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diff --git a/bzrlib/tests/test_tuned_gzip.py b/bzrlib/tests/test_tuned_gzip.py
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+++ b/bzrlib/tests/test_tuned_gzip.py
@@ -0,0 +1,130 @@
+# Copyright (C) 2006, 2009, 2010, 2011 Canonical Ltd
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+
+"""Tests for tuned_gzip."""
+
+
+# do not use bzrlib test cases here - this should be suitable for sending
+# upstream.
+from cStringIO import StringIO
+import zlib
+
+
+from bzrlib import (
+    symbol_versioning,
+    tuned_gzip,
+    tests,
+    )
+
+
+class FakeDecompress(object):
+    """A fake decompressor for testing GzipFile."""
+
+    def __init__(self):
+        self.unused_data=''
+
+    def decompress(self, buf):
+        """Return an empty string as though we are at eof."""
+        # note that the zlib module *overwrites* unused data
+        # on writes after EOF.
+        self.unused_data = buf
+        return ''
+
+
+class TestFakeDecompress(tests.TestCase):
+    """We use a fake decompressor to test GzipFile.
+
+    This class tests the behaviours we want from it.
+    """
+
+    def test_decompress(self):
+        # decompressing returns no data.
+        decompress = FakeDecompress()
+        self.assertEqual('', decompress.decompress('0'))
+
+    def test_unused_data(self):
+        # after decompressing, we have 1 unused byte.
+        # this is normally set by decompressors when they
+        # detect the end of a compressed stream.
+        decompress = FakeDecompress()
+        decompress.decompress('0')
+        self.assertEqual('0', decompress.unused_data)
+        # decompressing again (when the short read is read)
+        # will give us the latest input in the unused_data
+        # this is arguably a bug in zlib but ...
+        decompress.decompress('1234567')
+        self.assertEqual('1234567', decompress.unused_data)
+
+
+class TestGzip(tests.TestCase):
+
+    def test__read_short_remainder(self):
+        # a _read call at the end of a compressed hunk should
+        # read more bytes if there is less than 8 bytes (the
+        # gzip trailer) unread.
+        stream = StringIO('\0\0\0\0\0\0\0\0')
+        myfile = self.applyDeprecated(
+            symbol_versioning.deprecated_in((2, 3, 0)),
+            tuned_gzip.GzipFile, fileobj=stream)
+        # disable the _new_member check, we are microtesting.
+        myfile._new_member = False
+        myfile.crc = zlib.crc32('')
+        myfile.decompress = FakeDecompress()
+        myfile.size = 0
+        myfile._read(1)
+        # all the data should have been read now
+        self.assertEqual('', stream.read())
+        # and it should be new member time in the stream.
+        self.assertTrue(myfile._new_member)
+
+    def test_negative_crc(self):
+        """Content with a negative crc should not break when written"""
+        sio = StringIO()
+        gfile = self.applyDeprecated(
+            symbol_versioning.deprecated_in((2, 3, 0)),
+            tuned_gzip.GzipFile, mode="w", fileobj=sio)
+        gfile.write("\xFF")
+        gfile.close()
+        self.assertEqual(gfile.crc & 0xFFFFFFFFL, 0xFF000000L)
+        self.assertEqual(sio.getvalue()[-8:-4], "\x00\x00\x00\xFF")
+
+
+class TestToGzip(tests.TestCase):
+
+    def assertToGzip(self, chunks):
+        bytes = ''.join(chunks)
+        gzfromchunks = tuned_gzip.chunks_to_gzip(chunks)
+        gzfrombytes = tuned_gzip.bytes_to_gzip(bytes)
+        self.assertEqual(gzfrombytes, gzfromchunks)
+        decoded = self.applyDeprecated(
+            symbol_versioning.deprecated_in((2, 3, 0)),
+            tuned_gzip.GzipFile, fileobj=StringIO(gzfromchunks)).read()
+        self.assertEqual(bytes, decoded)
+
+    def test_single_chunk(self):
+        self.assertToGzip(['a modest chunk\nwith some various\nbits\n'])
+
+    def test_simple_text(self):
+        self.assertToGzip(['some\n', 'strings\n', 'to\n', 'process\n'])
+
+    def test_large_chunks(self):
+        self.assertToGzip(['a large string\n'*1024])
+        self.assertToGzip(['a large string\n']*1024)
+
+    def test_enormous_chunks(self):
+        self.assertToGzip(['a large string\n'*1024*256])
+        self.assertToGzip(['a large string\n']*1024*256)