gh-154690: pathlib: Resolve paths in ensure_distinct_paths() before comparing

Lib/pathlib/_os.py

@@ -268,16 +268,24 @@ def ensure_distinct_paths(source, target):
     # Note: there is no straightforward, foolproof algorithm to determine
     # if one directory is within another (a particularly perverse example
     # would be a single network share mounted in one location via NFS, and
-    # in another location via CIFS), so we simply checks whether the
-    # other path is lexically equal to, or within, this path.
+    # in another location via CIFS). Resolve when possible so relative vs
+    # absolute paths and symlink aliases are compared like shutil._destinsrc
+    # (realpath); otherwise fall back to a lexical check.
+    source_orig, target_orig = source, target
+    try:
+        source = source.resolve(strict=False)
+        target = target.resolve(strict=False)
+    except (AttributeError, OSError):
+        # No resolve(), or realpath failed.
+        pass
     if source == target:
         err = OSError(EINVAL, "Source and target are the same path")
     elif source in target.parents:
         err = OSError(EINVAL, "Source path is a parent of target path")
     else:
         return
-    err.filename = vfspath(source)
-    err.filename2 = vfspath(target)
+    err.filename = vfspath(source_orig)
+    err.filename2 = vfspath(target_orig)
     raise err
 
 

Lib/test/test_pathlib/test_pathlib.py

@@ -1453,6 +1453,49 @@ def test_copy_directory_symlink_into_itself(self):
         self.assertRaises(OSError, source.copy, target, follow_symlinks=False)
         self.assertFalse(target.exists())
 
+    def test_copy_dir_into_itself_relative_source_absolute_target(self):
+        with os_helper.change_cwd(self.base):
+            source = self.cls('dirC')
+            target = source.resolve() / 'copyC'
+            self.assertRaises(OSError, source.copy, target)
+            self.assertRaises(OSError, source.copy, target, follow_symlinks=False)
+            self.assertFalse(target.exists())
+
+    def test_copy_dir_into_itself_absolute_source_relative_target(self):
+        with os_helper.change_cwd(self.base):
+            source = self.cls(self.base) / 'dirC'
+            target = self.cls('dirC') / 'copyC'
+            self.assertRaises(OSError, source.copy, target)
+            self.assertRaises(OSError, source.copy, target, follow_symlinks=False)
+            self.assertFalse((source / 'copyC').exists())
+
+    @needs_symlinks
+    def test_copy_dir_into_itself_via_symlink(self):
+        base = self.cls(self.base)
+        source = base / 'dirC'
+        link = base / 'linkToDirC'
+        link.symlink_to(source, target_is_directory=True)
+        target = link / 'copyC'
+        self.assertRaises(OSError, source.copy, target)
+        self.assertRaises(OSError, source.copy, target, follow_symlinks=False)
+        self.assertFalse((source / 'copyC').exists())
+
+    def test_copy_dir_into_itself_when_no_resolve(self):
+        base = self.cls(self.base)
+        source = base / 'dirC'
+        target = source / 'copyC'
+        with mock.patch.object(self.cls, 'resolve', side_effect=AttributeError):
+            self.assertRaises(OSError, source.copy, target)
+            self.assertFalse(target.exists())
+
+    def test_copy_dir_into_itself_when_resolve_raises_oserror(self):
+        base = self.cls(self.base)
+        source = base / 'dirC'
+        target = source / 'copyC'
+        with mock.patch.object(self.cls, 'resolve', side_effect=OSError):
+            self.assertRaises(OSError, source.copy, target)
+            self.assertFalse(target.exists())
+
     @needs_symlinks
     def test_copy_directory_symlink_to_existing_symlink(self):
         base = self.cls(self.base)

Misc/NEWS.d/next/Library/2026-07-25-12-06-55.gh-issue-154690.cHNanK.rst

@@ -0,0 +1,3 @@
+:meth:`pathlib.Path.copy` now resolves paths when checking whether the
+destination is inside the source directory, preventing bypasses via
+relative/absolute path aliases and symlinks.