GH-117546: Fix symlink resolution in os.path.realpath('loop/../link') (#117568)

Continue resolving symlink targets after encountering a symlink loop, which
matches coreutils `realpath` behaviour.

Author: barneygale

Doc/library/os.path.rst

@@ -409,9 +409,8 @@ the :mod:`glob` module.)
    style names such as ``C:\\PROGRA~1`` to ``C:\\Program Files``.
 
    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.
+   ``True``, :exc:`OSError` is raised. If *strict* is ``False`` these errors
+   are ignored, and so the result might be missing or otherwise inaccessible.
 
    .. note::
       This function emulates the operating system's procedure for making a path

Lib/posixpath.py

@@ -431,11 +431,6 @@ def realpath(filename, *, strict=False):
     # the same links.
     seen = {}
 
-    # Whether we're calling lstat() and readlink() to resolve symlinks. If we
-    # encounter an OSError for a symlink loop in non-strict mode, this is
-    # switched off.
-    querying = True
-
     while rest:
         name = rest.pop()
         if name is None:
@@ -453,9 +448,6 @@ def realpath(filename, *, strict=False):
             newpath = path + name
         else:
             newpath = path + sep + name
-        if not querying:
-            path = newpath
-            continue
         try:
             st = os.lstat(newpath)
             if not stat.S_ISLNK(st.st_mode):
@@ -477,11 +469,8 @@ def realpath(filename, *, strict=False):
             if strict:
                 # Raise OSError(errno.ELOOP)
                 os.stat(newpath)
-            else:
-                # Return already resolved part + rest of the path unchanged.
-                path = newpath
-                querying = False
-                continue
+            path = newpath
+            continue
         seen[newpath] = None # not resolved symlink
         target = os.readlink(newpath)
         if target.startswith(sep):

Lib/test/test_posixpath.py

@@ -484,7 +484,7 @@ def test_realpath_symlink_loops(self):
             self.assertEqual(realpath(ABSTFN+"1/../x"), dirname(ABSTFN) + "/x")
             os.symlink(ABSTFN+"x", ABSTFN+"y")
             self.assertEqual(realpath(ABSTFN+"1/../" + basename(ABSTFN) + "y"),
-                             ABSTFN + "y")
+                             ABSTFN + "x")
             self.assertEqual(realpath(ABSTFN+"1/../" + basename(ABSTFN) + "1"),
                              ABSTFN + "1")
 

Misc/NEWS.d/next/Library/2024-04-05-13-38-53.gh-issue-117546.lWjhHE.rst

@@ -0,0 +1,2 @@
+Fix issue where :func:`os.path.realpath` stopped resolving symlinks after
+encountering a symlink loop on POSIX.