From f3017b51dcc5b4fe7b6e186af80760d91d1058ed Mon Sep 17 00:00:00 2001
From: Aryan <aryangupta050903@gmail.com>
Date: Thu, 15 Oct 2026 22:27:41 +0530
Subject: [PATCH] Preserve spooled text decoder buffers during rollover

Retain pending UTF-8 bytes, decoded characters and buffered lines so rollover and fileno do not skip unread text or start decoding mid-character.

Fixes #581.

AI assistance: OpenAI Codex (GPT-6) investigated, implemented and validated this change at the account owner request.
---
 boltons/ioutils.py    |  4 ++++
 tests/test_ioutils.py | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 45 insertions(+)

diff --git a/boltons/ioutils.py b/boltons/ioutils.py
index 76afdec0..24c6d870 100644
--- a/boltons/ioutils.py
+++ b/boltons/ioutils.py
@@ -490,6 +490,10 @@ def rollover(self):
             pos = self.buffer.tell()
             tmp.write(self.buffer.getvalue())
             tmp.seek(pos)
+            # The decoder may have read ahead of the logical text position.
+            tmp.reader.bytebuffer = self.buffer.reader.bytebuffer
+            tmp.reader.charbuffer = self.buffer.reader.charbuffer
+            tmp.reader.linebuffer = self.buffer.reader.linebuffer
             self.buffer.close()
             self._buffer = tmp
 
diff --git a/tests/test_ioutils.py b/tests/test_ioutils.py
index f81fdd38..5bf3d465 100644
--- a/tests/test_ioutils.py
+++ b/tests/test_ioutils.py
@@ -376,6 +376,47 @@ def test_use_as_context_mgr(self):
             f.write(test_str)
             self.assertEqual(f.getvalue(), test_str)
 
+    def test_rollover_preserves_read_ahead(self):
+        for text in ('first\nlast', 'first\nsecond\nlast',
+                     'head\n' + '\u2603' * 100):
+            for method in ('rollover', 'fileno'):
+                with self.subTest(text=text, method=method):
+                    with ioutils.SpooledStringIO() as stream:
+                        stream.write(text)
+                        stream.seek(0)
+                        first, remaining = text.split('\n', 1)
+                        self.assertEqual(stream.readline(), first + '\n')
+                        position = stream.tell()
+                        getattr(stream, method)()
+                        self.assertEqual(stream.tell(), position)
+                        self.assertEqual(stream.read(), remaining)
+                        stream.seek(0)
+                        self.assertEqual(stream.read(), text)
+
+    def test_rollover_after_length_query(self):
+        with ioutils.SpooledStringIO() as stream:
+            stream.write('first\nlast')
+            stream.seek(2)
+            self.assertEqual(len(stream), 10)
+            position = stream.tell()
+            stream.rollover()
+            self.assertEqual(stream.tell(), position)
+            self.assertEqual(stream.read(), 'rst\nlast')
+
+    def test_rollover_preserves_mixed_read_write_state(self):
+        with ioutils.SpooledStringIO() as control:
+            with ioutils.SpooledStringIO(max_size=11) as stream:
+                for current in (control, stream):
+                    current.write('first\nlast')
+                    current.seek(0)
+                    current.readline()
+                for current in (control, stream):
+                    current.write('!')
+                self.assertTrue(stream._rolled)
+                self.assertEqual(stream.tell(), control.tell())
+                self.assertEqual(stream.read(), control.read())
+                self.assertEqual(stream.getvalue(), control.getvalue())
+
     def test_len_no_rollover(self):
         """Make sure len property works with in-memory flo"""
         self.spooled_flo.write(self.test_str)