging lock, the child process may deadlock when it tries to log.  This fixes that by acquiring all logging locks before fork and releasing them afterwards.

A regression test that fails before this change is included.

Within the new unittest itself: There is a small _potential_ due to mixing of fork and a thread in the child process if the parent's thread happened to hold a non-reentrant library call lock (malloc?) when the os.fork() happens.  buildbots and time will tell if this actually manifests itself in this test or not.  :/  A functionality test that avoids that would be a challenge.

An alternate test that isn't trying to produce the deadlock itself but just checking that the release and acquire calls are made would be the next best alternative if so.

Some updates to ancient text about comparisons; fixes bp-34552.

Second pass for asyncio subprocess and sync docs.


https://bugs.python.org/issue33649

This test is doesn't work when the test process is privledged, which is hard to detect.





https://bugs.python.org/issue34668

This function was not in any .h file and was not used by Python, so removing it is safe.


https://bugs.python.org/issue34653

[bpo-34658](https://www.bugs.python.org/issue34658): Fix a rare interpreter unhandled exception state SystemError only
seen when using subprocess with a preexec_fn while an after_parent handler has
been registered with os.register_at_fork and the fork system call fails.



https://bugs.python.org/issue34658

This causes the tearDown code to only unimport the test modules specifically created as part of each test via the self.mkhier method rather than abusing test.support.modules_setup() and the scary test.support.modules_cleanup() code.


https://bugs.python.org/issue34200

A fix for 883702ebb8bbfa749ef0040d1b58d6222bf589ee.

closes bpo-34650: Check if sched_getscheduler returns ENOSYS before declaring it supported. (GH-9228)

musl doesn't support the scheduler API, but declares stubs that alway return ENOSYS.

Reported by Svace static analyzer.

Returning EINTR from pthread semaphore or lock acquisition is an optional POSIX
feature. musl does not provide this feature, so some threadsignal tests fail
when Python is built against it.

There's no good way to test for musl, so we skip if we're on Linux and not using
glibc pthreads.

Also, hedge in the threading documentation about when we can provide interrupts
from lock acquisition.

* raise exception if _missing_ returns None or invalid type

Store a weak reference to stream readerfor breaking strong references

It breaks the strong reference loop between reader and protocol and allows to detect and close the socket if the stream is deleted (garbage collected)