Define a :config mode, dont run external pipeline on config mode. Fixes #1844

middleman-cli/bin/middleman

@@ -46,6 +46,7 @@ end
 
 ::Middleman::Logger.singleton(3)
 ::Middleman::Cli.config = ::Middleman::Application.new do
+  config[:mode] = :config
   config[:exit_before_ready] = true
   config[:watcher_disable] = true
   config[:disable_sitemap] = true

middleman-cli/lib/middleman-cli/config.rb

@@ -23,6 +23,7 @@ module Middleman::Cli
       ::Middleman::Logger.singleton(2, false)
 
       app = ::Middleman::Application.new do
+        config[:mode] = :config
         config[:disable_sitemap] = true
         config[:watcher_disable] = true
         config[:exit_before_ready] = true

middleman-core/lib/middleman-core/extensions/external_pipeline.rb

@@ -10,6 +10,8 @@ class Middleman::Extensions::ExternalPipeline < ::Middleman::Extension
   def initialize(app, config={}, &block)
     super
 
+    return if app.mode?(:config)
+
     require 'thread'
 
     @watcher = app.files.watch :source,