GH-92737: Corrected posixpath behaviour to not assume '//' is equivalent to '/'.

Lib/posixpath.py

@@ -524,16 +524,27 @@ def relpath(path, start=None):
         start = os.fspath(start)
 
     try:
-        start_list = [x for x in abspath(start).split(sep) if x]
-        path_list = [x for x in abspath(path).split(sep) if x]
+        start_abs = abspath(normpath(start))
+        path_abs = abspath(normpath(path))
+
+        _, start_root, start_rest = splitroot(start_abs)
+        _, path_root, path_rest = splitroot(path_abs)
+
+        if start_root != path_root:
+            raise ValueError(
+                f"Path is on root {path_root!r}, start on root {start_root!r}"
+            )
+
+        start_list = [x for x in start_rest.split(sep) if x]
+        path_list = [x for x in path_rest.split(sep) if x]
         # Work out how much of the filepath is shared by start and path.
         i = len(commonprefix([start_list, path_list]))
 
         rel_list = [pardir] * (len(start_list)-i) + path_list[i:]
         if not rel_list:
             return curdir
         return join(*rel_list)
-    except (TypeError, AttributeError, BytesWarning, DeprecationWarning):
+    except (TypeError, ValueError, AttributeError, BytesWarning, DeprecationWarning):
         genericpath._check_arg_types('relpath', path, start)
         raise
 

Lib/test/test_posixpath.py

@@ -652,6 +652,12 @@ def test_relpath(self):
             self.assertEqual(posixpath.relpath("/", "/"), '.')
             self.assertEqual(posixpath.relpath("/a", "/a"), '.')
             self.assertEqual(posixpath.relpath("/a/b", "/a/b"), '.')
+            self.assertRaises(ValueError, posixpath.relpath, "/foo/bar", "//foo")
+            self.assertRaises(ValueError, posixpath.relpath, "//foo/bar", "/foo")
+            self.assertRaises(ValueError, posixpath.relpath, "//foo/bar", "///foo")
+            self.assertEqual(posixpath.relpath("//foo/bar", "//foo"), "bar")
+            self.assertEqual(posixpath.relpath("///foo/bar", "/foo"), "bar")
+            self.assertEqual(posixpath.relpath("/foo/bar/baz", "///foo"), "bar/baz")
         finally:
             os.getcwd = real_getcwd
 

Misc/NEWS.d/next/Library/2023-04-24-15-23-49.gh-issue-92737.ZK2sK3.rst

@@ -0,0 +1,2 @@
+Fixed posixpath.relpath to not assume '//' is equivalent to '/'. Patch by
+Adam Chhina.