Fix venv path discovery for symlinked executables

Use the resolved base executable when locating the base installation
instead of relying on the home directory in pyvenv.cfg.

Add tests covering relocated Python installations and misleading
standard-library landmarks near executable symlinks.

Author: jjhelmus

Lib/test/test_getpath.py

@@ -407,6 +407,74 @@ def test_venv_changed_name_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_venv_relocated_base_installation_posix(self):
+        "Test a venv whose base installation was relocated from PREFIX."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/build/install",
+        )
+        ns.add_known_xfile("/relocated/bin/python3")
+        ns.add_known_xfile("/public/bin/python")
+        ns.add_known_link("/public/bin/python", "/relocated/bin/python3")
+        ns.add_known_xfile("/venv/bin/python")
+        ns.add_known_link("/venv/bin/python", "/public/bin/python")
+        ns.add_known_file("/relocated/lib/python9.8/os.py")
+        ns.add_known_dir("/relocated/lib/python9.8/lib-dynload")
+        ns.add_known_file("/venv/pyvenv.cfg", [
+            "home = /public/bin",
+        ])
+        expected = dict(
+            executable="/venv/bin/python",
+            prefix="/venv",
+            exec_prefix="/venv",
+            base_executable="/relocated/bin/python3",
+            base_prefix="/relocated",
+            base_exec_prefix="/relocated",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/relocated/lib/python98.zip",
+                "/relocated/lib/python9.8",
+                "/relocated/lib/python9.8/lib-dynload",
+            ],
+        )
+        actual = getpath(ns, expected)
+        self.assertEqual(expected, actual)
+
+    def test_venv_symlink_with_misleading_landmarks_posix(self):
+        "Test that landmarks near an external symlink do effect resolution."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/opt/python",
+        )
+        ns.add_known_xfile("/opt/python/bin/python3")
+        ns.add_known_xfile("/public/bin/python")
+        ns.add_known_link("/public/bin/python", "/opt/python/bin/python3")
+        ns.add_known_xfile("/venv/bin/python")
+        ns.add_known_link("/venv/bin/python", "/public/bin/python")
+        ns.add_known_file("/opt/python/lib/python9.8/os.py")
+        ns.add_known_dir("/opt/python/lib/python9.8/lib-dynload")
+        ns.add_known_file("/public/lib/python9.8/os.py")
+        ns.add_known_dir("/public/lib/python9.8/lib-dynload")
+        ns.add_known_file("/venv/pyvenv.cfg", [
+            "home = /public/bin",
+        ])
+        expected = dict(
+            executable="/venv/bin/python",
+            prefix="/venv",
+            exec_prefix="/venv",
+            base_executable="/opt/python/bin/python3",
+            base_prefix="/opt/python",
+            base_exec_prefix="/opt/python",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/opt/python/lib/python98.zip",
+                "/opt/python/lib/python9.8",
+                "/opt/python/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

@@ -409,6 +409,10 @@ def search_up(prefix, *landmarks, test=isfile):
                             if isfile(candidate):
                                 base_executable = candidate
                                 break
+                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)
             # home key found; stop iterating over lines
             break