Add support for notify-reload systemd unit type (patroni#3450)

This allows systemd-reload command to wait for the configuration to
actually have been processed.

Encapsulate the import logic in a separate function to be used by the
already present notify implementation.

Author: rdunklau

patroni/daemon.py

@@ -18,6 +18,18 @@
 
 logger = logging.getLogger(__name__)
 
+try:  # pragma: no cover
+    from systemd import daemon  # pyright: ignore
+
+    def notify_systemd(msg: str) -> None:
+        daemon.notify(msg)  # pyright: ignore
+
+except ImportError:  # pragma: no cover
+    logger.info("Systemd integration is not supported")
+
+    def notify_systemd(msg: str) -> None:
+        pass
+
 
 def get_base_arg_parser() -> argparse.ArgumentParser:
     """Create a basic argument parser with the arguments used for both patroni and raft controller daemon.
@@ -67,6 +79,7 @@ def sighup_handler(self, *_: Any) -> None:
         Flag the daemon as "SIGHUP received".
         """
         self._received_sighup = True
+        notify_systemd("RELOADING=1")
 
     def api_sigterm(self) -> bool:
         """Guarantee only a single SIGTERM is being processed.
@@ -135,16 +148,13 @@ def run(self) -> None:
         Start the logger thread and keep running execution cycles until a SIGTERM is eventually received. Also reload
         configuration upon receiving SIGHUP.
         """
-        try:  # pragma: no cover
-            from systemd import daemon  # pyright: ignore
-            daemon.notify("READY=1")  # pyright: ignore
-        except ImportError:  # pragma: no cover
-            logger.info("Systemd integration is not supported")
+        notify_systemd("READY=1")
         self.logger.start()
         while not self.received_sigterm:
             if self._received_sighup:
                 self._received_sighup = False
                 self.reload_config(True, self.config.reload_local_configuration())
+                notify_systemd("READY=1")
 
             self._run_cycle()