gh-106045: Keep searching from venv home when the resolved base executable has no stdlib

GH-156483 searches for a venv's base installation relative to the
resolved base executable. When that executable lives outside the
installation tree that 'home' in pyvenv.cfg points into (e.g. a launcher
binary symlinked into it), nothing is found there and discovery falls
back to the compiled-in PREFIX.

Only prefer the resolved executable's directory when a standard library
landmark is found from there; otherwise keep searching from 'home'.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A93JzpSGH27i6PNX8re4YV

Author: itamaro

Lib/test/test_getpath.py

@@ -475,6 +475,39 @@ def test_venv_symlink_with_misleading_landmarks_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_venv_symlink_to_launcher_outside_runtime_posix(self):
+        "Test a venv whose base executable resolves away from its stdlib."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/usr",
+        )
+        ns.add_known_xfile("/out/launcher")
+        ns.add_known_xfile("/runtime/bin/python")
+        ns.add_known_link("/runtime/bin/python", "/out/launcher")
+        ns.add_known_xfile("/venv/bin/python")
+        ns.add_known_link("/venv/bin/python", "/runtime/bin/python")
+        ns.add_known_file("/runtime/lib/python9.8/os.py")
+        ns.add_known_dir("/runtime/lib/python9.8/lib-dynload")
+        ns.add_known_file("/venv/pyvenv.cfg", [
+            "home = /runtime/bin",
+        ])
+        expected = dict(
+            executable="/venv/bin/python",
+            prefix="/venv",
+            exec_prefix="/venv",
+            base_executable="/out/launcher",
+            base_prefix="/runtime",
+            base_exec_prefix="/runtime",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/runtime/lib/python98.zip",
+                "/runtime/lib/python9.8",
+                "/runtime/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
     def test_venv_non_installed_zip_path_posix(self):
         "Test a venv created from non-installed python has correct zip path."""
         ns = MockPosixNamespace(

Modules/getpath.py

@@ -412,7 +412,15 @@ def search_up(prefix, *landmarks, test=isfile):
                 if base_executable and isfile(base_executable):
                     # Search relative to the resolved base executable rather
                     # than the potentially external symlink in pyvenv.cfg.
-                    executable_dir = real_executable_dir = dirname(base_executable)
+                    # If nothing there locates a standard library (e.g. a
+                    # launcher binary stored outside the installation that
+                    # 'home' points into), keep searching from 'home'.
+                    base_executable_dir = dirname(base_executable)
+                    if (
+                        isfile(joinpath(base_executable_dir, BUILDDIR_TXT))
+                        or search_up(base_executable_dir, ZIP_LANDMARK, *STDLIB_LANDMARKS)
+                    ):
+                        executable_dir = real_executable_dir = base_executable_dir
             # home key found; stop iterating over lines
             break