GH-114847: Speed up `posixpath.realpath()` (#114848)

Apply the following optimizations to `posixpath.realpath()`:

- Remove use of recursion
- Construct child paths directly rather than using `join()`
- Use `os.getcwd[b]()` rather than `abspath()`
- Use `startswith(sep)` rather than `isabs()`
- Use slicing rather than `split()`

Co-authored-by: Petr Viktorin <encukou@gmail.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/Lib/test/test_posixpath.py b/Lib/test/test_posixpath.py
index cbb7c4c..807f985 100644
--- a/Lib/test/test_posixpath.py
+++ b/Lib/test/test_posixpath.py
@@ -458,6 +458,15 @@ class PosixPathTest(unittest.TestCase):
 
     @os_helper.skip_unless_symlink
     @skip_if_ABSTFN_contains_backslash
+    def test_realpath_missing_pardir(self):
+        try:
+            os.symlink(os_helper.TESTFN + "1", os_helper.TESTFN)
+            self.assertEqual(realpath("nonexistent/../" + os_helper.TESTFN), ABSTFN + "1")
+        finally:
+            os_helper.unlink(os_helper.TESTFN)
+
+    @os_helper.skip_unless_symlink
+    @skip_if_ABSTFN_contains_backslash
     def test_realpath_symlink_loops(self):
         # Bug #930024, return the path unchanged if we get into an infinite
         # symlink loop in non-strict mode (default).