Inconsistent `Popen.communicate()` behavior if stdin/stdout/stderr is closed `PIPE`

Bug report

Bug description:

The behavior with Popen.communicate() if stdin/stdout/stderr are PIPEs and they are closed before calling communicate() is rather strange.

If both stdout and stderr are PIPEs and one (or both) is closed, communicate() succeeds. If only one of them is a PIPE and it is closed, the behavior changes and there's a ValueError instead:

>>> from subprocess import Popen, PIPE
>>>
>>> p = Popen(['python', '-V'], stdout=PIPE, stderr=PIPE)
>>> p.stdout.close()
>>> p.communicate()
(b'', b'')
>>>
>>> p = Popen(['python', '-V'], stdout=PIPE)
>>> p.stdout.close()
>>> p.communicate()
Traceback (most recent call last):
  File "<python-input-7>", line 1, in <module>
    p.communicate()
    ~~~~~~~~~~~~~^^
  File "/usr/lib/python3.14/subprocess.py", line 1207, in communicate
    stdout = self.stdout.read()
ValueError: read of closed file

If stdin is a closed PIPE, the behavior is pretty much the opposite of the above. If other streams are not PIPEs, communicate() succeeds, but if the are other PIPEs, we get a ValueError:

>>> from subprocess import Popen, PIPE
>>>
>>> p = Popen(['python', '-c', 'pass'], stdin=PIPE)
>>> p.stdin.close()
>>> p.communicate()
(None, None)
>>> 
>>> p = Popen(['python', '-c', 'pass'], stdin=PIPE, stdout=PIPE)
>>> p.stdin.close()
>>> p.communicate()
Traceback (most recent call last):
  File "<python-input-14>", line 1, in <module>
    p.communicate()
    ~~~~~~~~~~~~~^^
  File "/usr/lib/python3.14/subprocess.py", line 1220, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
                     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.14/subprocess.py", line 2081, in _communicate
    self.stdin.flush()
    ~~~~~~~~~~~~~~~~^^
ValueError: flush of closed file

This inconsistency seems to be caused by an optimization that leads to two different code paths depending on the number of PIPEs. I needed to spend some time to understand why our tests behaved seemingly inconsistently after code was refactored.

It would be easy to fix problems by changing code like if self.stdout: to if self.stdout and not self.stdout.closed:. I guess it could be argued that raising a ValueError is the correct way to handle these situations, but in that case it should be raised always.

Tested with Python 3.14 alpha 5. Occurs also with earlier ones.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

先复现 closed-PIPE 示例，然后检查 Lib/subprocess.py 中第 1194 行附近与 communicate() 优化相关的部分，以及 traceback 中显示的 stdin/stdout 处理。确定并确认对已关闭流的预期一致行为，然后为受影响的组合添加回归测试覆盖，并验证现有的 subprocess 测试。