Support binstubs with --enable-load-relative prolog

lib/rubygems/installer.rb

@@ -220,7 +220,17 @@ def check_executable_overwrite(filename) # :nodoc:
     existing = nil
 
     File.open generated_bin, 'rb' do |io|
-      next unless io.gets =~ /^#!/ # shebang
+      line = io.gets
+      shebang = /^#!.*ruby/
+
+      if load_relative_enabled?
+        until line.nil? || line =~ shebang do
+          line = io.gets
+        end
+      end
+
+      next unless line =~ shebang
+
       io.gets # blankline
 
       # TODO detect a specially formatted comment instead of trying
@@ -585,7 +595,6 @@ def generate_bin_symlink(filename, bindir)
   #
 
   def shebang(bin_file_name)
-    ruby_name = 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 } || ""
 
@@ -614,14 +623,12 @@ def shebang(bin_file_name)
       end
 
       "#!#{which}"
-    elsif not ruby_name
-      "#!#{Gem.ruby}#{opts}"
-    elsif opts
-      "#!/bin/sh\n'exec' #{ruby_name.dump} '-x' \"$0\" \"$@\"\n#{shebang}"
-    else
+    elsif @env_shebang
       # Create a plain shebang line.
       @env_path ||= ENV_PATHS.find {|env_path| File.executable? env_path }
-      "#!#{@env_path} #{ruby_name}"
+      "#!#{@env_path} #{ruby_install_name}"
+    else
+      "#{bash_prolog_script}#!#{Gem.ruby}#{opts}"
     end
   end
 
@@ -980,4 +987,29 @@ def rb_config
   def ruby_install_name
     rb_config["ruby_install_name"]
   end
+
+  def load_relative_enabled?
+    rb_config["LIBRUBY_RELATIVE"] == 'yes'
+  end
+
+  def bash_prolog_script
+    if load_relative_enabled?
+      script = +<<~EOS
+        bindir="${0%/*}"
+      EOS
+
+      script << %Q(exec "$bindir/#{ruby_install_name}" "-x" "$0" "$@"\n)
+
+      <<~EOS
+        #!/bin/sh
+        # -*- ruby -*-
+        _=_\\
+        =begin
+        #{script.chomp}
+        =end
+      EOS
+    else
+      ""
+    end
+  end
 end