arbiter: clean up main loop

* Look up handlers in __init__() to induce run-time error early on if
  something is wrong.

* Since we now know that all handlers exist, we can simplify the main
  loop in arbiter, in such a way that we don't need to call wakeup().

So after this commit, the pipe in arbiter is only used to deliver
which signal was sent.

gunicorn/arbiter.py

@@ -42,7 +42,6 @@ class Arbiter(object):
     SIG_QUEUE = queue.SimpleQueue()
     SIGNALS = [getattr(signal.Signals, "SIG%s" % x)
                for x in "CHLD HUP QUIT INT TERM TTIN TTOU USR1 USR2 WINCH".split()]
-    SIG_NAMES = dict((sig, sig.name[3:].lower()) for sig in SIGNALS)
 
     def __init__(self, app):
         os.environ["SERVER_SOFTWARE"] = SERVER_SOFTWARE
@@ -72,6 +71,11 @@ def __init__(self, app):
             0: sys.executable
         }
 
+        self.SIG_HANDLERS = dict(
+            (sig, getattr(self, "handle_%s" % sig.name[3:].lower()))
+            for sig in self.SIGNALS
+        )
+
     def _get_num_workers(self):
         return self._num_workers
 
@@ -194,17 +198,10 @@ def run(self):
 
                 try:
                     sig = self.SIG_QUEUE.get(timeout=1)
+                    self.log.info("Handling signal: %s", signal.Signals(sig).name)
+                    self.SIG_HANDLERS[sig]()
                 except queue.Empty:
-                    sig = None
-
-                if sig:
-                    signame = self.SIG_NAMES.get(sig)
-                    handler = getattr(self, "handle_%s" % signame, None)
-                    if not handler:
-                        self.log.error("Unhandled signal: %s", signame)
-                        continue
-                    self.log.info("Handling signal: %s", signame)
-                    handler()
+                    pass
 
                 self.murder_workers()
                 self.manage_workers()