[3.14] pythongh-143602: Fix duplicate buffer exports in io.BytesIO.write (python#143629) (python#143872)

pythongh-143602: Fix duplicate buffer exports in io.BytesIO.write (python#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 c461aa9)

Co-authored-by: zhong <[email protected]>
(cherry picked from commit 1241432)

Author: vstinner

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):

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:
 

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.