Fix setting of GEM_HOME

In 1.3.0 we started to launch the Spring server process via a
Process.spawn, rather than a fork
(ae175de).

This brought to light some problems with how we were handling the
GEM_HOME environment variable. In < 1.3 it was set to an empty string.
In 1.3.0 it was set to nil.

The bug was reported in #352 and #383.

After switching to Process.spawn, having no value in GEM_HOME in the
server process meant that Bundler was unable to find bundled git
repositories.

In actual fact, we do not need to set the gem home in the binstub. We
only need to set the gem path, so that the install of spring which is in
the bundle can be found. And we don't really need to mutate the ENV hash
either.

This makes the binstub clearer and simpler.

We then set the GEM_PATH and GEM_HOME as we need them for the env when
we spawn the server process.

Author: jonleighton

lib/spring/client/binstub.rb

@@ -29,18 +29,15 @@ class Binstub < Command
       SPRING = <<'CODE'
 #!/usr/bin/env ruby
 
-# This file loads spring without using Bundler, in order to be fast
-# It gets overwritten when you run the `spring binstub` command
+# This file loads spring without using Bundler, in order to be fast.
+# It gets overwritten when you run the `spring binstub` command.
 
 unless defined?(Spring)
   require "rubygems"
   require "bundler"
 
   if match = Bundler.default_lockfile.read.match(/^GEM$.*?^    (?:  )*spring \((.*?)\)$.*?^$/m)
-    ENV["GEM_PATH"] = ([Bundler.bundle_path.to_s] + Gem.path).join(File::PATH_SEPARATOR)
-    ENV["GEM_HOME"] = nil
-    Gem.paths = ENV
-
+    Gem.paths = { "GEM_PATH" => Bundler.bundle_path.to_s }
     gem "spring", match[1]
     require "spring/binstub"
   end

lib/spring/client/run.rb

@@ -1,5 +1,6 @@
 require "rbconfig"
 require "socket"
+require "bundler"
 
 module Spring
   module Client
@@ -64,10 +65,8 @@ def run
       def boot_server
         env.socket_path.unlink if env.socket_path.exist?
 
-        # The GEM_HOME handling is to work around a problem with spring binstubs
-        # generated prior to 1.3.0.
         pid = Process.spawn(
-          ENV["GEM_HOME"] == "" ? { "GEM_HOME" => nil } : {},
+          gem_env,
           "ruby",
           "-r", "spring/server",
           "-e", "Spring::Server.boot"
@@ -80,6 +79,16 @@ def boot_server
         end
       end
 
+      def gem_env
+        bundle = Bundler.bundle_path.to_s
+        paths  = ENV["GEM_PATH"].to_s.split(File::PATH_SEPARATOR)
+
+        {
+          "GEM_PATH" => [bundle, *paths].join(File::PATH_SEPARATOR),
+          "GEM_HOME" => bundle
+        }
+      end
+
       def stop_server
         server.close
         @server = nil