Python may startup fails if the `home` variable in `pyvenv.cfg` is inaccurate

Bug report

Bug description:

The problem comes from getpath.py. When getpath.py cannot find STDLIB_LANDMARKS from executable_dir, it will set prefix to the PREFIX. If the directory set in PREFIX does not exist, an error occurs.

See details: https://github.com/python/cpython/blob/49f15d8667e4755876698a8daa13bab6acee5fa1/Modules/getpath.py#L590-L599

How to reproduce

# 1. build cpython with prefix
./configure --prefix=/home/xx/tmp/install_dir
make
make install

# 2. copy install_dir to another
cd /home/xx/tmp
cp -r ./install_dir/ ./copy_dir

# 3. create a symlink from copy_dir/python
ln -s ./copy_dir/bin/python3 ./pylink

# 4. delete original install_dir
rm -rf ./install_dir/

# 5. use python symlink to create a venv
./pylink -m venv ./bad_venv

# 6. failed to start!
./bad_venv/bin/python3

Error message

Could not find platform independent libraries <prefix>
Could not find platform dependent libraries <exec_prefix>
Python path configuration:
  PYTHONHOME = (not set)
  PYTHONPATH = (not set)
  program name = './bad_venv/bin/python3'
  isolated = 0
  environment = 1
  user site = 1
  safe_path = 0
  import site = 1
  is in build tree = 0
  stdlib dir = '/home/xx/tmp/install_dir/lib/python3.12'
  sys._base_executable = '/home/xx/tmp/copy_dir/bin/python3.12'
  sys.base_prefix = '/home/xx/tmp/install_dir'
  sys.base_exec_prefix = '/home/xx/tmp/install_dir'
  sys.platlibdir = 'lib'
  sys.executable = '/home/xx/tmp/bad_venv/bin/python3'
  sys.prefix = '/home/xx/tmp/install_dir'
  sys.exec_prefix = '/home/xx/tmp/install_dir'
  sys.path = [
    '/home/xx/tmp/install_dir/lib/python312.zip',
    '/home/xx/tmp/install_dir/lib/python3.12',
    '/home/xx/tmp/install_dir/lib/python3.12/lib-dynload',
  ]
Fatal Python error: init_fs_encoding: failed to get the Python codec of the filesystem encoding
Python runtime state: core initialized
ModuleNotFoundError: No module named 'encodings'

We can find that sys._base_executable here is correct, but sys.base_prefix is ​​wrong.
We could configure base_prefix through the real interpreter path, which is used to search for libraries that Python depends on.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-127441

先從 Modules/getpath.py 第 590-599 行附近開始，然後在 Linux 上執行已回報的 build、複製、symlink 和 venv 重現步驟。比較得到的 sys._base_executable 和 sys.base_prefix 值，並檢視相關 PR gh-127441。當移動後的 venv 直譯器能夠啟動並載入 encodings，且不參照已刪除的安裝路徑時，即表示完成。