From 4b99d39bc45be50af8a4e3bdd77816597a5bf0f7 Mon Sep 17 00:00:00 2001
From: Daniel Li <daniel.li@deshaw.com>
Date: Wed, 29 Jan 2025 10:56:51 -0500
Subject: [PATCH] gh-120170: Exclude __mp_main__ in C version of whichmodule()

Importing multiprocessing adds an alias to __main__ named __mp_main__.
In #23403, the Python version of whichmodule() was fixed to exclude
__mp_main__. Apply the same fix to the C version of the function.
---
 .../Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst    | 3 +++
 Modules/_pickle.c                                             | 4 ++++
 2 files changed, 7 insertions(+)
 create mode 100644 Misc/NEWS.d/next/Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst

diff --git a/Misc/NEWS.d/next/Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst b/Misc/NEWS.d/next/Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst
new file mode 100644
index 00000000000000..ce7d874aa20375
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-06-06-17-49-07.gh-issue-120170.DUxhmT.rst
@@ -0,0 +1,3 @@
+Fix an issue in the :mod:`!_pickle` extension module in which importing
+:mod:`multiprocessing` could change how pickle identifies which module an
+object belongs to, potentially breaking the unpickling of those objects.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 754a326822e0f0..93f1b53bb25c8c 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -1888,6 +1888,10 @@ _checkmodule(PyObject *module_name, PyObject *module,
             _PyUnicode_EqualToASCIIString(module_name, "__main__")) {
         return -1;
     }
+    if (PyUnicode_Check(module_name) &&
+            _PyUnicode_EqualToASCIIString(module_name, "__mp_main__")) {
+        return -1;
+    }
 
     PyObject *candidate = get_deep_attribute(module, dotted_path, NULL);
     if (candidate == NULL) {