gh-119696: Find the standard library from the shared library path

Lib/test/test_getpath.py

@@ -290,6 +290,115 @@ def test_normal_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_shared_library_posix(self):
+        for os_name in ('posix', 'darwin'):
+            for platlibdir in ('lib', 'lib64'):
+                for argv0 in ('missing-python', '/application/bin/host'):
+                    with self.subTest(os_name=os_name, platlibdir=platlibdir,
+                                      argv0=argv0):
+                        ns = MockPosixNamespace(
+                            os_name=os_name,
+                            PREFIX='/configured',
+                            EXEC_PREFIX='/configured',
+                            PLATLIBDIR=platlibdir,
+                            argv0=argv0,
+                            library=f'/moved/bin/../{platlibdir}/libpython9.8.so',
+                        )
+                        ns.add_known_xfile('/application/bin/host')
+                        for prefix in ('/configured', '/moved'):
+                            ns.add_known_file(
+                                f'{prefix}/{platlibdir}/python9.8/os.py')
+                            ns.add_known_dir(
+                                f'{prefix}/{platlibdir}/python9.8/lib-dynload')
+                        expected = dict(
+                            prefix='/moved',
+                            exec_prefix='/moved',
+                            base_prefix='/moved',
+                            base_exec_prefix='/moved',
+                            module_search_paths=[
+                                f'/moved/{platlibdir}/python98.zip',
+                                f'/moved/{platlibdir}/python9.8',
+                                f'/moved/{platlibdir}/python9.8/lib-dynload',
+                            ],
+                        )
+                        self.assertEqual(expected, getpath(ns, expected))
+
+    def test_shared_library_home_posix(self):
+        ns = MockPosixNamespace(
+            PREFIX='/configured',
+            argv0='missing-python',
+            library='/moved/lib/libpython9.8.so',
+            ENV_PYTHONHOME='/pythonhome',
+        )
+        for prefix in ('/moved', '/pythonhome'):
+            ns.add_known_file(f'{prefix}/lib/python9.8/os.py')
+            ns.add_known_dir(f'{prefix}/lib/python9.8/lib-dynload')
+        expected = dict(
+            prefix='/pythonhome',
+            exec_prefix='/pythonhome',
+            base_prefix='/pythonhome',
+            base_exec_prefix='/pythonhome',
+        )
+        self.assertEqual(expected, getpath(ns, expected))
+
+    def test_shared_library_relative_path_posix(self):
+        def abspath(path):
+            return posixpath.normpath(posixpath.join('/foreign/work', path))
+
+        for library in ('libpython9.8.so', 'lib/libpython9.8.so'):
+            with self.subTest(library=library):
+                ns = MockPosixNamespace(
+                    argv0='/moved/bin/python',
+                    library=library,
+                    abspath=abspath,
+                )
+                ns.add_known_xfile('/moved/bin/python')
+                for prefix in ('/moved', '/foreign'):
+                    ns.add_known_file(f'{prefix}/lib/python9.8/os.py')
+                    ns.add_known_dir(f'{prefix}/lib/python9.8/lib-dynload')
+                expected = dict(prefix='/moved', exec_prefix='/moved')
+                self.assertEqual(expected, getpath(ns, expected))
+
+    def test_shared_library_without_cwd_posix(self):
+        def abspath(path):
+            raise FileNotFoundError('working directory has been removed')
+
+        ns = MockPosixNamespace(
+            PREFIX='/configured',
+            argv0='python',
+            config={'executable': '/moved/bin/python'},
+            library='libpython9.8.so',
+            abspath=abspath,
+        )
+        ns.add_known_xfile('/moved/bin/python')
+        ns.add_known_file('/moved/lib/python9.8/os.py')
+        ns.add_known_dir('/moved/lib/python9.8/lib-dynload')
+        expected = dict(prefix='/moved', exec_prefix='/moved')
+        self.assertEqual(expected, getpath(ns, expected))
+
+    def test_shared_library_venv_posix(self):
+        ns = MockPosixNamespace(
+            PREFIX='/configured',
+            argv0='python',
+            ENV_PATH='/venv/bin:/moved/bin',
+            library='/moved/lib/libpython9.8.so',
+        )
+        ns.add_known_xfile('/moved/bin/python')
+        ns.add_known_xfile('/venv/bin/python')
+        ns.add_known_link('/venv/bin/python', '/moved/bin/python')
+        ns.add_known_file('/moved/lib/python9.8/os.py')
+        ns.add_known_dir('/moved/lib/python9.8/lib-dynload')
+        ns.add_known_file('/venv/pyvenv.cfg', ['home = /moved/bin'])
+        expected = dict(
+            executable='/venv/bin/python',
+            base_executable='/moved/bin/python',
+            prefix='/venv',
+            exec_prefix='/venv',
+            base_prefix='/moved',
+            base_exec_prefix='/moved',
+        )
+        self.assertEqual(expected, getpath(ns, expected))
+
     def test_buildpath_posix(self):
         """Test an in-build-tree layout on POSIX.
 
