gh-91985: Ensure the same path calculations when repeated with PYTHONHOME

Lib/test/test_embed.py

@@ -1386,6 +1386,49 @@ def test_init_pybuilddir_win32(self):
                                    api=API_COMPAT, env=env,
                                    ignore_stderr=False, cwd=tmpdir)
 
+    @unittest.skipUnless(MS_WINDOWS, 'Windows only')
+    def test_repeated_init_pybuilddir_pythonhome_win32(self):
+        # Test an out-of-build-tree layout with PYTHONHOME override,
+        # repeating path calculation (gh-91985). This layout is cited
+        # from test_buildtree_pythonhome_win32 in test_getpath.
+        config = self._get_expected_config()
+        paths = config['config']['module_search_paths']
+
+        for path in paths:
+            if not os.path.isdir(path):
+                continue
+            if os.path.exists(os.path.join(path, 'os.py')):
+                home = os.path.dirname(path)
+                break
+        else:
+            self.fail(f"Unable to find home in {paths!r}")
+
+        with self.tmpdir_with_python() as tmpdir:
+            filename = os.path.join(tmpdir, 'pybuilddir.txt')
+            with open(filename, "w", encoding="utf8") as fp:
+                fp.write(tmpdir)
+
+            config = {
+                'home': home,
+                'base_exec_prefix': home,
+                'base_prefix': home,
+                'base_executable': self.test_exe,
+                'executable': self.test_exe,
+                'prefix': home,
+                'exec_prefix': home,
+                'stdlib_dir': os.path.join(home, 'Lib'),
+                'module_search_paths': [
+                    os.path.join(tmpdir, os.path.basename(paths[0])),  # .zip
+                    os.path.join(home, 'Lib'),
+                    os.path.join(tmpdir),
+                ],
+            }
+            pyd = import_helper.import_module('_testinternalcapi').__file__
+            shutil.copyfile(pyd, os.path.join(tmpdir, os.path.basename(pyd)))
+            self.check_all_configs("test_repeated_init_compat_config", config,
+                                   api=API_COMPAT, env=dict(PYTHONHOME=home),
+                                   ignore_stderr=False, cwd=tmpdir)
+
     def test_init_pyvenv_cfg(self):
         # Test path configuration with pyvenv.cfg configuration file
 

Lib/test/test_getpath.py

@@ -239,6 +239,54 @@ def test_buildtree_pythonhome_win32(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_custom_setpythonhome_win32(self):
+        """Test a deterministic layout on Windows.
+
+        PYTHONHOME, *._pth file, and pybuilddir.txt are ignored when 'home' is
+        explicitly set on a embedded Python.
+        """
+        ns = MockNTNamespace(
+            argv0=r"C:\a\embed.exe",
+            ENV_PYTHONHOME=r"C:\a",
+        )
+        # After PyConfig_SetString() or Py_SetPythonHome() is used
+        ns["config"]["home"] = r"C:\Python"
+
+        ns.add_known_file(r"C:\a\pybuilddir.txt", [""])
+        ns.add_known_file(r"C:\a\python._pth", [""])
+        expected = dict(
+            executable=r"C:\a\embed.exe",
+            base_executable=r"C:\a\embed.exe",
+            prefix=r"C:\Python",
+            exec_prefix=r"C:\Python",
+            module_search_paths_set=1,
+            module_search_paths=[
+                r"C:\a\python98.zip",
+                r"C:\Python\Lib",
+                r"C:\Python\DLLs",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
+    def test_custom_module_search_paths_set_win32(self):
+        "Test a deterministic layout on Windows with module_search_paths"
+        ns = MockNTNamespace(
+            argv0=r"C:\a\embed.exe",
+            ENV_PYTHONHOME=r"C:\a",
+        )
+        ns["config"]["home"] = r"C:\Python"
+        ns["config"]["module_search_paths"] = [r"C:\Python\Lib"]
+        ns["config"]["module_search_paths_set"] = 1
+        ns.add_known_file(r"C:\a\pybuilddir.txt", [""])
+        ns.add_known_file(r"C:\a\python._pth", [""])
+        expected = dict(
+            module_search_paths_set=1,
+            module_search_paths=[r"C:\Python\Lib"],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
     def test_normal_posix(self):
         "Test a 'standard' install layout on *nix"
         ns = MockPosixNamespace(

