home: add service ready notification support on Linux

internal/home/ready_linux.go

@@ -0,0 +1,49 @@
+//go:build linux
+
+package home
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"time"
+)
+
+// Notifies the service manager that the program is ready to serve
+func notifyReady() error {
+	return sdNotify("READY=1")
+}
+
+// Notifies the service manager that the program is beginning to reload its
+// configuration
+func notifyReload() error {
+	now := time.Now().UnixMicro()
+	return sdNotify(fmt.Sprintf("RELOADING=1\nMONOTONIC_USEC=%v", now))
+}
+
+// Implements the sd_notify mechanism
+//
+// Reference: https://www.freedesktop.org/software/systemd/man/latest/sd_notify.html
+func sdNotify(message string) error {
+	socketPath := os.Getenv("NOTIFY_SOCKET")
+	if socketPath == "" {
+		return nil
+	}
+	socketAddr := net.UnixAddr{
+		Name: socketPath,
+		Net:  "unixgram",
+	}
+
+	conn, err := net.DialUnix("unixgram", nil, &socketAddr)
+	if err != nil {
+		return fmt.Errorf("connecting to %q: %w", socketAddr.String(), err)
+	}
+	defer conn.Close()
+
+	_, err = conn.Write([]byte(message))
+	if err != nil {
+		return fmt.Errorf("sending %q: %w", message, err)
+	}
+
+	return nil
+}

internal/home/ready_others.go

@@ -0,0 +1,14 @@
+//go:build !linux
+
+package home
+
+// Notifies the service manager that the program is ready to serve
+func notifyReady() error {
+	return nil
+}
+
+// Notifies the service manager that the program is beginning to reload its
+// configuration
+func notifyReload() error {
+	return nil
+}

internal/home/service.go

@@ -466,13 +466,17 @@ var launchdConfig = `<?xml version='1.0' encoding='UTF-8'?>
 //  2. The StandardOutput and StandardError settings are set to redirect the
 //     output to the systemd journal, see
 //     https://man7.org/linux/man-pages/man5/systemd.exec.5.html#LOGGING_AND_STANDARD_INPUT/OUTPUT.
+//
+//  3. The Type setting has been configured to enable service readiness
+//     notification support.
 const systemdScript = `[Unit]
 Description={{.Description}}
 ConditionFileIsExecutable={{.Path|cmdEscape}}
 {{range $i, $dep := .Dependencies}}
 {{$dep}} {{end}}
 
 [Service]
+Type=notify
 StartLimitInterval=5
 StartLimitBurst=10
 ExecStart={{.Path|cmdEscape}}{{range .Arguments}} {{.|cmd}}{{end}}

internal/home/web.go

@@ -235,14 +235,26 @@ func (web *webAPI) start(ctx context.Context) {
 			errs <- web.httpServer.ListenAndServe()
 		}()
 
-		err := <-errs
+		// Tell the service manager that we are ready to serve requests
+		err := notifyReady()
+		if err != nil {
+			logger.ErrorContext(ctx, "sending service ready notification: %v", slogutil.KeyError, err)
+		}
+
+		err = <-errs
 		if !errors.Is(err, http.ErrServerClosed) {
 			cleanupAlways()
 			panic(err)
 		}
 
 		// We use ErrServerClosed as a sign that we need to rebind on a new
 		// address, so go back to the start of the loop.
+		//
+		// Let the service manager know that we are reloading
+		err = notifyReload()
+		if err != nil {
+			logger.ErrorContext(ctx, "sending service reload notification: %v", slogutil.KeyError, err)
+		}
 	}
 }