Process.kill to check for process is more portable.

On OpenBSD, you can only use getpgid(pid) if the calling process
belongs to the same session as the the process with the PID `pid'.
Otherwise getpgid() returns EPERM.

Using kill(0, pid) is a more portable way to check whether a process
exists.

bin/sidekiqctl

@@ -41,9 +41,13 @@ class Sidekiqctl
   end
 
   def fetch_process
-    Process.getpgid(pid)
+    Process.kill(0, pid)
   rescue Errno::ESRCH
     done "Process doesn't exist", :error
+  # We were not allowed to send a signal, but the process must have existed
+  # when Process.kill() was called.
+  rescue Errno::EPERM
+    return pid
   end
 
   def done(msg, error = nil)
@@ -67,10 +71,12 @@ class Sidekiqctl
     `kill -TERM #{pid}`
     kill_timeout.times do
       begin
-        Process.getpgid(pid)
+        Process.kill(0, pid)
       rescue Errno::ESRCH
         FileUtils.rm_f pidfile
         done 'Sidekiq shut down gracefully.'
+      rescue Errno::EPERM
+        done 'Not permitted to shut down Sidekiq.'
       end
       sleep 1
     end