Add Spring.spawn_on_env

Currently, the only way Spring can create multiple applications for a
given environment variable value is by changing the `SPRING_APPLICATION_ID`.
This has the tradeoff of not being visible in `bin/spring status` calls,
and being treated as a separate application, when it actually isn't. You
might want to run an application with and without certain features
enabled in the same rails env, and with this patch you can do that and
have Spring treat these as the same app.

Configuration must be set in config/spring_client.rb

lib/spring/application.rb

@@ -28,6 +28,11 @@ def state!(val)
       @interrupt.last.write "."
     end
 
+    def spawn_env
+      env = JSON.load(ENV["SPRING_SPAWN_ENV"].dup).map { |key, value| "#{key}=#{value}" }
+      env.join(", ") if env.any?
+    end
+
     def app_env
       ENV['RAILS_ENV']
     end

lib/spring/application/boot.rb

@@ -11,9 +11,15 @@
 
 Signal.trap("TERM") { app.terminate }
 
-Spring::ProcessTitleUpdater.run { |distance|
-  "spring app    | #{app.app_name} | started #{distance} ago | #{app.app_env} mode"
-}
+Spring::ProcessTitleUpdater.run do |distance|
+  attributes = [
+    app.app_name,
+    "started #{distance} ago",
+    "#{app.app_env} mode",
+    app.spawn_env,
+  ].compact
+  "spring app    | #{attributes.join(" | ")}"
+end
 
 app.eager_preload if ENV.delete("SPRING_PRELOAD") == "1"
 app.run

lib/spring/application_manager.rb

@@ -1,9 +1,10 @@
 module Spring
   class ApplicationManager
-    attr_reader :pid, :child, :app_env, :spring_env, :status
+    attr_reader :pid, :child, :app_env, :spawn_env, :spring_env, :status
 
-    def initialize(app_env, spring_env)
+    def initialize(app_env, spawn_env, spring_env)
       @app_env    = app_env
+      @spawn_env  = spawn_env
       @spring_env = spring_env
       @mutex      = Mutex.new
       @state      = :running
@@ -100,7 +101,9 @@ def start_child(preload = false)
             "RAILS_ENV"           => app_env,
             "RACK_ENV"            => app_env,
             "SPRING_ORIGINAL_ENV" => JSON.dump(Spring::ORIGINAL_ENV),
-            "SPRING_PRELOAD"      => preload ? "1" : "0"
+            "SPRING_PRELOAD"      => preload ? "1" : "0",
+            "SPRING_SPAWN_ENV"    => JSON.dump(spawn_env),
+            **spawn_env,
           },
           "ruby",
           *(bundler_dir != RbConfig::CONFIG["rubylibdir"] ? ["-I", bundler_dir] : []),

lib/spring/client/run.rb

@@ -132,7 +132,7 @@ 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, "spawn_env" => spawn_env
 
         if IO.select([server], [], [], CONNECT_TIMEOUT)
           server.gets or raise CommandNotFound
@@ -232,6 +232,10 @@ def send_json(socket, data)
       def default_rails_env
         ENV['RAILS_ENV'] || ENV['RACK_ENV'] || 'development'
       end
+
+      def spawn_env
+        ENV.slice(*Spring.spawn_on_env)
+      end
     end
   end
 end

lib/spring/configuration.rb

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

lib/spring/server.rb

@@ -19,7 +19,9 @@ def self.boot(options = {})
     def initialize(options = {})
       @foreground   = options.fetch(:foreground, false)
       @env          = options[:env] || default_env
-      @applications = Hash.new { |h, k| h[k] = ApplicationManager.new(k, env) }
+      @applications = Hash.new do |hash, key|
+        hash[key] = ApplicationManager.new(*key, env)
+      end
       @pidfile      = env.pidfile_path.open('a')
       @mutex        = Mutex.new
     end
@@ -57,12 +59,12 @@ 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, spawn_env = command.values_at('args', 'default_rails_env', 'spawn_env')
 
       if Spring.command?(args.first)
         log "running command #{args.first}"
         client.puts
-        client.puts @applications[rails_env_for(args, default_rails_env)].run(app_client)
+        client.puts @applications[rails_env_for(args, default_rails_env, spawn_env)].run(app_client)
       else
         log "command not found #{args.first}"
         client.close
@@ -73,8 +75,8 @@ def serve(client)
       redirect_output
     end
 
-    def rails_env_for(args, default_rails_env)
-      Spring.command(args.first).env(args.drop(1)) || default_rails_env
+    def rails_env_for(args, default_rails_env, spawn_env)
+      [Spring.command(args.first).env(args.drop(1)) || default_rails_env, spawn_env]
     end
 
     # Boot the server into the process group of the current session.