bridgev2/mxmain: add systemd Type=notify support

bridgev2/matrix/mxmain/main.go

@@ -115,7 +115,11 @@ func (br *BridgeMain) Run() {
 	br.PreInit()
 	br.Init()
 	br.Start()
+	sdNotifyReady(br.Log)
+	stopWatchdog := startSdWatchdog(br.Log)
 	exitCode := br.WaitForInterrupt()
+	sdNotifyStopping(br.Log)
+	stopWatchdog()
 	br.Stop()
 	os.Exit(exitCode)
 }

bridgev2/matrix/mxmain/systemd.go

@@ -0,0 +1,84 @@
+// Copyright (c) 2026 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package mxmain
+
+import (
+	"time"
+
+	"github.com/coreos/go-systemd/v22/daemon"
+	"github.com/rs/zerolog"
+)
+
+// sdNotifyReady tells the systemd service manager that the bridge has finished
+// starting up. The call is a no-op when the bridge is not running under a
+// systemd unit with `Type=notify`.
+//
+// See https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html.
+func sdNotifyReady(log *zerolog.Logger) {
+	if sent, err := daemon.SdNotify(false, daemon.SdNotifyReady); err != nil {
+		log.Warn().Err(err).Msg("Failed to send systemd READY=1 notification")
+	} else if sent {
+		log.Debug().Msg("Sent systemd READY=1 notification")
+	}
+}
+
+// sdNotifyStopping tells the systemd service manager that the bridge is
+// shutting down. The call is a no-op outside of systemd.
+func sdNotifyStopping(log *zerolog.Logger) {
+	if sent, err := daemon.SdNotify(false, daemon.SdNotifyStopping); err != nil {
+		log.Warn().Err(err).Msg("Failed to send systemd STOPPING=1 notification")
+	} else if sent {
+		log.Debug().Msg("Sent systemd STOPPING=1 notification")
+	}
+}
+
+// startSdWatchdog pings the systemd watchdog at half the configured interval
+// for as long as the returned stop function has not been called. The returned
+// stop function blocks until the watchdog goroutine has exited.
+//
+// Returns a no-op stop function when the process is not running under a
+// systemd unit with `WatchdogSec=` set (so the call is always safe).
+func startSdWatchdog(log *zerolog.Logger) (stop func()) {
+	interval, err := daemon.SdWatchdogEnabled(false)
+	if err != nil {
+		log.Warn().Err(err).Msg("Failed to query systemd watchdog setting")
+		return func() {}
+	}
+	if interval == 0 {
+		// No watchdog configured by the unit file.
+		return func() {}
+	}
+	// systemd recommends pinging at half the configured interval to leave
+	// headroom for scheduling jitter.
+	ping := interval / 2
+	log.Info().
+		Stringer("watchdog_interval", interval).
+		Stringer("ping_interval", ping).
+		Msg("Starting systemd watchdog pinger")
+
+	done := make(chan struct{})
+	stopped := make(chan struct{})
+	go func() {
+		defer close(stopped)
+		ticker := time.NewTicker(ping)
+		defer ticker.Stop()
+		for {
+			select {
+			case <-done:
+				return
+			case <-ticker.C:
+				if _, err := daemon.SdNotify(false, daemon.SdNotifyWatchdog); err != nil {
+					log.Warn().Err(err).Msg("Failed to send systemd WATCHDOG=1 notification")
+				}
+			}
+		}
+	}()
+	return func() {
+		close(done)
+		<-stopped
+	}
+}

bridgev2/matrix/mxmain/systemd_test.go

@@ -0,0 +1,39 @@
+// Copyright (c) 2026 Tulir Asokan
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package mxmain
+
+import (
+	"os"
+	"testing"
+
+	"github.com/rs/zerolog"
+)
+
+// TestSdNotifyHelpersAreNoOpWithoutSystemd verifies that the systemd helpers
+// are safe to call when the process is not running under a systemd unit (i.e.
+// NOTIFY_SOCKET is not set). They must not panic, must not block, and the
+// watchdog stop function must be idempotent and return promptly.
+func TestSdNotifyHelpersAreNoOpWithoutSystemd(t *testing.T) {
+	t.Setenv("NOTIFY_SOCKET", "")
+	t.Setenv("WATCHDOG_USEC", "")
+	t.Setenv("WATCHDOG_PID", "")
+	// Some test runners inherit a parent NOTIFY_SOCKET; explicitly unset it
+	// so the helpers really do see a non-systemd environment.
+	_ = os.Unsetenv("NOTIFY_SOCKET")
+	_ = os.Unsetenv("WATCHDOG_USEC")
+	_ = os.Unsetenv("WATCHDOG_PID")
+
+	log := zerolog.Nop()
+	sdNotifyReady(&log)
+	sdNotifyStopping(&log)
+
+	stop := startSdWatchdog(&log)
+	if stop == nil {
+		t.Fatal("startSdWatchdog returned a nil stop function")
+	}
+	stop()
+}