def long_filename
  sha1 = File.read('sha1').chomp
  "my_prog-#{sha1}"
end

file "archive.tgz" do
  Rake::Task[:build].invoke
  `tar cvzf archive.tgz #{long_filename}`
end

task :build => ["my_prog", "sha1"] do
  cp "my_prog", long_filename
end

file "my_prog" => ["foo"] do
  touch "my_prog"
end

file "sha1" do
  `sha1sum foo | awk '{ print $1 }' > sha1`
end