Correct end of tape marker

The POSIX standard for tar says that there must be at least
two 512-bytes zero blocks at the end of the tar archive file.

Our implementation would only emit a single 512-byte zero block if the
size of the last file was in the range 18*512 through 19*512-1 (modulo
20*512). GNU tar would correctly unpack such tar archive file, but
would emit a warning:

   tar: A lone zero block at 20

1 files changed, 11 insertions, 3 deletions

diff --git a/lib/stdlib/src/erl_tar.erl b/lib/stdlib/src/erl_tar.erl
index 89b4ea6c04..acf7a5cd40 100644
--- a/lib/stdlib/src/erl_tar.erl
+++ b/lib/stdlib/src/erl_tar.erl
@@ -397,9 +397,17 @@ to_string(Str0, Count) ->
 
 pad_file(File) ->
     {ok,Position} = file:position(File, {cur,0}),
-    %% There must be at least one empty record at the end of the file.
-    Zeros = zeroes(?block_size - (Position rem ?block_size)),
-    file:write(File, Zeros).
+    %% There must be at least two zero records at the end.
+    Fill = case ?block_size - (Position rem ?block_size) of
+	       Fill0 when Fill0 < 2*?record_size ->
+		   %% We need to another block here to ensure that there
+		   %% are at least two zero records at the end.
+		   Fill0 + ?block_size;
+	       Fill0 ->
+		   %% Large enough.
+		   Fill0
+	   end,
+    file:write(File, zeroes(Fill)).
 
 split_filename(Name) when length(Name) =< ?th_name_len ->
     {"", Name};