Dump the path configuration at Python startup if `import encodings` fails

[vstinner]

Bug report

When Python fails to locate the stdlib at startup, it fails with an unhelpful error message:

$ PYTHONHOME=/xyz ./python 
WARN: Could not find the standard library directory! The Python 'home' directory was set to '/xyz', is this correct?
Fatal Python error: Failed to import encodings module
Python runtime state: core initialized
Exception ignored in the internal traceback machinery:
ModuleNotFoundError: No module named 'traceback'
ModuleNotFoundError: No module named 'encodings'

Current thread 0x00007f58ef90c780 [python] (most recent call first):

Note: Well, the new WARN: Could not find the standard library directory! The Python 'home' directory was set to '/xyz', is this correct? warning is useful here :-)

In Python 3.12 and older, the Python path configuration is dumped at startup at the first import error:

$ PYTHONHOME=/xyz python3.12
Python path configuration:
  PYTHONHOME = '/xyz'
  PYTHONPATH = (not set)
  program name = 'python3.12'
  isolated = 0
  environment = 1
  user site = 1
  safe_path = 0
  import site = 1
  is in build tree = 0
  stdlib dir = '/xyz/lib64/python3.12'
  sys._base_executable = '/usr/bin/python3.12'
  sys.base_prefix = '/xyz'
  sys.base_exec_prefix = '/xyz'
  sys.platlibdir = 'lib64'
  sys.executable = '/usr/bin/python3.12'
  sys.prefix = '/xyz'
  sys.exec_prefix = '/xyz'
  sys.path = [
    '/xyz/lib64/python312.zip',
    '/xyz/lib64/python3.12',
    '/xyz/lib64/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'

Current thread 0x00007fdf1c0c0bc0 (most recent call first):
  <no Python frame>

I propose fixing this Python 3.12 regression by dumping the Python path configuration at the first import error (import encodings).

Linked PRs

• gh-151253: Dump the Python path configuration on _PyCodec_InitRegistry() failure #151250
• [3.15] gh-151253: Dump the Python path configuration on _PyCodec_InitRegistry() failure (#151250) #151269
• [3.14][3.15] gh-151253: Dump the Python path configuration on _PyCodec_InitRegistry() failure (#151250) (#151269) #151283
• [3.13] gh-151253: Dump the Python path configuration on _PyCodec_InitRegistry() failure (#151250) (#151269) (#151283) #151287

[vstinner]

In Python 3.15, the encodings module is frozen and frozen modules are used by default in release build. So the issue is less important in Python 3.15.

On Python 3.15, import site fails in _read_pthstart_file() on loading the utf-8-sig codec to parse a PTH file. The error is on the file /home/vstinner/.local/lib/python3.15/site-packages/a1_coverage.pth. The utf-8-sig codec module is not frozen.