`test_embed.test_init_pyvenv_cfg()` fails

[WolframAlph]

Bug report

Bug description:

Trying to build Python from main branch and running tests on MacOS 15.2 fails on test_embed.test_init_pyvenv_cfg.

./configure && make -j && ./python.exe -m test test_embed

FAIL: test_init_pyvenv_cfg (test.test_embed.InitConfigTests.test_init_pyvenv_cfg)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/yyanchii/Desktop/cpython/Lib/test/test_embed.py", line 1671, in test_init_pyvenv_cfg
    self.check_all_configs("test_init_compat_config", config,
    ~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
                           api=API_COMPAT, env=env,
                           ^^^^^^^^^^^^^^^^^^^^^^^^
                           ignore_stderr=True, cwd=tmpdir)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/yyanchii/Desktop/cpython/Lib/test/test_embed.py", line 939, in check_all_configs
    self.check_config(configs, expected_config)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/yyanchii/Desktop/cpython/Lib/test/test_embed.py", line 865, in check_config
    self.assertEqual(config, expected)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
AssertionError: {'argv': [''], 'base_exec_prefix': '/usr/lo[2028 chars]None} != {'_config_init': 1, 'isolated': False, 'use[2128 chars]alse}
  {'_config_init': 1,
   '_init_main': True,
   '_install_importlib': True,
   'argv': [''],
-  'base_exec_prefix': '/usr/local',
+  'base_exec_prefix': '/var/folders/z9/tt4sxxm159zbh6xlm2mhrg4r0000gp/T/tmp_o9bmpp_',
   'base_executable': '/var/folders/z9/tt4sxxm159zbh6xlm2mhrg4r0000gp/T/tmp_o9bmpp_/_testembed',
   'base_prefix': '/usr/local',
   'buffered_stdio': True,
   'bytes_warning': 0,
   'check_hash_pycs_mode': 'default',
   'code_debug_ranges': True,
   'configure_c_stdio': False,
   'cpu_count': -1,
   'dev_mode': False,
   'dump_refs': False,
   'dump_refs_file': None,
   'exec_prefix': '/private/var/folders/z9/tt4sxxm159zbh6xlm2mhrg4r0000gp/T/tmp_7n79vly',
   'executable': '/private/var/folders/z9/tt4sxxm159zbh6xlm2mhrg4r0000gp/T/tmp_7n79vly/_testembed',
   'faulthandler': False,
   'hash_seed': 0,
   'home': None,
   'import_time': False,
   'inspect': False,
   'install_signal_handlers': True,
   'int_max_str_digits': 4300,
   'interactive': False,
   'isolated': False,
   'malloc_stats': False,
   'module_search_paths': ['/usr/local/lib/python314.zip',
                           '/Users/yyanchii/Desktop/cpython/Lib',
                           '/Users/yyanchii/Desktop/cpython/build/lib.macosx-15.2-arm64-3.14',
                           '/usr/local/lib/python314.zip',
                           '/usr/local/lib/python3.14',
-                          '/usr/local/lib/python3.14/lib-dynload'],
+                          '/var/folders/z9/tt4sxxm159zbh6xlm2mhrg4r0000gp/T/tmp_o9bmpp_/lib/python3.14/lib-dynload'],
   'module_search_paths_set': True,
   'optimization_level': 0,
   'orig_argv': [],
   'parse_argv': False,
   'parser_debug': False,
   'pathconfig_warnings': True,
   'perf_profiling': 0,
   'platlibdir': 'lib',
   'prefix': '/private/var/folders/z9/tt4sxxm159zbh6xlm2mhrg4r0000gp/T/tmp_7n79vly',
   'program_name': './_testembed',
   'pycache_prefix': None,
   'pythonpath_env': '/usr/local/lib/python314.zip:/Users/yyanchii/Desktop/cpython/Lib:/Users/yyanchii/Desktop/cpython/build/lib.macosx-15.2-arm64-3.14',
   'quiet': False,
   'run_command': None,
   'run_filename': None,
   'run_module': None,
   'run_presite': None,
   'safe_path': False,
   'show_ref_count': False,
   'site_import': True,
   'skip_source_first_line': False,
   'sys_path_0': None,
   'tracemalloc': 0,
   'use_environment': True,
   'use_frozen_modules': False,
   'use_hash_seed': False,
   'use_system_logger': False,
   'user_site_directory': True,
   'verbose': 0,
   'warn_default_encoding': False,
   'warnoptions': [],
   'write_bytecode': True,
   'xoptions': {}}

----------------------------------------------------------------------
Ran 82 tests in 5.051s

FAILED (failures=1, skipped=3)
test test_embed failed
test_embed failed (1 failure)

== Tests result: FAILURE ==

1 test failed:
    test_embed

Total duration: 5.1 sec
Total tests: run=82 failures=1 skipped=3
Total test files: run=1/1 failed=1
Result: FAILURE

After bisecting found out 34e840f9ddd commit to be the reason. cc @FFY00

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• GH-128690: skip test_init_pyvenv_cfg on shared builds #128724
• gh-128690: temporarily do not use test_embed in PGO profile builds #128771
• gh-128690: Update test_embed for getpath.py exec_prefix change #129137
• gh-128690: Use test_embed in PGO profile builds again #129377

[FFY00]

The test is mocking an installation, and we now return more accurate information since the prefix comes from the runtime library instead of pyvenv.cfg, so we can just skip the test when running with a shared library, since pyvenv.cfg is no longer relevant here.

[ned-deily]

FWIW, this failure apparently can also arise in non-shared builds and can have a fatal impact on builds where it does fail since test_embed is one of the tests run for PGO profiling with --enable-optimizations. See #128771 for more details.

[ned-deily]

With release manager approval, #128771 was merged to temporarily avoid using test_embed in the PGO profile builds which are used in some release builds. I'm marking this as a deferred blocker so we don't forget to resolve the main issue and restore use of test_embed.

[vstinner]

Is it possible to reproduce this issue on operating systems other than macOS? Or is the issue specific to macOS?