Add ability to wakeup and cancel persistent connections

Without this, the graceful stop code has to wait for the persistent
timeout to expire even though there is actually no work going on.

lib/puma/server.rb

@@ -52,6 +52,7 @@ def initialize(app, events=Events::DEFAULT)
       @thread_pool = nil
 
       @persistent_timeout = PERSISTENT_TIMEOUT
+      @persistent_check, @persistent_wakeup = IO.pipe
 
       @proto_env = {
         "rack.version".freeze => Rack::VERSION,
@@ -219,9 +220,11 @@ def process_client(client)
                 env = @proto_env.dup
                 nparsed = 0
               else
-                unless IO.select([client], nil, nil, @persistent_timeout)
+                unless ret = IO.select([client, @persistent_check], nil, nil, @persistent_timeout)
                   raise EOFError, "Timed out persistent connection"
                 end
+
+                return if ret.first.include? @persistent_check
               end
             else
               # Parser is not done, queue up more data to read and continue parsing
@@ -514,12 +517,14 @@ def graceful_shutdown
     # off the request queue before finally exiting.
     #
     def stop(sync=false)
+      @persistent_wakeup.close
       @notify << STOP_COMMAND
 
       @thread.join if @thread && sync
     end
 
     def halt(sync=false)
+      @persistent_wakeup.close
       @notify << HALT_COMMAND
 
       @thread.join if @thread && sync