Programs/_testembed.c

@@ -481,6 +481,27 @@ static int test_init_compat_config(void)
 }
 
 
+static int test_repeated_init_compat_config(void)
+{
+    PyConfig config;
+    _PyConfig_InitCompatConfig(&config);
+    config_set_program_name(&config);
+
+    for (int i=0; i<3; i++) {
+        PyStatus status = Py_InitializeFromConfig(&config);
+        if (PyStatus_Exception(status)) {
+            Py_ExitStatusException(status);
+        }
+        Py_Finalize();
+    }
+    init_from_config_clear(&config);
+
+    dump_config();
+    Py_Finalize();
+    return 0;
+}
+
+
 static int test_init_global_config(void)
 {
     /* FIXME: test Py_IgnoreEnvironmentFlag */
@@ -1939,6 +1960,7 @@ static struct TestCase TestCases[] = {
     {"test_init_initialize_config", test_init_initialize_config},
     {"test_preinit_compat_config", test_preinit_compat_config},
     {"test_init_compat_config", test_init_compat_config},
+    {"test_repeated_init_compat_config", test_repeated_init_compat_config},
     {"test_init_global_config", test_init_global_config},
     {"test_init_from_config", test_init_from_config},
     {"test_init_parse_argv", test_init_parse_argv},

Python/pathconfig.c

@@ -42,7 +42,10 @@ typedef struct _PyPathConfig {
       {.module_search_path = NULL}
 
 
-_PyPathConfig _Py_path_config = _PyPathConfig_INIT;
+static _PyPathConfig _Py_path_config = _PyPathConfig_INIT;
+
+// Turned on when _Py_path_config.home is set directly by Py_SetPythonHome()
+static int home_is_original = 0;
 
 
 const wchar_t *
@@ -76,6 +79,7 @@ _PyPathConfig_ClearGlobal(void)
 #undef CLEAR
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
+    home_is_original = 0;
 }
 
 PyStatus
@@ -103,12 +107,18 @@ _PyPathConfig_ReadGlobal(PyConfig *config)
     COPY(exec_prefix);
     COPY(stdlib_dir);
     COPY(program_name);
-    COPY(home);
     COPY2(executable, program_full_path);
     // module_search_path must be initialised - not read
 #undef COPY
 #undef COPY2
 
+    // _Py_path_config.home cannot be reused in getpath.py except when
+    // the value is set by Py_SetPythonHome().
+    if (_Py_path_config.home && !config->home && home_is_original) {
+        status = PyConfig_SetString(config, &config->home, _Py_path_config.home);
+        if (_PyStatus_EXCEPTION(status)) goto done;
+    }
+
 done:
     return status;
 }
@@ -137,6 +147,12 @@ _PyPathConfig_UpdateGlobal(const PyConfig *config)
         } \
     } while (0)
 
+    if (!_Py_path_config.home ||
+        (config->home && wcscmp(_Py_path_config.home, config->home) != 0))
+    {
+        home_is_original = 0;
+    }
+
     COPY(prefix);
     COPY(exec_prefix);
     COPY(stdlib_dir);
@@ -242,6 +258,11 @@ Py_SetPythonHome(const wchar_t *home)
     PyMem_RawFree(_Py_path_config.home);
     if (has_value) {
         _Py_path_config.home = _PyMem_RawWcsdup(home);
+        home_is_original = 1;
+    }
+    else {
+        _Py_path_config.home = NULL;
+        home_is_original = 0;
     }
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);