From 69b85effc5923ad7c7beb85442b7958df47fabf5 Mon Sep 17 00:00:00 2001
From: Jelle Zijlstra <jelle@openai.com>
Date: Wed, 21 Oct 2026 20:57:07 -0700
Subject: [PATCH] gh-159739: Fully clear the unpickler memo

Clearing an Unpickler memo left its sequential index and sparse entries intact, so reusing the unpickler for an independent record could raise UnpicklingError or retrieve an object from a previous record. Replace the memo and reset all of its state before releasing the old entries. Backport to 3.15; manually backport only the counter reset to 3.14, which has no sparse memo.
---
 Lib/test/pickletester.py                      | 16 +++++++++++++
 ...-10-22-03-55-33.gh-issue-159739.a8bf30.rst |  2 ++
 Modules/_pickle.c                             | 24 ++++++++++++-------
 3 files changed, 33 insertions(+), 9 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-10-22-03-55-33.gh-issue-159739.a8bf30.rst

diff --git a/Lib/test/pickletester.py b/Lib/test/pickletester.py
index 9c0679d1d13d23d..e9d367f9ea95aa8 100644
--- a/Lib/test/pickletester.py
+++ b/Lib/test/pickletester.py
@@ -5299,6 +5299,22 @@ def test_clear_pickler_memo(self):
             self.assertNotEqual(first_pickled, second_pickled)
             self.assertEqual(first_pickled, third_pickled)
 
+    def test_clear_unpickler_memo(self):
+        value = [b'repeated'] * 2
+        data = pickle.dumps(value, protocol=4)
+        unpickler = self.unpickler_class(io.BytesIO(data * 2))
+        self.assertEqual(unpickler.load(), value)
+        unpickler.memo.clear()
+        self.assertEqual(unpickler.load(), value)
+
+    def test_clear_unpickler_memo_sparse(self):
+        # Store None at index 1000, then retrieve it in a second record.
+        unpickler = self.unpickler_class(io.BytesIO(b'Np1000\n.g1000\n.'))
+        self.assertIsNone(unpickler.load())
+        unpickler.memo.clear()
+        with self.assertRaises(pickle.UnpicklingError):
+            unpickler.load()
+
     def test_priming_pickler_memo(self):
         # Verify that we can set the Pickler's memo attribute.
         data = ["abcdefg", "abcdefg", 44]
diff --git a/Misc/NEWS.d/next/Library/2026-10-22-03-55-33.gh-issue-159739.a8bf30.rst b/Misc/NEWS.d/next/Library/2026-10-22-03-55-33.gh-issue-159739.a8bf30.rst
new file mode 100644
index 000000000000000..7052cdb6bd9682b
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-10-22-03-55-33.gh-issue-159739.a8bf30.rst
@@ -0,0 +1,2 @@
+Fix clearing a :class:`pickle.Unpickler` memo so that the unpickler can be
+reused for independent records.
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index f37c0a5bba55419..9703855e06bbb97 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -1762,16 +1762,16 @@ _Unpickler_NewMemo(Py_ssize_t new_size)
     return memo;
 }
 
-/* Free the unpickler's memo, taking care to decref any items left in it. */
+/* Replace the unpickler's memo and release the old entries. */
 static void
-_Unpickler_MemoCleanup(UnpicklerObject *self)
+_Unpickler_MemoCleanup(UnpicklerObject *self, PyObject **replacement)
 {
     Py_ssize_t i;
     PyObject **memo = self->memo;
 
-    if (self->memo == NULL)
+    self->memo = replacement;
+    if (memo == NULL)
         return;
-    self->memo = NULL;
     i = self->memo_size;
     while (--i >= 0) {
         Py_XDECREF(memo[i]);
@@ -7750,7 +7750,7 @@ Unpickler_clear(PyObject *op)
     }
 
     Py_CLEAR(self->memo_dict);
-    _Unpickler_MemoCleanup(self);
+    _Unpickler_MemoCleanup(self, NULL);
     PyMem_Free(self->marks);
     self->marks = NULL;
     PyMem_Free(self->input_line);
@@ -7897,10 +7897,16 @@ static PyObject *
 _pickle_UnpicklerMemoProxy_clear_impl(UnpicklerMemoProxyObject *self)
 /*[clinic end generated code: output=d20cd43f4ba1fb1f input=b1df7c52e7afd9bd]*/
 {
-    _Unpickler_MemoCleanup(self->unpickler);
-    self->unpickler->memo = _Unpickler_NewMemo(self->unpickler->memo_size);
-    if (self->unpickler->memo == NULL)
+    UnpicklerObject *unpickler = self->unpickler;
+    PyObject **memo = _Unpickler_NewMemo(unpickler->memo_size);
+    if (memo == NULL) {
         return NULL;
+    }
+    PyObject *memo_dict = unpickler->memo_dict;
+    unpickler->memo_dict = NULL;
+    unpickler->memo_len = 0;
+    _Unpickler_MemoCleanup(unpickler, memo);
+    Py_XDECREF(memo_dict);
     Py_RETURN_NONE;
 }
 
@@ -8105,7 +8111,7 @@ Unpickler_set_memo(PyObject *op, PyObject *obj, void *Py_UNUSED(closure))
         return -1;
     }
 
-    _Unpickler_MemoCleanup(self);
+    _Unpickler_MemoCleanup(self, NULL);
     self->memo_size = new_memo_size;
     self->memo = new_memo;