From 92dc2b3176c56073a0ed367652af31029ca17f71 Mon Sep 17 00:00:00 2001
From: barneygale <barney.gale@gmail.com>
Date: Sat, 9 Sep 2023 17:13:27 +0100
Subject: [PATCH] GH-109187: Improve symlink loop handling in
 `pathlib.Path.resolve()`

Treat symlink loops like other errors: in strict mode, raise `OSError`, and
in non-strict mode, do not raise any exception.
---
 Doc/library/pathlib.rst                       | 14 ++++++++-----
 Lib/pathlib.py                                | 21 +------------------
 Lib/test/test_pathlib.py                      |  8 +++++--
 ...-09-09-17-09-54.gh-issue-109187.dIayNW.rst |  3 +++
 4 files changed, 19 insertions(+), 27 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2023-09-09-17-09-54.gh-issue-109187.dIayNW.rst

diff --git a/Doc/library/pathlib.rst b/Doc/library/pathlib.rst
index 22360b22fd924b..48d6176d26bb8f 100644
--- a/Doc/library/pathlib.rst
+++ b/Doc/library/pathlib.rst
@@ -1381,15 +1381,19 @@ call fails (for example because the path doesn't exist).
       >>> p.resolve()
       PosixPath('/home/antoine/pathlib/setup.py')
 
-   If the path doesn't exist and *strict* is ``True``, :exc:`FileNotFoundError`
-   is raised.  If *strict* is ``False``, the path is resolved as far as possible
-   and any remainder is appended without checking whether it exists.  If an
-   infinite loop is encountered along the resolution path, :exc:`RuntimeError`
-   is raised.
+   If a path doesn't exist or a symlink loop is encountered, and *strict* is
+   ``True``, :exc:`OSError` is raised.  If *strict* is ``False``, the path is
+   resolved as far as possible and any remainder is appended without checking
+   whether it exists.
 
    .. versionchanged:: 3.6
       The *strict* parameter was added (pre-3.6 behavior is strict).
 
+   .. versionchanged:: 3.13
+      Symlink loops are treated like other errors: :exc:`OSError` is raised in
+      strict mode, and no exception is raised in non-strict mode. In previous
+      versions, :exc:`RuntimeError` is raised no matter the value of *strict*.
+
 .. method:: Path.rglob(pattern, *, case_sensitive=None, follow_symlinks=None)
 
    Glob the given relative *pattern* recursively.  This is like calling
diff --git a/Lib/pathlib.py b/Lib/pathlib.py
index f4ec315da6b4fa..bd5f61b0b7c878 100644
--- a/Lib/pathlib.py
+++ b/Lib/pathlib.py
@@ -1230,26 +1230,7 @@ def resolve(self, strict=False):
         normalizing it.
         """
 
-        def check_eloop(e):
-            winerror = getattr(e, 'winerror', 0)
-            if e.errno == ELOOP or winerror == _WINERROR_CANT_RESOLVE_FILENAME:
-                raise RuntimeError("Symlink loop from %r" % e.filename)
-
-        try:
-            s = os.path.realpath(self, strict=strict)
-        except OSError as e:
-            check_eloop(e)
-            raise
-        p = self.with_segments(s)
-
-        # In non-strict mode, realpath() doesn't raise on symlink loops.
-        # Ensure we get an exception by calling stat()
-        if not strict:
-            try:
-                p.stat()
-            except OSError as e:
-                check_eloop(e)
-        return p
+        return self.with_segments(os.path.realpath(self, strict=strict))
 
     def owner(self):
         """
diff --git a/Lib/test/test_pathlib.py b/Lib/test/test_pathlib.py
index 09df3fe471fc3e..b07e7ba04021f4 100644
--- a/Lib/test/test_pathlib.py
+++ b/Lib/test/test_pathlib.py
@@ -3180,8 +3180,12 @@ def test_absolute(self):
 
     def _check_symlink_loop(self, *args, strict=True):
         path = self.cls(*args)
-        with self.assertRaises(RuntimeError):
-            print(path.resolve(strict))
+        if strict:
+            with self.assertRaises(OSError) as cm:
+                path.resolve(strict=True)
+            self.assertEqual(cm.exception.errno, errno.ELOOP)
+        else:
+            path.resolve(strict=False)
 
     @unittest.skipIf(
         is_emscripten or is_wasi,
diff --git a/Misc/NEWS.d/next/Library/2023-09-09-17-09-54.gh-issue-109187.dIayNW.rst b/Misc/NEWS.d/next/Library/2023-09-09-17-09-54.gh-issue-109187.dIayNW.rst
new file mode 100644
index 00000000000000..31b3ef77807cde
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2023-09-09-17-09-54.gh-issue-109187.dIayNW.rst
@@ -0,0 +1,3 @@
+:meth:`pathlib.Path.resolve` now treats symlink loops like other errors: in
+strict mode, :exc:`OSError` is raised, and in non-strict mode, no exception
+is raised.