From efaba28dfcbc26fc84e7747c0aa5109e66b9e11c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Hilmi=20Tuna=20Say=C4=B1n?= <hilmitunasayin@icloud.com>
Date: Tue, 3 Nov 2026 22:28:10 +0100
Subject: [PATCH] gh-161271: Drop the read buffer in
 _pyio.BufferedRandom.truncate()

---
 Lib/_pyio.py                                          |  6 +++++-
 Lib/test/test_io/test_bufferedio.py                   | 11 +++++++++++
 .../2026-11-03-20-10-00.gh-issue-161271.Qk2Lm7.rst    |  4 ++++
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-11-03-20-10-00.gh-issue-161271.Qk2Lm7.rst

diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index 370b518f1e5e5a6..2be127985066b66 100644
--- a/Lib/_pyio.py
+++ b/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):
diff --git a/Lib/test/test_io/test_bufferedio.py b/Lib/test/test_io/test_bufferedio.py
index e83dd0d4e28d006..64ae4718be4f42a 100644
--- a/Lib/test/test_io/test_bufferedio.py
+++ b/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)
diff --git a/Misc/NEWS.d/next/Library/2026-11-03-20-10-00.gh-issue-161271.Qk2Lm7.rst b/Misc/NEWS.d/next/Library/2026-11-03-20-10-00.gh-issue-161271.Qk2Lm7.rst
new file mode 100644
index 000000000000000..619c88af7c043a3
--- /dev/null
+++ b/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.