Keep the resolved base executable directory for exec_prefix discovery

Even when stdlib discovery falls back to 'home', the resolved base
executable may live under a separate exec_prefix (e.g. ./configure
--exec-prefix), so search there for platform dependent libraries before
falling back to the compiled-in EXEC_PREFIX.

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

@@ -508,6 +508,40 @@ def test_venv_symlink_to_launcher_outside_runtime_posix(self):
         actual = getpath(ns, expected)
         self.assertEqual(expected, actual)
 
+    def test_venv_symlink_to_separate_exec_prefix_posix(self):
+        "Test a venv whose base executable lives under a separate exec_prefix."
+        ns = MockPosixNamespace(
+            argv0="/venv/bin/python",
+            PREFIX="/usr",
+            EXEC_PREFIX="/usr",
+        )
+        ns.add_known_xfile("/exec/bin/python")
+        ns.add_known_xfile("/runtime/bin/python")
+        ns.add_known_link("/runtime/bin/python", "/exec/bin/python")
+        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("/exec/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="/exec/bin/python",
+            base_prefix="/runtime",
+            base_exec_prefix="/exec",
+            module_search_paths_set=1,
+            module_search_paths=[
+                "/runtime/lib/python98.zip",
+                "/runtime/lib/python9.8",
+                "/exec/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

@@ -232,6 +232,7 @@ def search_up(prefix, *landmarks, test=isfile):
 stdlib_dir_was_set_in_config = bool(stdlib_dir)
 
 real_executable_dir = None
+base_executable_dir = None
 platstdlib_dir = None
 stdlib_zip = None
 
@@ -415,6 +416,8 @@ def search_up(prefix, *landmarks, test=isfile):
                     # 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 is still used to find platform
+                    # dependent libraries (exec_prefix) in that case.
                     base_executable_dir = dirname(base_executable)
                     if (
                         isfile(joinpath(base_executable_dir, BUILDDIR_TXT))
@@ -632,6 +635,11 @@ def search_up(prefix, *landmarks, test=isfile):
             exec_prefix = prefix
         if not exec_prefix and executable_dir:
             exec_prefix = search_up(executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
+        if not exec_prefix and base_executable_dir and base_executable_dir != executable_dir:
+            # The venv's resolved base executable may live under a separate
+            # exec_prefix (e.g. ./configure --exec-prefix) even when the
+            # stdlib was found from 'home'.
+            exec_prefix = search_up(base_executable_dir, PLATSTDLIB_LANDMARK, test=isdir)
         if not exec_prefix and EXEC_PREFIX:
             exec_prefix = EXEC_PREFIX
         if not exec_prefix or not isdir(joinpath(exec_prefix, PLATSTDLIB_LANDMARK)):