From 3fed0870a6fec72665068e09200c674b574dabdb Mon Sep 17 00:00:00 2001
From: Victor Stinner <victor.stinner@haypocalc.com>
Date: Sat, 22 May 2010 02:16:27 +0000
Subject: [PATCH] Merged revisions 81459 via svnmerge from
 svn+ssh://pythondev@svn.python.org/python/trunk

........
  r81459 | victor.stinner | 2010-05-22 04:11:07 +0200 (sam., 22 mai 2010) | 3 lines

  Issue #6268: Fix seek() method of codecs.open(), don't read the BOM twice
  after seek(0)
........
---
 Lib/codecs.py           |  4 ++++
 Lib/test/test_codecs.py | 21 +++++++++++++++++++++
 Misc/NEWS               |  3 +++
 3 files changed, 28 insertions(+)

diff --git a/Lib/codecs.py b/Lib/codecs.py
index f05c4f7e2db..9490602ad3c 100644
--- a/Lib/codecs.py
+++ b/Lib/codecs.py
@@ -699,6 +699,10 @@ class StreamReaderWriter:
         self.reader.reset()
         self.writer.reset()
 
+    def seek(self, offset, whence=0):
+        self.reader.seek(offset, whence)
+        self.writer.seek(offset, whence)
+
     def __getattr__(self, name,
                     getattr=getattr):
 
diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
index b17cded0814..6e7afc422b5 100644
--- a/Lib/test/test_codecs.py
+++ b/Lib/test/test_codecs.py
@@ -1594,6 +1594,26 @@ class SurrogateEscapeTest(unittest.TestCase):
                          b"\xe4\xeb\xef\xf6\xfc")
 
 
+class BomTest(unittest.TestCase):
+    def test_seek0(self):
+        data = "1234567890"
+        tests = ("utf-16",
+                 "utf-16-le",
+                 "utf-16-be",
+                 "utf-32",
+                 "utf-32-le",
+                 "utf-32-be")
+        for encoding in tests:
+            with codecs.open('foo', 'w+', encoding=encoding) as f:
+                # Check if the BOM is written only once
+                f.write(data)
+                f.write(data)
+                f.seek(0)
+                self.assertEquals(f.read(), data * 2)
+                f.seek(0)
+                self.assertEquals(f.read(), data * 2)
+
+
 def test_main():
     support.run_unittest(
         UTF32Test,
@@ -1621,6 +1641,7 @@ def test_main():
         WithStmtTest,
         TypesTest,
         SurrogateEscapeTest,
+        BomTest,
     )
 
 
diff --git a/Misc/NEWS b/Misc/NEWS
index 7c2cc5de938..58c2785ea9c 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -393,6 +393,9 @@ C-API
 Library
 -------
 
+- Issue #6268: Fix seek() method of codecs.open(), don't read the BOM twice
+  after seek(0)
+
 - Issue #3798: sys.exit(message) writes the message to sys.stderr file, instead
   of the C file stderr, to use stderr encoding and error handler