@@ -1328,7 +1437,7 @@ def __missing__(self, key):
 
     def abspath(self, path):
         if self.isabs(path):
-            return path
+            return posixpath.normpath(path)
         return self.joinpath("/Absolute", path)
 
     def basename(self, path):

Lib/test/test_venv.py

@@ -1138,6 +1138,17 @@ def test_zippath_from_non_installed_posix(self):
         ld_library_path = sysconfig.get_config_var("LIBDIR")
         if not ld_library_path or sysconfig.is_python_build():
             ld_library_path = os.path.abspath(os.path.dirname(sys.executable))
+        if (sysconfig.get_config_var('Py_ENABLE_SHARED')
+                and sys.platform != 'cygwin'):
+            # Startup can search relative to libpython before the executable.
+            # Load a copy from the temporary installation to select its
+            # standard library.
+            shared_libdir = os.path.join(non_installed_dir, 'lib')
+            os.makedirs(shared_libdir, exist_ok=True)
+            instsoname = sysconfig.get_config_var('INSTSONAME')
+            shutil.copy2(os.path.join(ld_library_path, instsoname),
+                         os.path.join(shared_libdir, instsoname))
+            ld_library_path = os.pathsep.join((shared_libdir, ld_library_path))
         if sys.platform == 'darwin':
             ld_library_path_env = "DYLD_LIBRARY_PATH"
         else:
@@ -1154,7 +1165,9 @@ def test_zippath_from_non_installed_posix(self):
         # correct zip path in pythonpath.
         target_python = os.path.join(self.env_dir, self.bindir, python_exe)
         cmd = [target_python, '-S', '-c', 'import sys; print(sys.path)']
-        out, err = check_output(cmd)
+        with EnvironmentVarGuard() as env:
+            env[ld_library_path_env] = ld_library_path
+            out, err = check_output(cmd)
         self.assertTrue(zip_landmark.encode() in out)
 
     @requireVenvCreate

Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-20-24-00.gh-issue-119696.p4ThLs.rst

@@ -0,0 +1,3 @@
+A relocated POSIX shared build can now find its unpacked standard library at
+startup when a launcher's ``argv[0]`` points outside the installation,
+provided the loader reports an absolute path to ``libpython``.

Modules/getpath.py

@@ -577,7 +577,7 @@ def search_up(prefix, *landmarks, test=isfile):
             stdlib_dir = None
 
 
-    # First try to detect prefix by looking alongside our runtime library, if known
+    # First try to detect prefix by searching from our runtime library, if known.
     if library and not prefix:
         library_dir = dirname(library)
         if ZIP_LANDMARK:
@@ -588,10 +588,14 @@ def search_up(prefix, *landmarks, test=isfile):
             else:
                 prefix = search_up(library_dir, ZIP_LANDMARK)
         if STDLIB_SUBDIR and STDLIB_LANDMARKS and not prefix:
-            if any(isfile(joinpath(library_dir, f)) for f in STDLIB_LANDMARKS):
+            # A relative loader path may have been resolved against an earlier
+            # working directory.
+            if os_name != 'nt' and isabs(library):
+                prefix = search_up(dirname(abspath(library)), *STDLIB_LANDMARKS)
+            elif any(isfile(joinpath(library_dir, f)) for f in STDLIB_LANDMARKS):
                 prefix = library_dir
-                if not stdlib_dir_was_set_in_config:
-                    stdlib_dir = joinpath(prefix, STDLIB_SUBDIR)
+            if prefix and not stdlib_dir_was_set_in_config:
+                stdlib_dir = joinpath(prefix, STDLIB_SUBDIR)
 
 
     # Detect prefix by looking for zip file