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diff --git a/exts/importer_omnibus_extensions.rb b/exts/importer_omnibus_extensions.rb
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+++ b/exts/importer_omnibus_extensions.rb
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+# Extensions for the Omnibus tool that allow using it to generate morphologies.
+#
+# Copyright (C) 2014  Codethink Limited
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; version 2 of the License.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+require 'omnibus'
+
+require 'optparse'
+require 'rubygems/commands/build_command'
+require 'rubygems/commands/install_command'
+require 'shellwords'
+
+class Omnibus::Builder
+  # It's possible to use `gem install` in build commands, which is a great
+  # way of subverting the dependency tracking Omnibus provides. It's done
+  # in `omnibus-chef/config/software/chefdk.rb`, for example.
+  #
+  # To handle this, here we extend the class that executes the build commands
+  # to detect when `gem install` is run. It uses the Gem library to turn the
+  # commandline back into a Bundler::Dependency object that we can use.
+  #
+  # We also trap `gem build` so we know when a software component is a RubyGem
+  # that should be handled by 'rubygems.to_chunk'.
+
+  class GemBuildCommandParser < Gem::Commands::BuildCommand
+    def gemspec_path(args)
+      handle_options args
+      if options[:args].length != 1
+        raise "Invalid `gem build` commandline: 1 argument expected, got " \
+               "#{options[:args]}."
+      end
+      options[:args][0]
+    end
+  end
+
+  class GemInstallCommandParser < Gem::Commands::InstallCommand
+    def dependency_list_from_commandline(args)
+      handle_options args
+
+      # `gem install foo*` is sometimes used when installing a locally built
+      # Gem, to avoid needing to know the exact version number that was built.
+      # We only care about remote Gems being installed, so anything with a '*'
+      # in its name can be ignored.
+      gem_names = options[:args].delete_if { |name| name.include?('*') }
+
+      gem_names.collect do |gem_name|
+        Bundler::Dependency.new(gem_name, options[:version])
+      end
+    end
+  end
+
+  def gem(command, options = {})
+    # This function re-implements the 'gem' function in the build-commands DSL.
+    if command.start_with? 'build'
+      parser = GemBuildCommandParser.new
+      args = Shellwords.split(command).drop(1)
+      if built_gemspec != nil
+        raise "More than one `gem build` command was run as part f the build " \
+              "process. The 'rubygems.to_chunk' program currently supports " \
+              "only one .gemspec build per chunk, so this can't be " \
+              "processed automatically."
+      end
+      @built_gemspec = parser.gemspec_path(args)
+    elsif command.start_with? 'install'
+      parser = GemInstallCommandParser.new
+      args = Shellwords.split(command).drop(1)
+      args_without_build_flags = args.take_while { |item| item != '--' }
+      gems = parser.dependency_list_from_commandline(args_without_build_flags)
+      manually_installed_rubygems.concat gems
+    end
+  end
+
+  def built_gemspec
+    @built_gemspec
+  end
+
+  def manually_installed_rubygems
+    @manually_installed_rubygems ||= []
+  end
+end