Speedup gunzipping

[pjump]

Gunzipping the specs.4.8.gz file on my PC takes about 16 seconds with Gem.gunzip.
It's virtually instantaneous (80ms) with zcat/gunzip (i.e., it it 200
times faster).

This patch uses zcat instead of Gem.gunzip, if it is available. (The
availability check could probably be done better, but I don't know how
to do it properly.)

Just a suggestion.

[raggi]

This really just probably needs to use a stream implementation rather than a slurp. Both implementations are using zlib, so there shouldn't be this huge difference if the allocations are no longer being done.

[raggi]

General concept:

Zlib::GzipReader.open(input) { |in| open(output, 'w') { |out| out << in.readpartial(65536) while true rescue nil; end } } # TODO rescue EOFError specifically

[pjump]

I looked at it in a little more detail now and it's the Gem.gunzip method.
File.write('specs.4.8', Zlib::GzipReader.open('specs.4.8.gz').read)
is virtually instantaneous too. (Doesn't even use streaming, although streaming might make it more memory-friendly, depending on how GzipReader works internally).

[pjump]

Thanks for your attention. This https://github.com/rubygems/rubygems/pulls should fix it upstream if they take it. Basically somebody once made Gem::gunzip switch from StringIO to a 300 times slower, in-ruby implementation, which is why the unzipping of the specs takes 16 seconds.