gh-139001: Fix thread-safety issue in pathlib.Path

Don't cache the joined path in `_raw_path` because the caching isn't thread safe.

Author: colesbury

Lib/pathlib/__init__.py

@@ -335,13 +335,8 @@ def _raw_path(self):
             return paths[0]
         elif paths:
             # Join path segments from the initializer.
-            path = self.parser.join(*paths)
-            # Cache the joined path.
-            paths.clear()
-            paths.append(path)
-            return path
+            return self.parser.join(*paths)
         else:
-            paths.append('')
             return ''
 
     @property

Misc/NEWS.d/next/Library/2025-09-17-12-07-21.gh-issue-139001.O6tseN.rst

@@ -0,0 +1,2 @@
+Fix race condition in :class:`pathlib.Path` on the internal ``_raw_paths``
+field.