Factor out common application start

We have 10 copies of this code, and I don't really want to update each one
separately.

synapse/app/_base.py

@@ -0,0 +1,92 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import gc
+import logging
+
+from daemonize import Daemonize
+from synapse.util import PreserveLoggingContext
+from synapse.util.rlimit import change_resource_limit
+from twisted.internet import reactor
+
+
+def start_worker_reactor(appname, config):
+    """ Run the reactor in the main process
+
+    Daemonizes if necessary, and then configures some resources, before starting
+    the reactor. Pulls configuration from the 'worker' settings in 'config'.
+
+    Args:
+        appname (str): application name which will be sent to syslog
+        config (synapse.config.Config): config object
+    """
+
+    logger = logging.getLogger(config.worker_app)
+
+    start_reactor(
+        appname,
+        config.soft_file_limit,
+        config.gc_thresholds,
+        config.worker_pid_file,
+        config.worker_daemonize,
+        logger
+    )
+
+
+def start_reactor(
+        appname,
+        soft_file_limit,
+        gc_thresholds,
+        pid_file,
+        daemonize,
+        logger,
+):
+    """ Run the reactor in the main process
+
+    Daemonizes if necessary, and then configures some resources, before starting
+    the reactor
+
+    Args:
+        appname (str): application name which will be sent to syslog
+        soft_file_limit (int):
+        gc_thresholds:
+        pid_file (str): name of pid file to write to if daemonize is True
+        daemonize (bool): true to run the reactor in a background process
+        logger (logging.Logger): logger instance to pass to Daemonize
+    """
+
+    def run():
+        # make sure that we run the reactor with the sentinel log context,
+        # otherwise other PreserveLoggingContext instances will get confused
+        # and complain when they see the logcontext arbitrarily swapping
+        # between the sentinel and `run` logcontexts.
+        with PreserveLoggingContext():
+            logger.info("Running")
+            change_resource_limit(soft_file_limit)
+            if gc_thresholds:
+                gc.set_threshold(*gc_thresholds)
+            reactor.run()
+
+    if daemonize:
+        daemon = Daemonize(
+            app=appname,
+            pid=pid_file,
+            action=run,
+            auto_close_fds=False,
+            verbose=True,
+            logger=logger,
+        )
+        daemon.start()
+    else:
+        run()