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The installer installs the files contained in the .gem into the Gem.home.
Gem::Installer does the work of putting files in all the right places on the filesystem including unpacking the gem into its gem dir, installing the gemspec in the specifications dir, storing the cached gem in the cache dir, and installing either wrappers or symlinks for executables.
The installer invokes pre and post install hooks.  Hooks can be added either through a rubygems_plugin.rb file in an installed gem or via a rubygems/defaults/#{RUBY_ENGINE}.rb or rubygems/defaults/operating_system.rb file.  See Gem.pre_install and Gem.post_install for details.
Paths where env(1) might live. Some systems are broken and have it in /bin
Overrides the executable format.
This is a sprintf format with a “%s” which will be replaced with the executable name. It is based off the ruby executable name's difference from “ruby”.
Construct an installer object for the gem file located at path
 
               # File rubygems/installer.rb, line 101
def self.at path, options = {}
  security_policy = options[:security_policy]
  package = Gem::Package.new path, security_policy
  new package, options
end
             
            Defaults to use Ruby's program prefix and suffix.
 
               # File rubygems/installer.rb, line 92
def exec_format
  @exec_format ||= Gem.default_exec_format
end
             
            Construct an installer object for an ephemeral gem (one where we don't actually have a .gem file, just a spec)
 
               # File rubygems/installer.rb, line 133
def self.for_spec spec, options = {}
  # FIXME: we should have a real Package class for this
  new FakePackage.new(spec), options
end
             
            Constructs an Installer instance that will install the gem located at gem.  options is a Hash with the following keys:
Where to put a bin wrapper if needed.
Whether or not development dependencies should be installed.
Use /usr/bin/env in bin wrappers.
Overrides all version checks and security policy checks, except for a signed-gems-only policy.
Format the executable the same as the Ruby executable. If your Ruby is ruby18, foo_exec will be installed as foo_exec18.
Don't raise if a dependency is missing.
The directory to install the gem into.
Use the specified security policy.  See Gem::Security
Indicate that the gem should be unpacked into the users personal gem directory.
Only validate dependencies against what is in the install_dir
Install wrappers if true, symlinks if false.
An Array of arguments to pass to the extension builder process. If not set, then Gem::Command.build_args is used
 
               # File rubygems/installer.rb, line 161
def initialize(package, options={})
  require 'fileutils'
  @options = options
  if package.is_a? String
    security_policy = options[:security_policy]
    @package = Gem::Package.new package, security_policy
    if $VERBOSE
      warn "constructing an Installer object with a string is deprecated. Please use Gem::Installer.at (called from: #{caller.first})"
    end
  else
    @package = package
  end
  process_options
  if options[:user_install] and not options[:unpack] then
    @gem_home = Gem.user_dir
    @bin_dir = Gem.bindir gem_home unless options[:bin_dir]
    check_that_user_bin_dir_is_in_path
  end
end
             
            Return the text for an application file.
 
               # File rubygems/installer.rb, line 708
  def app_script_text(bin_file_name)
    # note that the `load` lines cannot be indented, as old RG versions match
    # against the beginning of the line
    return <<-TEXT
#{shebang bin_file_name}
#
# This file was generated by RubyGems.
#
# The application '#{spec.name}' is installed as part of a gem, and
# this file is here to facilitate running it.
#
require 'rubygems'
version = "#{Gem::Requirement.default}.a"
if ARGV.first
  str = ARGV.first
  str = str.dup.force_encoding("BINARY") if str.respond_to? :force_encoding
  if str =~ /\\A_(.*)_\\z/ and Gem::Version.correct?($1) then
    version = $1
    ARGV.shift
  end
end
if Gem.respond_to?(:activate_bin_path)
load Gem.activate_bin_path('#{spec.name}', '#{bin_file_name}', version)
else
gem #{spec.name.dump}, version
load Gem.bin_path(#{spec.name.dump}, #{bin_file_name.dump}, version)
end
TEXT
  end
             
            Builds extensions. Valid types of extensions are extconf.rb files, configure scripts and rakefiles or mkrf_conf files.
 
