Lib/pydoc.py's _is_stdlib_module() fails when source path contains symlinks

Bug report

Bug description:

In my particular setup, both my cpython source and build dirs are normally referred to via symlinks.

With this setup, test.test_pydoc.test_pydoc fails. This seems to have happed since 78a50ee10ef73296eba98899d496bff3915ddc41

The output is below, but I think it boils down to _is_stdlib_module() - the file.startswith(basedir) call failing. In my setup, basedir is set by the call basedir = os.path.join(srcdir, 'Lib') just above.

basedir then effectively has had its symlinks resolved (or moreover, srcdir has). file has not. So the .startswith() call returns False.

This fixes it, but I have no idea what the broader implications are. For example, what if sysconfig.is_python_build() is false? Then builddir will probably remain with unresolved symlinks.

@@ -536,6 +537,7 @@ def _is_stdlib_module(self, object, basedir=None):
         try:
             file = inspect.getabsfile(object)
+            file = str(pathlib.Path(file).resolve())
         except TypeError:
             file = '(built-in)'

Here's the failing test cases.

======================================================================
ERROR: test_mixed_case_module_names_are_lower_cased (test.test_pydoc.test_pydoc.PydocDocTest.test_mixed_case_module_names_are_lower_cased)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/nick/src/cpython-trunk/Lib/test/test_pydoc/test_pydoc.py", line 469, in test_mixed_case_module_names_are_lower_cased
    self.assertIn('xml.etree.elementtree', doc_link)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/nick/src/cpython-trunk/Lib/unittest/case.py", line 1189, in assertIn
    if member not in container:
       ^^^^^^^^^^^^^^^^^^^^^^^
TypeError: argument of type 'NoneType' is not a container or iterable

======================================================================
FAIL: test_online_docs_link (test.test_pydoc.test_pydoc.PydocDocTest.test_online_docs_link)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/nick/src/cpython-trunk/Lib/test/test_pydoc/test_pydoc.py", line 490, in test_online_docs_link
    self.assertIsNotNone(doc_link)
    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^
AssertionError: unexpectedly None

======================================================================
FAIL: test__get_version (test.test_pydoc.test_pydoc.TestInternalUtilities.test__get_version)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/nick/src/cpython-trunk/Lib/test/test_pydoc/test_pydoc.py", line 2391, in test__get_version
    self.assertEqual(len(w), 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^
AssertionError: 1 != 0

----------------------------------------------------------------------
Ran 118 tests in 2.274s

FAILED (failures=2, errors=1)
test test.test_pydoc.test_pydoc failed
0:00:02 load avg: 2.42 [1/1/1] test.test_pydoc.test_pydoc failed (1 error, 2 failures)

== Tests result: FAILURE ==

1 test failed:
    test.test_pydoc.test_pydoc

Total duration: 2.4 sec
Total tests: run=118 failures=2
Total test files: run=1/1 failed=1
Result: FAILURE

CPython versions tested on:

CPython main branch

Operating systems tested on:

macOS

Linked PRs

• gh-151897