gh-102381: don't call watcher callback with dead object (#102382)

Co-authored-by: T. Wouters <thomas@python.org>

1 files changed, 14 insertions, 3 deletions

diff --git a/Doc/c-api/function.rst b/Doc/c-api/function.rst
index bc7569d..947ed70 100644
--- a/Doc/c-api/function.rst
+++ b/Doc/c-api/function.rst
@@ -173,8 +173,19 @@ There are a few functions specific to Python functions.
    runtime behavior depending on optimization decisions, it does not change
    the semantics of the Python code being executed.
 
-   If the callback returns with an exception set, it must return ``-1``; this
-   exception will be printed as an unraisable exception using
-   :c:func:`PyErr_WriteUnraisable`. Otherwise it should return ``0``.
+   If *event* is ``PyFunction_EVENT_DESTROY``,  Taking a reference in the
+   callback to the about-to-be-destroyed function will resurrect it, preventing
+   it from being freed at this time. When the resurrected object is destroyed
+   later, any watcher callbacks active at that time will be called again.
+
+   If the callback sets an exception, it must return ``-1``; this exception will
+   be printed as an unraisable exception using :c:func:`PyErr_WriteUnraisable`.
+   Otherwise it should return ``0``.
+
+   There may already be a pending exception set on entry to the callback. In
+   this case, the callback should return ``0`` with the same exception still
+   set. This means the callback may not call any other API that can set an
+   exception unless it saves and clears the exception state first, and restores
+   it before returning.
 
    .. versionadded:: 3.12