From 6d5c37084d16566ffda835e5ec7017a012f9773e Mon Sep 17 00:00:00 2001
From: Petr Viktorin <encukou@gmail.com>
Date: Wed, 28 May 2025 09:53:04 +0200
Subject: [PATCH] gh-134160: Block multiple module initialization (GH-134773)
 (cherry picked from commit 469a56470b1d8df03862d8ceb73aebca14772dcf)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
---
 Doc/extending/extending.rst | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/Doc/extending/extending.rst b/Doc/extending/extending.rst
index 918c751b009761..fd63495674651b 100644
--- a/Doc/extending/extending.rst
+++ b/Doc/extending/extending.rst
@@ -204,17 +204,32 @@ value must be in a particular range or must satisfy other conditions,
 :c:data:`PyExc_ValueError` is appropriate.
 
 You can also define a new exception that is unique to your module.
-For this, you can declare a static global object variable at the beginning
-of the file::
+The simplest way to do this is to declare a static global object variable at
+the beginning of the file::
 
-   static PyObject *SpamError;
+   static PyObject *SpamError = NULL;
 
-and initialize it with an exception object in the module's
+and initialize it by calling :c:func:`PyErr_NewException` in the module's
 :c:data:`Py_mod_exec` function (:c:func:`!spam_module_exec`)::
 
+   SpamError = PyErr_NewException("spam.error", NULL, NULL);
+
+Since :c:data:`!SpamError` is a global variable, it will be overwitten every time
+the module is reinitialized, when the :c:data:`Py_mod_exec` function is called.
+
+For now, let's avoid the issue: we will block repeated initialization by raising an
+:py:exc:`ImportError`::
+
+   static PyObject *SpamError = NULL;
+
    static int
    spam_module_exec(PyObject *m)
    {
+       if (SpamError != NULL) {
+           PyErr_SetString(PyExc_ImportError,
+                           "cannot initialize spam module more than once");
+           return -1;
+       }
        SpamError = PyErr_NewException("spam.error", NULL, NULL);
        if (PyModule_AddObjectRef(m, "SpamError", SpamError) < 0) {
            return -1;
@@ -253,6 +268,11 @@ needed to ensure that it will not be discarded, causing :c:data:`!SpamError` to
 become a dangling pointer. Should it become a dangling pointer, C code which
 raises the exception could cause a core dump or other unintended side effects.
 
+For now, the :c:func:`Py_DECREF` call to remove this reference is missing.
+Even when the Python interpreter shuts down, the global :c:data:`!SpamError`
+variable will not be garbage-collected. It will "leak".
+We did, however, ensure that this will happen at most once per process.
+
 We discuss the use of :c:macro:`PyMODINIT_FUNC` as a function return type later in this
 sample.