Extend 'gem fetch' command. Fetch all dependencies. Handles OS (Windows vs non-Windows) and Ruby Platform (JRuby vs Ruby).

gem-fetch-dependencies.rb

#!/usr/bin/ruby

=begin
  ## USAGE:
  
    # Ruby:
      ruby gem-fetch-dependencies.rb fetch <gem_name> --dependencies
      
    # JRuby:
      jruby gem-fetch-dependencies.rb fetch <gem_name> --dependencies
      
=end

require 'rubygems'
require 'rubygems/commands/fetch_command'
require 'rbconfig'

class Gem::Commands::FetchCommand
  def add_version_option_with_fetch_depends
    add_version_option_without_fetch_depends
    add_option('-y', '--[no-]dependencies',
               "Fetch dependent gems") do |value, options|
      options[:dependencies] = value
    end
  end

  def get_all_gem_names_with_fetch_depends
    @dependent_gem_names || get_all_gem_names_without_fetch_depends
  end

  def execute_with_fetch_depends
    execute_without_fetch_depends
    if options[:dependencies] then
      @dependent_gem_names = get_dependent_gem_names
      options[:version] = nil
      execute_without_fetch_depends
    end
  end

  [:add_version_option, :get_all_gem_names, :execute].each do |target|
    feature = "fetch_depends"
    alias_method "#{target}_without_#{feature}", target
    alias_method target, "#{target}_with_#{feature}"
  end

private
  def get_dependent_gem_names
    version = options[:version] || Gem::Requirement.default
    request_gem_names = get_all_gem_names_without_fetch_depends.uniq
    to_do = fetch_specs_by_names_and_version(request_gem_names, version)
    seen = {}

    until to_do.empty? do
      spec = to_do.shift
      next if spec.nil? or seen[spec.name]
      seen[spec.name] = true
      deps = spec.runtime_dependencies
      deps.each do |dep|
        requirements = dep.requirement.requirements.map { |req,| req }
        all = !dep.prerelease? and
              (requirements.length > 1 or
                (requirements.first != ">=" and requirements.first != ">"))
        result = fetch_spec(dep, all)
        to_do.push(result)
      end
    end

    gem_names = seen.map { |name,| name }
    gem_names.reject { |name| request_gem_names.include? name }
  end

  def fetch_specs_by_names_and_version(gem_names, version)
    all = Gem::Requirement.default != version
    specs = gem_names.map do |gem_name|
      dep = Gem::Dependency.new(gem_name, version)
      dep.prerelease = options[:prerelease]
      fetch_spec(dep, all)
    end
    specs.compact
  end

  def fetch_spec(dep, all)
    # See "https://github.com/puppetlabs/Razor-Microkernel/pull/76/files":
    
    # specs_and_sources, errors = Gem::SpecFetcher.fetcher.fetch_with_errors(dep, all, true, dep.prerelease?)
      
    # Use the Ruby 2.0 RubyGems API if it's available
    if Gem::SpecFetcher.fetcher.respond_to? :spec_for_dependency
      specs_and_sources, errors =
        Gem::SpecFetcher.fetcher.spec_for_dependency(dep, true)
    else # Default to the 1.x API
      specs_and_sources, errors =
        # Gem::SpecFetcher.fetcher.fetch_with_errors(dep, all, true, false)
        Gem::SpecFetcher.fetcher.fetch_with_errors(dep, all, true, dep.prerelease?)
    end
      
    if platform = Gem.platforms.last then
      filtered = specs_and_sources.select { |s,| s.platform == platform }
      specs_and_sources = filtered unless filtered.empty?
    end
    spec, source_uri = specs_and_sources.sort_by { |s,| s.version }.last
    spec
  end
end

# load `which gem`.rstrip
_is_jruby = (RUBY_PLATFORM == 'java')
_is_windows = (RbConfig::CONFIG['host_os'] =~ /mswin|msys|mingw|cygwin|bccwin|wince|emc/)
_which_or_where_cmd = (_is_windows ? 'where' : 'which')
_gem_cmd = ((_is_jruby) ? 'jgem' : 'gem')
load `#{_which_or_where_cmd} #{_gem_cmd}`.split("\n").first