Add process hooks to tell systemd our state.

Fixes #5676.

synapse/app/_base.py

@@ -15,10 +15,12 @@
 
 import gc
 import logging
+import os
 import signal
 import sys
 import traceback
 
+import sdnotify
 from daemonize import Daemonize
 
 from twisted.internet import defer, error, reactor
@@ -242,9 +244,16 @@ def start(hs, listeners=None):
         if hasattr(signal, "SIGHUP"):
 
             def handle_sighup(*args, **kwargs):
+                # Tell systemd our state, if we're using it. This will silently fail if
+                # we're not using systemd.
+                sd_channel = sdnotify.SystemdNotifier()
+                sd_channel.notify("RELOADING=1")
+
                 for i in _sighup_callbacks:
                     i(hs)
 
+                sd_channel.notify("READY=1")
+
             signal.signal(signal.SIGHUP, handle_sighup)
 
             register_sighup(refresh_certificate)
@@ -260,6 +269,7 @@ def start(hs, listeners=None):
         hs.get_datastore().start_profiling()
 
         setup_sentry(hs)
+        setup_sdnotify(hs)
     except Exception:
         traceback.print_exc(file=sys.stderr)
         reactor = hs.get_reactor()
@@ -292,6 +302,25 @@ def setup_sentry(hs):
         scope.set_tag("worker_name", name)
 
 
+def setup_sdnotify(hs):
+    """Adds process state hooks to tell systemd what we are up to.
+    """
+
+    # Tell systemd our state, if we're using it. This will silently fail if
+    # we're not using systemd.
+    sd_channel = sdnotify.SystemdNotifier()
+
+    hs.get_reactor().addSystemEventTrigger(
+        "after",
+        "startup",
+        lambda: sd_channel.notify("READY=1\nMAINPID=%s" % (os.getpid())),
+    )
+
+    hs.get_reactor().addSystemEventTrigger(
+        "before", "shutdown", lambda: sd_channel.notify("STOPPING=1")
+    )
+
+
 def install_dns_limiter(reactor, max_dns_requests_in_flight=100):
     """Replaces the resolver with one that limits the number of in flight DNS
     requests.

synapse/python_dependencies.py

@@ -72,6 +72,7 @@ REQUIREMENTS = [
     "netaddr>=0.7.18",
     "Jinja2>=2.9",
     "bleach>=1.4.3",
+    "sdnotify>=0.3",
 ]
 
 CONDITIONAL_REQUIREMENTS = {