               # File rubygems/installer.rb, line 761
def build_extensions
  builder = Gem::Ext::Builder.new spec, @build_args
  builder.build_extensions
end
             
            The location of the default spec file for default gems.
 
               # File rubygems/installer.rb, line 419
def default_spec_file
  File.join Gem::Specification.default_specifications_dir, "#{spec.full_name}.gemspec"
end
             
            Return the target directory where the gem is to be installed. This directory is not guaranteed to be populated.
 
               # File rubygems/installer.rb, line 813
def dir
  gem_dir.to_s
end
             
            Ensure that the dependency is satisfied by the current installation of gem. If it is not an exception is raised.
 
               # File rubygems/installer.rb, line 382
def ensure_dependency(spec, dependency)
  unless installation_satisfies_dependency? dependency then
    raise Gem::InstallError, "#{spec.name} requires #{dependency}"
  end
  true
end
             
            Ensures the Gem::Specification written out for this gem is loadable upon installation.
 
               # File rubygems/installer.rb, line 596
def ensure_loadable_spec
  ruby = spec.to_ruby_for_cache
  ruby.untaint
  begin
    eval ruby
  rescue StandardError, SyntaxError => e
    raise Gem::InstallError,
          "The specification for #{spec.full_name} is corrupt (#{e.class})"
  end
end
             
            Extracts only the bin/ files from the gem into the gem directory. This is used by default gems to allow a gem-aware stub to function without the full gem installed.
 
               # File rubygems/installer.rb, line 792
def extract_bin
  @package.extract_files gem_dir, "bin/*"
end
             
            Reads the file index and extracts each file into the gem directory.
Ensures that files can't be installed outside the gem directory.
 
               # File rubygems/installer.rb, line 783
def extract_files
  @package.extract_files gem_dir
end
             
            Prefix and suffix the program filename the same as ruby.
 
               # File rubygems/installer.rb, line 799
def formatted_program_filename(filename)
  if @format_executable then
    self.class.exec_format % File.basename(filename)
  else
    filename
  end
end
             
            Lazy accessor for the spec's gem directory.
 
               # File rubygems/installer.rb, line 254
def gem_dir
  @gem_dir ||= File.join(gem_home, "gems", spec.full_name)
end
             
            Creates the scripts to run the applications in the gem.
 
               # File rubygems/installer.rb, line 499
def generate_bin_script(filename, bindir)
  bin_script_path = File.join bindir, formatted_program_filename(filename)
  FileUtils.rm_f bin_script_path # prior install may have been --no-wrappers
  File.open bin_script_path, 'wb', 0755 do |file|
    file.print app_script_text(filename)
  end
  verbose bin_script_path
  generate_windows_script filename, bindir
end
             
            Creates the symlinks to run the applications in the gem. Moves the symlink if the gem being installed has a newer version.
 
               # File rubygems/installer.rb, line 517
def generate_bin_symlink(filename, bindir)
  src = File.join gem_dir, spec.bindir, filename
  dst = File.join bindir, formatted_program_filename(filename)
  if File.exist? dst then
    if File.symlink? dst then
      link = File.readlink(dst).split File::SEPARATOR
      cur_version = Gem::Version.create(link[-3].sub(/^.*-/, ''))
      return if spec.version < cur_version
    end
    File.unlink dst
  end
  FileUtils.symlink src, dst, :verbose => Gem.configuration.really_verbose
rescue NotImplementedError, SystemCallError
  alert_warning "Unable to use symlinks, installing wrapper"
  generate_bin_script filename, bindir
end
             
            Creates windows .bat files for easy running of commands
 
               # File rubygems/installer.rb, line 450
def generate_windows_script(filename, bindir)
  if Gem.win_platform? then
    script_name = filename + ".bat"
    script_path = File.join bindir, File.basename(script_name)
    File.open script_path, 'w' do |file|
      file.puts windows_stub_script(bindir, filename)
    end
    verbose script_path
  end
end
             
