diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 9d40ace..bd3fd8e 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -205,6 +205,94 @@
 class WriteStreamTest(WriteTest):
     sep = '|'
 
+class WriteGNULongTest(unittest.TestCase):
+    """This testcase checks for correct creation of GNU Longname
+       and Longlink extensions.
+
+       It creates a tarfile and adds empty members with either
+       long names, long linknames or both and compares the size
+       of the tarfile with the expected size.
+
+       It checks for SF bug #812325 in TarFile._create_gnulong().
+
+       While I was writing this testcase, I noticed a second bug
+       in the same method:
+       Long{names,links} weren't null-terminated which lead to
+       bad tarfiles when their length was a multiple of 512. This
+       is tested as well.
+    """
+
+    def setUp(self):
+        self.tar = tarfile.open(tmpname(), "w")
+        self.tar.posix = False
+
+    def tearDown(self):
+        self.tar.close()
+
+    def _length(self, s):
+        blocks, remainder = divmod(len(s) + 1, 512)
+        if remainder:
+            blocks += 1
+        return blocks * 512
+
+    def _calc_size(self, name, link=None):
+        # initial tar header
+        count = 512
+
+        if len(name) > tarfile.LENGTH_NAME:
+            # gnu longname extended header + longname
+            count += 512
+            count += self._length(name)
+
+        if link is not None and len(link) > tarfile.LENGTH_LINK:
+            # gnu longlink extended header + longlink
+            count += 512
+            count += self._length(link)
+
+        return count
+
+    def _test(self, name, link=None):
+        tarinfo = tarfile.TarInfo(name)
+        if link:
+            tarinfo.linkname = link
+            tarinfo.type = tarfile.LNKTYPE
+
+        self.tar.addfile(tarinfo)
+
+        v1 = self._calc_size(name, link)
+        v2 = self.tar.offset
+        self.assertEqual(v1, v2, "GNU longname/longlink creation failed")
+
+    def test_longname_1023(self):
+        self._test(("longnam/" * 127) + "longnam")
+
+    def test_longname_1024(self):
+        self._test(("longnam/" * 127) + "longname")
+
+    def test_longname_1025(self):
+        self._test(("longnam/" * 127) + "longname_")
+
+    def test_longlink_1023(self):
+        self._test("name", ("longlnk/" * 127) + "longlnk")
+
+    def test_longlink_1024(self):
+        self._test("name", ("longlnk/" * 127) + "longlink")
+
+    def test_longlink_1025(self):
+        self._test("name", ("longlnk/" * 127) + "longlink_")
+
+    def test_longnamelink_1023(self):
+        self._test(("longnam/" * 127) + "longnam",
+                   ("longlnk/" * 127) + "longlnk")
+
+    def test_longnamelink_1024(self):
+        self._test(("longnam/" * 127) + "longname",
+                   ("longlnk/" * 127) + "longlink")
+
+    def test_longnamelink_1025(self):
+        self._test(("longnam/" * 127) + "longname_",
+                   ("longlnk/" * 127) + "longlink_")
+
 # Gzip TestCases
 class ReadTestGzip(ReadTest):
     comp = "gz"
@@ -245,7 +333,8 @@
         ReadTest,
         ReadStreamTest,
         WriteTest,
-        WriteStreamTest
+        WriteStreamTest,
+        WriteGNULongTest,
     ]
 
     if gzip: