gh-141831: Document behavior of functools.lru_cache with concurrent access (gh-141869)

diff --git a/Doc/library/functools.rst b/Doc/library/functools.rst
index 97136b234084fc176b4a8ed7c00b0f2df5873e43..221c0712c7c96a15375bd5ec55f35e53d6e15492 100644 (file)
--- a/Doc/library/functools.rst
+++ b/Doc/library/functools.rst
@@ -57,6 +57,10 @@ The :mod:`functools` module defines the following functions:
    another thread makes an additional call before the initial call has been
    completed and cached.
 
+   Call-once behavior is not guaranteed because locks are not held during the
+   function call. Potentially another call with the same arguments could
+   occur while the first call is still running.
+
    .. versionadded:: 3.9