arbiter: Do basic rate limiting of signals

In the theoretical case that there are many signals directed to the
arbiter process for whatever reason, this change should keep the
backlog of signals fairly short to make the arbiter able to catch up.

gunicorn/arbiter.py

@@ -202,14 +202,10 @@ def run(self):
             while True:
                 self.maybe_promote_master()
 
-                try:
-                    sig = self.SIG_QUEUE.get(timeout=1)
-                    if sig != self.WAKEUP_REQUEST:
-                        if sig != signal.SIGCHLD:
-                            self.log.info("Handling signal: %s", signal.Signals(sig).name)
-                        self.SIG_HANDLERS[sig]()
-                except queue.Empty:
-                    pass
+                for sig in self.wait_for_signals(timeout=1):
+                    if sig != signal.SIGCHLD:
+                        self.log.info("Handling signal: %s", signal.Signals(sig).name)
+                    self.SIG_HANDLERS[sig]()
 
                 self.murder_workers()
                 self.manage_workers()
@@ -227,6 +223,16 @@ def run(self):
                 self.pidfile.unlink()
             sys.exit(-1)
 
+    def wait_for_signals(self, timeout=1):
+        signals = {}  # Use dict() instead of set() to keep the insertion order
+        try:
+            signals.setdefault(self.SIG_QUEUE.get(timeout))
+            while True:
+                signals.setdefault(self.SIG_QUEUE.get_nowait())
+        except queue.Empty:
+            signals.pop(self.WAKEUP_REQUEST, None)
+            return signals.keys()
+
     def handle_chld(self):
         "SIGCHLD handling"
         self.reap_workers()