add check_config instance method to workers

Add a method to check if config requirements are OK for a specific worker by
adding the `check_config` instance method. This method takes 2 arguments: the
config instance followed by the logger instance to log.

This method is right now used in the thread worker to display a warning in
the case it can't handle keep alive connections. This change is related to #979.

gunicorn/arbiter.py

@@ -134,6 +134,9 @@ class Arbiter(object):
         self.log.info("Listening at: %s (%s)", listeners_str, self.pid)
         self.log.info("Using worker: %s", self.cfg.worker_class_str)
 
+        # check worker class requirements
+        self.worker_class.check_config(self.cfg, self.log)
+
         self.cfg.when_ready(self)
 
     def init_signals(self):

gunicorn/workers/gthread.py

@@ -86,7 +86,6 @@ class ThreadWorker(base.Worker):
         super(ThreadWorker, self).__init__(*args, **kwargs)
         self.worker_connections = self.cfg.worker_connections
         self.max_keepalived = self.cfg.worker_connections - self.cfg.threads
-
         # initialise the pool
         self.tpool = None
         self.poller = None
@@ -94,6 +93,14 @@ class ThreadWorker(base.Worker):
         self.futures = deque()
         self._keep = deque()
 
+    @classmethod
+    def check_config(cls, cfg, log):
+        max_keepalived = cfg.worker_connections - cfg.threads
+
+        if max_keepalived <= 0 and cfg.keepalive:
+            log.warning("No keepalived connections can be handled. " +
+                    "Check the number of worker connections and threads.")
+
     def init_process(self):
         self.tpool = futures.ThreadPoolExecutor(max_workers=self.cfg.threads)
         self.poller = selectors.DefaultSelector()