            Installs the gem and returns a loaded Gem::Specification for the installed gem.
The gem will be installed with the following structure:
@gem_home/ cache/<gem-version>.gem #=> a cached copy of the installed gem gems/<gem-version>/... #=> extracted files specifications/<gem-version>.gemspec #=> the Gem::Specification
 
               # File rubygems/installer.rb, line 278
def install
  pre_install_checks
  FileUtils.rm_f File.join gem_home, 'specifications', spec.spec_name
  run_pre_install_hooks
  # Set loaded_from to ensure extension_dir is correct
  if @options[:install_as_default] then
    spec.loaded_from = default_spec_file
  else
    spec.loaded_from = spec_file
  end
  # Completely remove any previous gem files
  FileUtils.rm_rf gem_dir
  FileUtils.rm_rf spec.extension_dir
  FileUtils.mkdir_p gem_dir
  if @options[:install_as_default] then
    extract_bin
    write_default_spec
  else
    extract_files
    build_extensions
    write_build_info_file
    run_post_build_hooks
    generate_bin
    write_spec
    write_cache_file
  end
  say spec.post_install_message if options[:post_install_message] && !spec.post_install_message.nil?
  Gem::Installer.install_lock.synchronize { Gem::Specification.reset }
  run_post_install_hooks
  spec
# TODO This rescue is in the wrong place. What is raising this exception?
# move this rescue to around the code that actually might raise it.
rescue Zlib::GzipFile::Error
  raise Gem::InstallError, "gzip error installing #{gem}"
end
             
            True if the gems in the system satisfy dependency.
 
               # File rubygems/installer.rb, line 392
def installation_satisfies_dependency?(dependency)
  return true if @options[:development] and dependency.type == :development
  return true if installed_specs.detect { |s| dependency.matches_spec? s }
  return false if @only_install_dir
  not dependency.matching_specs.empty?
end
             
            Return an Array of Specifications contained within the gem_home we'll be installing into.
 
               # File rubygems/installer.rb, line 362
def installed_specs
  @specs ||= begin
    specs = []
    Dir[File.join(gem_home, "specifications", "*.gemspec")].each do |path|
      spec = Gem::Specification.load path.untaint
      specs << spec if spec
    end
    specs
  end
end
             
            Performs various checks before installing the gem such as the install repository is writable and its directories exist, required Ruby and rubygems versions are met and that dependencies are installed.
Version and dependency checks are skipped if this install is forced.
The dependent check will be skipped this install is ignoring dependencies.
 
               # File rubygems/installer.rb, line 826
def pre_install_checks
  verify_gem_home options[:unpack]
  ensure_loadable_spec
  verify_spec_name
  if options[:install_as_default]
    Gem.ensure_default_gem_subdirectories gem_home
  else
    Gem.ensure_gem_subdirectories gem_home
  end
  return true if @force
  ensure_required_ruby_version_met
  ensure_required_rubygems_version_met
  ensure_dependencies_met unless @ignore_dependencies
  true
end
             
            Generates a #! line for bin_file_name's wrapper copying arguments if necessary.
If the :custom_shebang config is set, then it is used as a template for how to create the shebang used for to run a gem's executables.
The template supports 4 expansions:
$env the path to the unix env utility $ruby the path to the currently running ruby interpreter $exec the path to the gem's executable $name the name of the gem the executable is for
 
