gzip --rsyncable via pigz?

[Paul Eggert]

I thought of another way to add support for --rsyncable
to GNU gzip.  We could use pigz's implementation
<http://zlib.net/pigz/>.

pigz has many improvements over gzip.
In particular, its --rsyncable implementation appears
to be better than the Debian / Fedora implementation
of gzip --rsyncable.

A downside is that pigz does not generate output that is
byte-for-byte the same as gzip output.  The output is
compatible with gzip, in the sense that gzip can read
pigz output and generate the identical uncompressed version,
and likewise for pigz reading gzip output; but the point is
that "gzip foo" and "pigz foo" generate compressed
files that differ slightly.

Another downside is that, in my tests, pigz -9 generates
output that's a bit longer than gzip's.  I'm not sure
why this should be, but there it is, at least with pigz 2.3:

  $ pigz -p 1 -9 -n -T <pigz.c >pigz.c.pigz
  $ gzip -9 -n <pigz.c >pigz.c.gzip
  $ ls -l pigz.c.????
  -rw-r--r--. 1 eggert eggert 38263 Jun 12 12:27 pigz.c.gzip
  -rw-r--r--. 1 eggert eggert 38354 Jun 12 12:26 pigz.c.pigz

Would this be a major concern?  If so, we could work
around this problem, by keeping the current gzip implementation
unless the user specifies options that only pigz supports.
These new options would include --rsyncable and --processes.

But I'd rather get rid of the current gzip implmementation
almost entirely, and simply substitute pigz's, as that should
simplify maintenance for us.