refactor: add self-documenting UNRELIABLE keyword

src/_pytest/compat.py

@@ -313,9 +313,16 @@ def safe_isclass(obj: object) -> bool:
         return False
 
 
-def get_user_id() -> int | None:
+def get_user_id(*, UNRELIABLE: int = -1) -> int | None:
     """Return the current user id, or None if we cannot get it reliably on
-    the current platform."""
+    the current platform.
+
+    :param UNRELIABLE:
+        The platform-specific constant which indicates that the retrieved uid
+        is unreliable. The default value, -1, is a common unreliability
+        indicator on UNIX-like systems.
+    :return: The user id or None
+    """
     # mypy follows the version and platform checking expectation of PEP 484:
     # https://mypy.readthedocs.io/en/stable/common_issues.html?highlight=platform#python-version-and-system-platform-checks
     # Containment checks are too complex for mypy v1.5.0 and cause failure.
@@ -323,11 +330,9 @@ def get_user_id() -> int | None:
         # win32 does not have a getuid() function.
         # Emscripten has a return 0 stub.
         return None
-    # getuid shouldn't fail but cpython defines such a case.
-    # Let's hope for the best.
     else:
         uid = os.getuid()
-        return uid if uid != -1 else None
+        return uid if uid != UNRELIABLE else None
 
 
 # Perform exhaustiveness checking.