add nworkers_changed hook

This hook will be called whenever the number of workers is set.

gunicorn/arbiter.py

@@ -51,6 +51,7 @@ class Arbiter(object):
     def __init__(self, app):
         os.environ["SERVER_SOFTWARE"] = SERVER_SOFTWARE
 
+        self._num_workers = None
         self.setup(app)
 
         self.pidfile = None
@@ -79,6 +80,14 @@ class Arbiter(object):
             0: sys.executable
         }
 
+    def _get_num_workers(self):
+        return self._num_workers
+    def _set_num_workers(self, value):
+        old_value = self._num_workers
+        self._num_workers = value
+        self.cfg.nworkers_changed(self, value, old_value)
+    num_workers = property(_get_num_workers, _set_num_workers)
+
     def setup(self, app):
         self.app = app
         self.cfg = app.cfg

gunicorn/config.py

@@ -1056,6 +1056,24 @@ class WorkerExit(Setting):
         the just-exited Worker.
         """
 
+class NumWorkersChanged(Setting):
+    name = "nworkers_changed"
+    section = "Server Hooks"
+    validator = validate_callable(3)
+    type = "callable"
+    def nworkers_changed(server, new_value, old_value):
+        pass
+    default = staticmethod(nworkers_changed)
+    desc = """\
+        Called just after num_workers has been changed.
+
+        The callable needs to accept an instance variable of the Arbiter and
+        two integers of number of workers after and before change.
+
+        If the number of workers is set for the first time, old_value would be
+        None.
+        """
+
 class ProxyProtocol(Setting):
     name = "proxy_protocol"
     section = "Server Mechanics"

tests/test_003-config.py

@@ -214,3 +214,12 @@ def test_post_request():
 
     c.set("post_request", post_request_2)
     t.eq(2, c.post_request(1, 2, 3, 4))
+
+
+def test_nworkers_changed():
+    c = config.Config()
+    def nworkers_changed_3(server, new_value, old_value):
+        return 3
+
+    c.set("nworkers_changed", nworkers_changed_3)
+    t.eq(3, c.nworkers_changed(1, 2, 3))