Add Spring.reset_on_env

Spring needs to restart when certain important environment variables
change (eg. DATABASE_URL). Without this, comamnds connect to a stale
application instance that has been booted with incorrect config.

Configuration must be set in config/spring_client.rb.

lib/spring/client/run.rb

@@ -73,7 +73,7 @@ def run
       def boot_server
         env.socket_path.unlink if env.socket_path.exist?
 
-        pid     = Process.spawn(gem_env, env.server_command, out: File::NULL)
+        pid     = Process.spawn(server_process_env, env.server_command, out: File::NULL)
         timeout = Time.now + BOOT_TIMEOUT
 
         @server_booted = true
@@ -107,6 +107,14 @@ def gem_env
         }
       end
 
+      def reset_env
+        ENV.slice(*Spring.reset_on_env)
+      end
+
+      def server_process_env
+        reset_env.merge(gem_env)
+      end
+
       def stop_server
         server.close
         @server = nil
@@ -132,7 +140,7 @@ def verify_server_version
 
       def connect_to_application(client)
         server.send_io client
-        send_json server, "args" => args, "default_rails_env" => default_rails_env
+        send_json server, "args" => args, "default_rails_env" => default_rails_env, "reset_env" => reset_env
 
         if IO.select([server], [], [], CONNECT_TIMEOUT)
           server.gets or raise CommandNotFound

lib/spring/configuration.rb

@@ -32,6 +32,10 @@ def after_fork(&block)
       after_fork_callbacks << block
     end
 
+    def reset_on_env
+      @reset_on_env ||= []
+    end
+
     def verify_environment
       application_root_path
     end

lib/spring/server.rb

@@ -57,12 +57,15 @@ def serve(client)
       app_client = client.recv_io
       command    = JSON.load(client.read(client.gets.to_i))
 
-      args, default_rails_env = command.values_at('args', 'default_rails_env')
+      args, default_rails_env, reset_env = command.values_at('args', 'default_rails_env', 'reset_env')
 
       if Spring.command?(args.first)
+        application = @applications[rails_env_for(args, default_rails_env)]
+        reset_if_env_changed(application, reset_env)
+
         log "running command #{args.first}"
         client.puts
-        client.puts @applications[rails_env_for(args, default_rails_env)].run(app_client)
+        client.puts application.run(app_client)
       else
         log "command not found #{args.first}"
         client.close
@@ -136,6 +139,10 @@ def set_process_title
 
     private
 
+    def reset_if_env_changed(application, reset_env)
+      application.stop if ENV.slice(*reset_env.keys) != reset_env
+    end
+
     def default_env
       Env.new(log_file: default_log_file)
     end