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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 109
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 100
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 149
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 at package which can either be a path or an
instance of Gem::Package.  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
Print gem post install message if true
 
               # File rubygems/installer.rb, line 179
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
  @package.dir_mode = options[:dir_mode]
  @package.prog_mode = options[:prog_mode]
  @package.data_mode = options[:data_mode]
  if options[:user_install]
    @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 767
  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_prerelease}"
str = ARGV.first
if str
  str = str.b[/\\A_(.*)_\\z/, 1]
  if str and Gem::Version.correct?(str)
    version = str
    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 841
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 448
def default_spec_file
  File.join Gem.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 894
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 410
def ensure_dependency(spec, dependency)
  unless installation_satisfies_dependency? dependency
    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 632
def ensure_loadable_spec
  ruby = spec.to_ruby_for_cache
  ruby.tap(&Gem::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 873
def extract_bin
  @package.extract_files gem_dir, "#{spec.bindir}/*"
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 864
def extract_files
  @package.extract_files gem_dir
end
             
            Prefix and suffix the program filename the same as ruby.
 
               # File rubygems/installer.rb, line 880
def formatted_program_filename(filename)
  if @format_executable
    self.class.exec_format % File.basename(filename)
  else
    filename
  end
end
             
            Lazy accessor for the spec’s gem directory.
 
               # File rubygems/installer.rb, line 276
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 534
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)
    file.chmod(options[:prog_mode] || 0755)
  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 553
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
    if File.symlink? dst
      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 479
def generate_windows_script(filename, bindir)
  if Gem.win_platform?
    script_name = formatted_program_filename(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 300
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]
    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
  dir_mode = options[:dir_mode]
  FileUtils.mkdir_p gem_dir, :mode => dir_mode && 0755
  if @options[:install_as_default]
    extract_bin
    write_default_spec
  else
    extract_files
    build_extensions
    write_build_info_file
    run_post_build_hooks
  end
  generate_bin
  unless @options[:install_as_default]
    write_spec
    write_cache_file
  end
  File.chmod(dir_mode, gem_dir) if dir_mode
  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 420
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 390
def installed_specs
  @specs ||= begin
    specs = []
    Gem::Util.glob_files_in_dir("*.gemspec", File.join(gem_home, "specifications")).each do |path|
      spec = Gem::Specification.load path.tap(&Gem::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 if the install is ignoring dependencies.
 
               # File rubygems/installer.rb, line 907
def pre_install_checks
  verify_gem_home
  # The name and require_paths must be verified first, since it could contain
  # ruby code that would be eval'ed in #ensure_loadable_spec
  verify_spec
  ensure_loadable_spec
  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 587
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
    # 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
    "#!#{Gem.ruby}#{opts}"
  elsif opts
    "#!/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 283
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 440
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 430
def unpack(directory)
  @gem_dir = directory
  extract_files
end
             
             
               # File rubygems/installer.rb, line 738
def verify_spec
  unless spec.name =~ Gem::Specification::VALID_NAME_PATTERN
    raise Gem::InstallError, "#{spec} has an invalid name"
  end
  if spec.raw_require_paths.any?{|path| path =~ /\R/ }
    raise Gem::InstallError, "#{spec} has an invalid require_paths"
  end
  if spec.extensions.any?{|ext| ext =~ /\R/ }
    raise Gem::InstallError, "#{spec} has an invalid extensions"
  end
  unless spec.specification_version.to_s =~ /\A\d+\z/
    raise Gem::InstallError, "#{spec} has an invalid specification_version"
  end
  if spec.dependencies.any? {|dep| dep.type != :runtime && dep.type != :development }
    raise Gem::InstallError, "#{spec} has an invalid dependencies"
  end
  if spec.dependencies.any? {|dep| dep.name =~ /(?:\R|[<>])/ }
    raise Gem::InstallError, "#{spec} has an invalid dependencies"
  end
end
             
            return the stub script text used to launch the true Ruby script
 
               # File rubygems/installer.rb, line 804
  def windows_stub_script(bindir, bin_file_name)
    rb_config = RbConfig::CONFIG
    rb_topdir = RbConfig::TOPDIR || File.dirname(rb_config["bindir"])
    # get ruby executable file name from RbConfig
    ruby_exe = "#{rb_config['RUBY_INSTALL_NAME']}#{rb_config['EXEEXT']}"
    ruby_exe = "ruby.exe" if ruby_exe.empty?
    if File.exist?(File.join bindir, ruby_exe)
      # stub & ruby.exe withing same folder.  Portable
      <<-TEXT
@ECHO OFF
@"%~dp0#{ruby_exe}" "%~dpn0" %*
      TEXT
    elsif bindir.downcase.start_with? rb_topdir.downcase
      # stub within ruby folder, but not standard bin.  Portable
      require 'pathname'
      from = Pathname.new bindir
      to   = Pathname.new "#{rb_topdir}/bin"
      rel  = to.relative_path_from from
      <<-TEXT
@ECHO OFF
@"%~dp0#{rel}/#{ruby_exe}" "%~dpn0" %*
      TEXT
    else
      # outside ruby folder, maybe -user-install or bundler.  Portable, but ruby
      # is dependent on PATH
      <<-TEXT
@ECHO OFF
@#{ruby_exe} "%~dpn0" %*
      TEXT
    end
  end
             
            Writes the file containing the arguments for building this gem’s extensions.
 
               # File rubygems/installer.rb, line 935
def write_build_info_file
  return if @build_args.empty?
  build_info_dir = File.join gem_home, 'build_info'
  dir_mode = options[:dir_mode]
  FileUtils.mkdir_p build_info_dir, :mode => dir_mode && 0755
  build_info_file = File.join build_info_dir, "#{spec.full_name}.info"
  File.open build_info_file, 'w' do |io|
    @build_args.each do |arg|
      io.puts arg
    end
  end
  File.chmod(dir_mode, build_info_dir) if dir_mode
end
             
            Writes the .gem file to the cache directory
 
               # File rubygems/installer.rb, line 957
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 470
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 456
def write_spec
  File.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