diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 41257f1..d20107e 100644
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -353,7 +353,6 @@
         if self.mode == "w" and self.buf:
             if self.type != "tar":
                 self.buf += self.cmp.flush()
-            self.__write("")            # Write remaining blocks to output
             self.fileobj.write(self.buf)
             self.buf = ""
             if self.type == "gz":
@@ -1775,6 +1774,8 @@
            of the longname as size, followed by data blocks,
            which contain the longname as a null terminated string.
         """
+        name += NUL
+
         tarinfo = TarInfo()
         tarinfo.name = "././@LongLink"
         tarinfo.type = type
@@ -1783,6 +1784,7 @@
 
         # write extended header
         self.fileobj.write(tarinfo.tobuf())
+        self.offset += BLOCKSIZE
         # write name blocks
         self.fileobj.write(name)
         blocks, remainder = divmod(tarinfo.size, BLOCKSIZE)
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:
diff --git a/Misc/NEWS b/Misc/NEWS
index c395576..fd6d244 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -39,6 +39,9 @@
 Library
 -------
 
+- Patch #846659.  Fix an error in tarfile.py when using
+  GNU longname/longlink creation. 
+
 - The obsolete FCNTL.py has been deleted.  The builtin fcntl module
   has been available (on platforms that support fcntl) since Python
   1.5a3, and all FCNTL.py did is export fcntl's names, after generating