               # File rubygems/installer.rb, line 551
def shebang(bin_file_name)
  ruby_name = RbConfig::CONFIG['ruby_install_name'] if @env_shebang
  path = File.join gem_dir, spec.bindir, bin_file_name
  first_line = File.open(path, "rb") {|file| file.gets}
  if /\A#!/ =~ first_line then
    # Preserve extra words on shebang line, like "-w".  Thanks RPA.
    shebang = first_line.sub(/\A\#!.*?ruby\S*((\s+\S+)+)/, "#!#{Gem.ruby}")
    opts = $1
    shebang.strip! # Avoid nasty ^M issues.
  end
  if which = Gem.configuration[:custom_shebang]
    # replace bin_file_name with "ruby" to avoid endless loops
    which = which.gsub(/ #{bin_file_name}$/," #{RbConfig::CONFIG['ruby_install_name']}")
    which = which.gsub(/\$(\w+)/) do
      case $1
      when "env"
        @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
      when "ruby"
        "#{Gem.ruby}#{opts}"
      when "exec"
        bin_file_name
      when "name"
        spec.name
      end
    end
    "#!#{which}"
  elsif not ruby_name then
    "#!#{Gem.ruby}#{opts}"
  elsif opts then
    "#!/bin/sh\n'exec' #{ruby_name.dump} '-x' \"$0\" \"$@\"\n#{shebang}"
  else
    # Create a plain shebang line.
    @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
    "#!#{@env_path} #{ruby_name}"
  end
end
             
            Lazy accessor for the installer's spec.
 
               # File rubygems/installer.rb, line 261
def spec
  @package.spec
rescue Gem::Package::Error => e
  raise Gem::InstallError, "invalid gem: #{e.message}"
end
             
            The location of the spec file that is installed.
 
               # File rubygems/installer.rb, line 411
def spec_file
  File.join gem_home, "specifications", "#{spec.full_name}.gemspec"
end
             
            Unpacks the gem into the given directory.
 
               # File rubygems/installer.rb, line 402
def unpack(directory)
  @gem_dir = directory
  extract_files
end
             
             
               # File rubygems/installer.rb, line 700
def verify_spec_name
  return if spec.name =~ Gem::Specification::VALID_NAME_PATTERN
  raise Gem::InstallError, "#{spec} has an invalid name"
end
             
            return the stub script text used to launch the true Ruby script
 
               # File rubygems/installer.rb, line 745
  def windows_stub_script(bindir, bin_file_name)
    ruby = Gem.ruby.gsub(/^\"|\"$/, "").tr(File::SEPARATOR, "\\")
    return <<-TEXT
@ECHO OFF
IF NOT "%~f0" == "~f0" GOTO :WinNT
@"#{ruby}" "#{File.join(bindir, bin_file_name)}" %1 %2 %3 %4 %5 %6 %7 %8 %9
GOTO :EOF
:WinNT
@"#{ruby}" "%~dpn0" %*
TEXT
  end
             
            Writes the file containing the arguments for building this gem's extensions.
 
               # File rubygems/installer.rb, line 852
def write_build_info_file
  return if @build_args.empty?
  build_info_dir = File.join gem_home, 'build_info'
  FileUtils.mkdir_p build_info_dir
  build_info_file = File.join build_info_dir, "#{spec.full_name}.info"
  open build_info_file, 'w' do |io|
    @build_args.each do |arg|
      io.puts arg
    end
  end
end
             
            Writes the .gem file to the cache directory
 
               # File rubygems/installer.rb, line 871
def write_cache_file
  cache_file = File.join gem_home, 'cache', spec.file_name
  @package.copy_to cache_file
end
             
            Writes the full .gemspec specification (in Ruby) to the gem home's specifications/default directory.
 
               # File rubygems/installer.rb, line 441
def write_default_spec
  File.open(default_spec_file, "w") do |file|
    file.puts spec.to_ruby
  end
end
             
            Writes the .gemspec specification (in Ruby) to the gem home's specifications directory.
 
               # File rubygems/installer.rb, line 427
def write_spec
  open spec_file, 'w' do |file|
    spec.installed_by_version = Gem.rubygems_version
    file.puts spec.to_ruby_for_cache
    file.fsync rescue nil # for filesystems without fsync(2)
  end
end