Add prune_bundler option

This option makes it possible to specify puma in a Gemfile, but then
have puma use future Gemfiles in future releases when those releases are
seen via cluster workers.

Basically, this implements the feature that many people want: the
ability to specify puma in a context file and have restarts pickup
Gemfile changes.

lib/puma/cli.rb

@@ -162,6 +162,10 @@ module Puma
           @options[:preload_app] = true
         end
 
+        o.on "--prune-bundler", "Prune out the bundler env if possible" do
+          @options[:prune_bundler] = true
+        end
+
         o.on "-q", "--quiet", "Quiet down the output" do
           @options[:quiet] = true
         end
@@ -437,6 +441,10 @@ module Puma
       end
     end
 
+    def prune_bundler?
+      @options[:prune_bundler] && clustered? && !@options[:preload_app]
+    end
+
     # Parse the options, load the rackup, start the server and wait
     # for it to finish.
     #
@@ -447,6 +455,21 @@ module Puma
         exit 1
       end
 
+      if prune_bundler? && defined?(Bundler)
+        puma_lib_dir = $:.detect { |d| File.exist? File.join(d, "puma", "const.rb") }
+
+        if puma_lib_dir
+          log "* Pruning Bundler environment"
+          Bundler.with_clean_env do
+            Kernel.exec(Gem.ruby, "-I", puma_lib_dir,
+                        File.expand_path(puma_lib_dir + "/../bin/puma"),
+                        *@original_argv)
+          end
+        end
+
+        log "! Unable to prune Bundler environment, continuing"
+      end
+
       if dir = @options[:directory]
         Dir.chdir dir
       end

lib/puma/cluster.rb

@@ -185,6 +185,12 @@ module Puma
         end
       end
 
+      # If we're not running under a Bundler context, then
+      # report the info about the context we will be using
+      if !ENV['BUNDLER_GEMFILE'] and File.exist?("Gemfile")
+        log "+ Gemfile in context: #{File.expand_path("Gemfile")}"
+      end
+
       # Invoke any worker boot hooks so they can get
       # things in shape before booting the app.
       hooks = @options[:before_worker_boot]

lib/puma/configuration.rb

@@ -346,6 +346,22 @@ module Puma
         raise "Provide either a #call'able or a block" unless obj
         @options[:lowlevel_error_handler] = obj
       end
+
+      # This option is used to allow your app and it's gems to be
+      # properly reloaded when not using preload.
+      #
+      # When set, if puma detects that it's been invoked in the
+      # context of Bundler, it will cleanup the environment and
+      # re-run itself outside the Bundler environment, but directly
+      # using the files that Bundler has setup.
+      #
+      # This means that puma is now decoupled from your Bundler
+      # context and when each worker loads, it will be loading a
+      # new Bundler context and thus can float around as the release
+      # dictates.
+      def prune_bundler(answer=true)
+        @options[:prune_bundler] = answer
+      end
     end
   end
 end