From 6db63cd700f55b8ff220136f12df9ddf9fcf7c4c Mon Sep 17 00:00:00 2001
From: Victor Stinner <vstinner@python.org>
Date: Thu, 15 Jan 2026 17:02:46 +0100
Subject: [PATCH] [3.14] gh-143602: Fix duplicate buffer exports in
 io.BytesIO.write (#143629) (#143872)

gh-143602: Fix duplicate buffer exports in io.BytesIO.write (#143629)

Fix an inconsistency issue in io.BytesIO.write() where the buffer was exported
twice, which could lead to unexpected data overwrites and position drift when
the buffer changes between exports.

(cherry picked from commit c461aa99e2fabbaf5859c0a8a93e08306ee8115d)

Co-authored-by: zhong <60600792+superboy-zjc@users.noreply.github.com>
(cherry picked from commit 1241432150f6342e3d38c5a80a19c8c157a4ebe8)
---
 Lib/_pyio.py                                  | 24 ++++++++++---------
 Lib/test/test_memoryio.py                     | 22 +++++++++++++++++
 ...-01-09-12-37-19.gh-issue-143602.V8vQpj.rst |  2 ++
 3 files changed, 37 insertions(+), 11 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-01-09-12-37-19.gh-issue-143602.V8vQpj.rst

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 59f4d06f1f5bf2..9ed2e5b2aa0aac 100644
--- a/Lib/_pyio.py
+++ b/Lib/_pyio.py
@@ -944,19 +944,21 @@ def write(self, b):
         if isinstance(b, str):
             raise TypeError("can't write str to binary stream")
         with memoryview(b) as view:
-            n = view.nbytes  # Size of any bytes-like object
             if self.closed:
                 raise ValueError("write to closed file")
-        if n == 0:
-            return 0
-        pos = self._pos
-        if pos > len(self._buffer):
-            # Inserts null bytes between the current end of the file
-            # and the new write position.
-            padding = b'\x00' * (pos - len(self._buffer))
-            self._buffer += padding
-        self._buffer[pos:pos + n] = b
-        self._pos += n
+
+            n = view.nbytes  # Size of any bytes-like object
+            if n == 0:
+                return 0
+
+            pos = self._pos
+            if pos > len(self._buffer):
+                # Inserts null bytes between the current end of the file
+                # and the new write position.
+                padding = b'\x00' * (pos - len(self._buffer))
+                self._buffer += padding
+            self._buffer[pos:pos + n] = view
+            self._pos += n
         return n
 
     def seek(self, pos, whence=0):
diff --git a/Lib/test/test_memoryio.py b/Lib/test/test_memoryio.py
index 91b9c87160af56..a1b881dff16244 100644
--- a/Lib/test/test_memoryio.py
+++ b/Lib/test/test_memoryio.py
@@ -629,6 +629,28 @@ def __buffer__(self, flags):
         memio = self.ioclass()
         self.assertRaises(BufferError, memio.writelines, [B()])
 
+    def test_write_mutating_buffer(self):
+        # Test that buffer is exported only once during write().
+        # See: https://github.com/python/cpython/issues/143602.
+        class B:
+            count = 0
+            def __buffer__(self, flags):
+                self.count += 1
+                if self.count == 1:
+                    return memoryview(b"AAA")
+                else:
+                    return memoryview(b"BBBBBBBBB")
+
+        memio = self.ioclass(b'0123456789')
+        memio.seek(2)
+        b = B()
+        n = memio.write(b)
+
+        self.assertEqual(b.count, 1)
+        self.assertEqual(n, 3)
+        self.assertEqual(memio.getvalue(), b"01AAA56789")
+        self.assertEqual(memio.tell(), 5)
+
 
 class TextIOTestMixin:
 
diff --git a/Misc/NEWS.d/next/Library/2026-01-09-12-37-19.gh-issue-143602.V8vQpj.rst b/Misc/NEWS.d/next/Library/2026-01-09-12-37-19.gh-issue-143602.V8vQpj.rst
new file mode 100644
index 00000000000000..0eaec9029221ba
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-01-09-12-37-19.gh-issue-143602.V8vQpj.rst
@@ -0,0 +1,2 @@
+Fix a inconsistency issue in :meth:`~io.RawIOBase.write` that leads to
+unexpected buffer overwrite by deduplicating the buffer exports.