gh-91985: Fix sys.path calculation with PYTHONHOME on Windows (alt)

Lib/test/test_embed.py

@@ -1264,6 +1264,14 @@ def test_init_setpythonhome(self):
             # Because we are specifying 'home', module search paths
             # are fairly static
             expected_paths = [paths[0], stdlib, os.path.join(home, 'DLLs')]
+            try:
+                # gh-91985 Replace the DLLs path according to pybuilddir.txt
+                exedir = os.path.dirname(self.test_exe)
+                with open(os.path.join(exedir, 'pybuilddir.txt'), encoding='utf8') as f:
+                    expected_paths[-1] = os.path.normpath(
+                        os.path.join(exedir, f'{f.read()}\n$'.splitlines()[0]))
+            except FileNotFoundError:
+                pass
         else:
             version = f'{sys.version_info.major}.{sys.version_info.minor}'
             stdlib = os.path.join(home, sys.platlibdir, f'python{version}')
@@ -1284,6 +1292,39 @@ def test_init_setpythonhome(self):
         self.check_all_configs("test_init_setpythonhome", config,
                                api=API_COMPAT, env=env)
 
+    @unittest.skipUnless(MS_WINDOWS, 'specific to Windows')
+    def test_init_setpythonhome_in_tmpdir(self):
+        # Test Py_SetPythonHome(home) with PYTHONPATH env var,
+        # running in a temp directory where pybuilddir.txt does not exist.
+        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}")
+
+        pyddir = os.path.dirname(
+            import_helper.import_module('_testinternalcapi').__file__)
+        with self.tmpdir_with_python() as tmpdir:
+            config = {
+                'home': home,
+                'pythonpath_env': pyddir,
+                'module_search_paths': [
+                    os.path.join(tmpdir, os.path.basename(paths[0])),  # zip
+                    os.path.join(home, 'Lib'),
+                    os.path.join(home, 'DLLs'),
+                ],
+            }
+            self.default_program_name(config)
+            env = {'TESTHOME': home, 'PYTHONPATH': pyddir}
+            self.check_all_configs("test_init_setpythonhome", config,
+                                   api=API_COMPAT, env=env)
+
     def copy_paths_by_env(self, config):
         all_configs = self._get_expected_config()
         paths = all_configs['config']['module_search_paths']

Modules/getpath.py

@@ -603,7 +603,16 @@ def search_up(prefix, *landmarks, test=isfile):
         stdlib_dir = ''
 
 if not platstdlib_dir:
-    if exec_prefix:
+    if os_name == 'nt' and real_executable_dir and \
+            isfile(joinpath(real_executable_dir, BUILDDIR_TXT)):
+        try:
+            platstdlib_dir = joinpath(
+                real_executable_dir,
+                readlines(joinpath(real_executable_dir, BUILDDIR_TXT))[0],
+            )
+        except IndexError:
+            platstdlib_dir = real_executable_dir
+    elif exec_prefix:
         platstdlib_dir = joinpath(exec_prefix, PLATSTDLIB_LANDMARK)
     else:
         platstdlib_dir = ''