`$COLUMNS` environment variable present in subprocess only when `env=None`

從最小的 subprocess.check_output 範例開始，重現互動式與非互動式執行，以及 env=None 與 env=os.environ 的全部四種組合。從該進入點追蹤 subprocess 的環境處理；當 COLUMNS 的不同行為獲得解釋，並且已修正或透過相應的測試清楚記錄時，即表示完成。

Bug report

for some reason which I cannot determine (perhaps a subtle difference between execvp and execvpe?) the $COLUMNS variable is mysteriously present in subprocesses when using env=None but not when using env=os.environ (which I expect to be identical in behaviour). this also seems to only happen with an interactive session?

here's a minimal case:

import sys, subprocess, os

subprocess.check_output(
    (sys.executable, '-c', 'import os; print(os.environ["COLUMNS"])'),
    # env=os.environ,
)

$ python3.11 -i t4.py
>>> 
$ python3.11 t4.py
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<frozen os>", line 679, in __getitem__
KeyError: 'COLUMNS'
Traceback (most recent call last):
  File "/home/asottile/workspace/cpython/t4.py", line 3, in <module>
    subprocess.check_output(
  File "/usr/lib/python3.11/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('/usr/bin/python3.11', '-c', 'import os; print(os.environ["COLUMNS"])')' returned non-zero exit status 1.

if I un-comment the line in the example script above -- it fails in both places:

$ sed -i 's/#//g' -- t4.py
$ python3.11 -i t4.py
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<frozen os>", line 679, in __getitem__
KeyError: 'COLUMNS'
Traceback (most recent call last):
  File "/home/asottile/workspace/cpython/t4.py", line 3, in <module>
    subprocess.check_output(
  File "/usr/lib/python3.11/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('/usr/bin/python3.11', '-c', 'import os; print(os.environ["COLUMNS"])')' returned non-zero exit status 1.
>>> 
$ python3.11 t4.py
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "<frozen os>", line 679, in __getitem__
KeyError: 'COLUMNS'
Traceback (most recent call last):
  File "/home/asottile/workspace/cpython/t4.py", line 3, in <module>
    subprocess.check_output(
  File "/usr/lib/python3.11/subprocess.py", line 466, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '('/usr/bin/python3.11', '-c', 'import os; print(os.environ["COLUMNS"])')' returned non-zero exit status 1.

normally $COLUMNS is a shell variable and not an environment variable, but that doesn't really explain where it's coming from and why it's different depending both on interactiveness and on env= parameter:

Your environment

• CPython versions tested on: 3.12a3, 3.11.1, 3.10.6
• Operating system and architecture: ubuntu 22.04, x86_64 -- I also tried in docker and it reproduces there as well -- kernel is 5.15.0-56-generic