[#1135] sdnotify: Send MONOTONIC_USEC on reload

Fixes #1135

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

pkg/util/sdnotify/clock.go

@@ -0,0 +1,10 @@
+package sdnotify
+
+import (
+	// For go:linkname to work.
+	_ "unsafe"
+)
+
+//go:noescape
+//go:linkname nanotime runtime.nanotime
+func nanotime() int64

pkg/util/sdnotify/clock.s

@@ -0,0 +1,2 @@
+// The file is intentionally empty.
+// It is a workaround for https://github.com/golang/go/issues/15006

pkg/util/sdnotify/sdnotify.go

@@ -38,6 +38,21 @@ func InitSocket() error {
 // FlagAndStatus sends systemd a combination of a
 // well-known status and STATUS=%s{status}, separated by newline.
 func FlagAndStatus(status string) error {
+	if status == ReloadingEnabled {
+		// From https://www.man7.org/linux/man-pages/man5/systemd.service.5.html
+		//
+		//    When initiating the reload process the service is
+		//    expected to reply with a notification message via
+		//    sd_notify(3) that contains the "RELOADING=1" field in
+		//    combination with "MONOTONIC_USEC=" set to the current
+		//    monotonic time (i.e.  CLOCK_MONOTONIC in
+		//    clock_gettime(2)) in μs, formatted as decimal string.
+		//    Once reloading is complete another notification message
+		//    must be sent, containing "READY=1".
+		//
+		// For MONOTONIC_USEC format refer to https://www.man7.org/linux/man-pages/man3/sd_notify.3.html
+		status += fmt.Sprintf("\nMONOTONIC_USEC=%d", uint64(nanotime())/1e3 /* microseconds in nanoseconds */)
+	}
 	status += "\nSTATUS=" + strings.TrimSuffix(status, "=1")
 	return Send(status)
 }