Fix confluent startup error if dead

Clean up stale dbg.sock if present rather than fail to start.  Also, if the pid indicated in pid file doesn't exist, ignore
the pidfile contents.

confluent_server/confluent/main.py

@@ -1,7 +1,7 @@
 # vim: tabstop=4 shiftwidth=4 softtabstop=4
 
 # Copyright 2014 IBM Corporation
-# Copyright 2015 Lenovo
+# Copyright 2015-2017 Lenovo
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -95,9 +95,15 @@ def _checkpidfile():
         fcntl.flock(pidfile, fcntl.LOCK_EX)
         pid = pidfile.read()
         if pid != '':
-            print ('/var/run/confluent/pid exists and indicates %s is still '
-                   'running' % pid)
-            sys.exit(1)
+            try:
+                os.kill(int(pid), 0)
+                print ('/var/run/confluent/pid exists and indicates %s is still '
+                       'running' % pid)
+                sys.exit(1)
+            except OSError:
+                # There is no process running by that pid, must be stale
+                pass
+        pidfile.seek(0)
         pidfile.write(str(os.getpid()))
         fcntl.flock(pidfile, fcntl.LOCK_UN)
         pidfile.close()
@@ -199,10 +205,12 @@ def run():
     auth.init_auth()
     signal.signal(signal.SIGINT, terminate)
     signal.signal(signal.SIGTERM, terminate)
-    #TODO(jbjohnso): eventlet has a bug about unix domain sockets, this code
-    #works with bugs fixed
     if dbgif:
         oumask = os.umask(0077)
+        try:
+            os.remove('/var/run/confluent/dbg.sock')
+        except OSError:
+            pass  # We are not expecting the file to exist
         dbgsock = eventlet.listen("/var/run/confluent/dbg.sock",
                                    family=socket.AF_UNIX)
         eventlet.spawn_n(backdoor.backdoor_server, dbgsock)