[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>

Lib/_pyio.py

@@ -938,17 +938,19 @@ class BytesIO(BufferedIOBase):
         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):
-            # Pad buffer to pos with null bytes.
-            self._buffer.resize(pos)
-        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):
+                # Pad buffer to pos with null bytes.
+                self._buffer.resize(pos)
+            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 @@ class PyBytesIOTest(MemoryTestMixin, MemorySeekTestMixin, unittest.TestCase):
         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.