add systemd sd_notify support (#1897)

* add systemd sd_notify support

roughly based on sd_notify() from systemd and https://github.com/bb4242/sdnotify

only implements `READY=1` and `STATUS=Gunicorn arbiter booted` of the
protocol in the arbiter. in the future, reloads can be notified, and
possibly also other statuses.

see https://www.freedesktop.org/software/systemd/man/sd_notify.html for
more info

sd_notify() is a noop when not run in a systemd service (i.e
NOTIFY_SOCKET environment variable is not set)

docs/source/deploy.rst

@@ -227,7 +227,7 @@ unix socket:
     After=network.target
 
     [Service]
-    PIDFile=/run/gunicorn/pid
+    Type=notify
     User=someuser
     Group=someuser
     RuntimeDirectory=gunicorn

gunicorn/arbiter.py

@@ -158,6 +158,7 @@ class Arbiter(object):
         self.log.debug("Arbiter booted")
         self.log.info("Listening at: %s (%s)", listeners_str, self.pid)
         self.log.info("Using worker: %s", self.cfg.worker_class_str)
+        systemd.sd_notify("READY=1\nSTATUS=Gunicorn arbiter booted", self.log)
 
         # check worker class requirements
         if hasattr(self.worker_class, "check_config"):

gunicorn/systemd.py

@@ -4,6 +4,7 @@
 # See the NOTICE for more information.
 
 import os
+import socket
 
 SD_LISTEN_FDS_START = 3
 
@@ -43,3 +44,34 @@ def listen_fds(unset_environment=True):
         os.environ.pop('LISTEN_FDS', None)
 
     return fds
+
+
+def sd_notify(state, logger, unset_environment=False):
+    """Send a notification to systemd. state is a string; see
+    the man page of sd_notify (http://www.freedesktop.org/software/systemd/man/sd_notify.html)
+    for a description of the allowable values.
+
+    If the unset_environment parameter is True, sd_notify() will unset
+    the $NOTIFY_SOCKET environment variable before returning (regardless of
+    whether the function call itself succeeded or not). Further calls to
+    sd_notify() will then fail, but the variable is no longer inherited by
+    child processes.
+    """
+
+
+    addr = os.environ.get('NOTIFY_SOCKET')
+    if addr is None:
+        # not run in a service, just a noop
+        return
+    try:
+        sock = socket.socket(socket.AF_UNIX, socket.SOCK_DGRAM | socket.SOCK_CLOEXEC)
+        if addr[0] == '@':
+            addr = '\0' + addr[1:]
+        sock.connect(addr)
+        sock.sendall(state.encode('utf-8'))
+    except:
+        logger.debug("Exception while invoking sd_notify()", exc_info=True)
+    finally:
+        if unset_environment:
+            os.environ.pop('NOTIFY_SOCKET')
+        sock.close()