gh-161271: Drop the read buffer in _pyio.BufferedRandom.truncate()

Lib/_pyio.py

@@ -1470,7 +1470,11 @@ def tell(self):
     def truncate(self, pos=None):
         if pos is None:
             pos = self.tell()
-        # Use seek to flush the read buffer.
+        # Use seek to flush the read buffer and to rewind the raw stream, as
+        # _io's buffered_flush_and_rewind_unlocked() does.  Without this a
+        # read after truncate() can still return data that was truncated
+        # away.
+        self.seek(self.tell())
         return BufferedWriter.truncate(self, pos)
 
     def read(self, size=None):

Lib/test/test_io/test_bufferedio.py

@@ -1410,6 +1410,17 @@ def test_truncate_after_read_or_write(self):
         self.assertEqual(bufio.write(b"BB"), 2) # the write buffer increases
         self.assertEqual(bufio.truncate(), 4)
 
+    def test_read_after_truncate(self):
+        # gh-161271: truncate() must drop the read-ahead buffer, so that a
+        # read after it cannot return data that is no longer in the stream.
+        raw = self.BytesIO(b"A" * 10)
+        bufio = self.tp(raw, 100)
+        self.assertEqual(bufio.read(2), b"AA")  # the read buffer gets filled
+        self.assertEqual(bufio.truncate(0), 0)
+        self.assertEqual(bufio.tell(), 2)
+        self.assertEqual(bufio.read(), b"")
+        self.assertEqual(raw.getvalue(), b"")
+
     def test_misbehaved_io(self):
         BufferedReaderTest.test_misbehaved_io(self)
         BufferedWriterTest.test_misbehaved_io(self)

Misc/NEWS.d/next/Library/2026-11-03-20-10-00.gh-issue-161271.Qk2Lm7.rst

@@ -0,0 +1,4 @@
+Fix the pure Python implementation of :class:`io.BufferedRandom` (``_pyio``):
+:meth:`!truncate` now drops the read-ahead buffer and rewinds the raw stream,
+as the C implementation does, so that a read after :meth:`!truncate` no longer
+returns data that was truncated away.