Add child_exit() callback

This is similar to worker_exit() in that it is called just after a
worker has terminated, but it's called in the Gunicorn *master* process,
not the *child* process.

gunicorn/arbiter.py

@@ -498,8 +498,9 @@ class Arbiter(object):
                 if self.reexec_pid == wpid:
                     self.reexec_pid = 0
                 else:
-                    # A worker said it cannot boot. We'll shutdown
-                    # to avoid infinite start/stop cycles.
+                    # A worker was terminated. If the termination reason was
+                    # that it could not boot, we'll shut it down to avoid
+                    # infinite start/stop cycles.
                     exitcode = status >> 8
                     if exitcode == self.WORKER_BOOT_ERROR:
                         reason = "Worker failed to boot."
@@ -507,10 +508,12 @@ class Arbiter(object):
                     if exitcode == self.APP_LOAD_ERROR:
                         reason = "App failed to load."
                         raise HaltServer(reason, self.APP_LOAD_ERROR)
+
                     worker = self.WORKERS.pop(wpid, None)
                     if not worker:
                         continue
                     worker.tmp.close()
+                    self.cfg.child_exit(self, worker)
         except OSError as e:
             if e.errno != errno.ECHILD:
                 raise

gunicorn/config.py

@@ -1634,6 +1634,23 @@ class PostRequest(Setting):
         """
 
 
+class ChildExit(Setting):
+    name = "child_exit"
+    section = "Server Hooks"
+    validator = validate_callable(2)
+    type = six.callable
+
+    def child_exit(server, worker):
+        pass
+    default = staticmethod(child_exit)
+    desc = """\
+        Called just after a worker has been exited, in the master process.
+
+        The callable needs to accept two instance variables for the Arbiter and
+        the just-exited Worker.
+        """
+
+
 class WorkerExit(Setting):
     name = "worker_exit"
     section = "Server Hooks"
@@ -1644,7 +1661,7 @@ class WorkerExit(Setting):
         pass
     default = staticmethod(worker_exit)
     desc = """\
-        Called just after a worker has been exited.
+        Called just after a worker has been exited, in the worker process.
 
         The callable needs to accept two instance variables for the Arbiter and
         the just-exited Worker.

tests/test_arbiter.py

@@ -55,6 +55,18 @@ def test_arbiter_calls_worker_exit(mock_os_fork):
     arbiter.cfg.worker_exit.assert_called_with(arbiter, mock_worker)
 
 
+@mock.patch('os.waitpid')
+def test_arbiter_reap_workers(mock_os_waitpid):
+    mock_os_waitpid.side_effect = [(42, 0), (0, 0)]
+    arbiter = gunicorn.arbiter.Arbiter(DummyApplication())
+    arbiter.cfg.settings['child_exit'] = mock.Mock()
+    mock_worker = mock.Mock()
+    arbiter.WORKERS = {42: mock_worker}
+    arbiter.reap_workers()
+    mock_worker.tmp.close.assert_called_with()
+    arbiter.cfg.child_exit.assert_called_with(arbiter, mock_worker)
+
+
 class PreloadedAppWithEnvSettings(DummyApplication):
     """
     Simple application that makes use of the 'preload' feature to