fix(core): stop atomic writes from colliding on a constant PID suffix (#5721)

atomic_write_json/atomic_write_text build their temp filename as
"{path}.tmp.{os.getpid()}". os.getpid() is constant for the life of a
process, so it only ever distinguishes concurrent writers that live in
different OS processes. Odysseus runs as a single long-lived process
per container, so two concurrent writers to the same path (e.g. two
request handlers racing a settings save) always compute the identical
temp path. Whichever finishes os.replace() first removes the shared
tmp file out from under the other, which then raises FileNotFoundError
on its own os.replace() instead of landing its write.

Fix: derive the temp suffix from uuid4() instead of the PID, so every
call gets a distinct temp path regardless of process/thread identity.

routes/prefs_routes.py's _save() had an independent, hand-rolled copy
of the exact same PID-suffix logic (not the shared core.atomic_io
helper other routes already use, e.g. routes/auth_routes.py) with the
same bug. Replaced it with a call to atomic_write_json.

Fixes #5596

Co-authored-by: Alexandre Teixeira <111787685+alteixeira20@users.noreply.github.com>

core/atomic_io.py

@@ -15,17 +15,21 @@ from __future__ import annotations
 
 import json
 import os
+import uuid
 from typing import Any, Optional
 
 
 def atomic_write_json(path: str, data: Any, *, indent: Optional[int] = None) -> None:
     """Atomically persist `data` as JSON at `path`.
 
-    The temp file uses the live PID as a suffix so two processes saving the
-    same file (e.g. unit tests) don't collide on the rename target.
+    The temp file uses a random suffix so two concurrent writers saving the
+    same file don't collide on the rename target. A PID suffix does not do
+    this: the PID is constant for the life of a process, so two writers on
+    the same path within one process (or one single-process container, where
+    the PID never changes at all) still race for the same temp file.
     """
     os.makedirs(os.path.dirname(path) or ".", exist_ok=True)
-    tmp = f"{path}.tmp.{os.getpid()}"
+    tmp = f"{path}.tmp.{uuid.uuid4().hex}"
     with open(tmp, "w", encoding="utf-8") as f:
         json.dump(data, f, indent=indent)
         f.flush()
@@ -37,7 +41,7 @@ def atomic_write_text(path: str, text: str) -> None:
     if not isinstance(text, str):
         raise TypeError("atomic_write_text expects a string")
     os.makedirs(os.path.dirname(path) or ".", exist_ok=True)
-    tmp = f"{path}.tmp.{os.getpid()}"
+    tmp = f"{path}.tmp.{uuid.uuid4().hex}"
     with open(tmp, "w", encoding="utf-8") as f:
         f.write(text)
         f.flush()