Fixed generation of .unitypackage with tarfile.

When using gzip and tarfile python modules to create a
.unitypackage, by default the gzip module will place the
output filename in the gzip header which results in Unity
on Windows (just Windows!) refusing to list the contents of
the file. To workaround this issue, this change replaces
the output filename in the gzip header with OUTPUT.tar.gz
when writing, for example, OUTPUT.unitypackage.

Tested:
* All unit test pass.
* Verified this works with Unity 2018.4.17f1 on Windows and
  still works in Unity on macOS and Linux.

Bug: 165661723
Change-Id: I38580b98e44a23c8eeb71790f57184901f7d46fd

Author: Stewart Miles

source/ExportUnityPackage/export_unity_package.py

@@ -2539,31 +2539,36 @@ def create_archive(archive_filename, input_directory, timestamp):
         finally:
           shutil.rmtree(os.path.dirname(list_filename))
       else:
-        with gzip.GzipFile(
-            archive_filename, "wb",
-            mtime=(timestamp if timestamp >= 0 else None)) as gzip_file:
-          with tarfile.open(mode="w|", fileobj=gzip_file,
-                            format=tarfile.USTAR_FORMAT, dereference=True,
-                            errorlevel=2) as tar_file:
-            def reproducible_tarinfo(tarinfo):
-              """Patch TarInfo so that it generates a reproducible archive.
-
-              Args:
-                tarinfo: TarInfo to modify.
-
-              Returns:
-                Modified tarinfo.
-              """
-              tarinfo.mtime = timestamp if timestamp >= 0 else tarinfo.mtime
-              tarinfo.uid = 0
-              tarinfo.gid = 0
-              tarinfo.uname = FLAGS.owner
-              tarinfo.gname = FLAGS.group
-              return tarinfo
-
-            for filename in input_filenames:
-              tar_file.add(filename, recursive=False,
-                           filter=reproducible_tarinfo)
+        with open(archive_filename, "wb") as gzipped_tar_file:
+          with gzip.GzipFile(
+              # The filename in the archive must end with .tar or .tar.gz for
+              # Unity to open it on Windows.
+              os.path.splitext(archive_filename)[0] + ".tar.gz",
+              fileobj=gzipped_tar_file, mode="wb",
+              mtime=(timestamp if timestamp >= 0 else None)) as gzip_file:
+            with tarfile.open(mode="w|", fileobj=gzip_file,
+                              format=tarfile.USTAR_FORMAT, dereference=True,
+                              errorlevel=2) as tar_file:
+              def reproducible_tarinfo(tarinfo):
+                """Patch TarInfo so that it generates a reproducible archive.
+
+                Args:
+                  tarinfo: TarInfo to modify.
+
+                Returns:
+                  Modified tarinfo.
+                """
+                tarinfo.mtime = timestamp if timestamp >= 0 else tarinfo.mtime
+                tarinfo.uid = 0
+                tarinfo.gid = 0
+                tarinfo.uname = FLAGS.owner
+                tarinfo.gname = FLAGS.group
+                return tarinfo
+
+              for filename in input_filenames:
+                tar_file.add(filename, recursive=False,
+                             filter=reproducible_tarinfo)
+
     finally:
       os.chdir(cwd)