Refactor signal handling to use a pipe

This commit removes the need to poll a global array for received signals every
second by introducing an internal pipe(2).

The writing end of the pipe is being used in the block trapping the signals.
Whenever a signal is received, the signal's name gets written to the pipe.

Instead of polling for received signals, the code now uses select(2) to
check whether the internal pipe has been written to and can be read from.
select(2) is blocking until one of the passed file descriptors is ready for
reading/writing or has an exception, which means the code does not need to use
`sleep 1` anymore.

So when a signal is sent to the sidekiq process, the signal name gets written to
the internal pipe, select(2) returns and the process can react according to the
received signal. After handling a signal, select(2) gets called again and blocks
until receiving another signal.

lib/sidekiq/cli.rb

@@ -1,11 +1,11 @@
-$sidekiq_signals = []
+$self_read, $self_write = IO.pipe
 
 # Signal handlers should do as little as humanly possible
 # and defer all work to a non-trap context.  We'll have
 # the main thread poll for signals and handle them there.
 %w(INT TERM USR1 USR2 TTIN).each do |sig|
   trap sig do
-    $sidekiq_signals << sig
+    $self_write.puts(sig)
   end
 end
 
@@ -69,9 +69,9 @@ module Sidekiq
         end
         launcher.run
 
-        while true
-          handle_signals
-          sleep 1
+        while readable_io = IO.select([$self_read])
+          signal = readable_io.first[0].gets.strip
+          handle_signal(signal)
         end
       rescue Interrupt
         logger.info 'Shutting down'
@@ -82,8 +82,7 @@ module Sidekiq
       end
     end
 
-    def handle_signals
-      while sig = $sidekiq_signals.shift
+    def handle_signal(sig)
       Sidekiq.logger.debug "Got #{sig} signal"
       case sig
       when 'INT'
@@ -119,7 +118,6 @@ module Sidekiq
         end
       end
     end
-    end
 
     private