Add support for SPRING_SERVER_COMMAND env variable

This extension point enables starting the spring server in a different
way, e.g. inside a docker container.

Redirecting output because starting the server should be a background
thing, so if there is output we don't want to see it.

Author: jonleighton

README.md

@@ -389,6 +389,9 @@ The following environment variables are used by Spring:
   the long-running Spring server process. By default this is related to
   the socket path; if the socket path is `/foo/bar/spring.sock` the
   pidfile will be `/foo/bar/spring.pid`.
+* `SPRING_SERVER_COMMAND` - The command to run to start up the spring
+  server when it is not already running. Defaults to `spring _[version]_
+  server --background`.
 
 ## Troubleshooting
 

lib/spring/client/run.rb

@@ -69,12 +69,7 @@ def run
 
       def boot_server
         env.socket_path.unlink if env.socket_path.exist?
-
-        pid = Process.spawn(
-          gem_env,
-          "ruby",
-          "-e", "gem 'spring', '#{Spring::VERSION}'; require 'spring/server'; Spring::Server.boot"
-        )
+        pid = Process.spawn(gem_env, env.server_command, out: File::NULL)
 
         until env.socket_path.exist?
           _, status = Process.waitpid2(pid, Process::WNOHANG)

lib/spring/client/server.rb

@@ -1,13 +1,17 @@
 module Spring
   module Client
     class Server < Command
+      def self.description
+        "Explicitly start a Spring server in the foreground"
+      end
+
       def call
         require "spring/server"
-        Spring::Server.boot(foreground: true)
+        Spring::Server.boot(foreground: foreground?)
       end
 
-      def self.description
-        "Explicitly start a Spring server in the foreground"
+      def foreground?
+        !args.include?("--background")
       end
     end
   end

lib/spring/env.rb

@@ -108,5 +108,9 @@ def kill(sig)
     rescue Errno::ESRCH
       # already dead
     end
+
+    def server_command
+      ENV["SPRING_SERVER_COMMAND"] || "spring _#{Spring::VERSION}_ server --background"
+    end
   end
 end