wakey wakey

wake_detector.go

@@ -0,0 +1,47 @@
+package nebula
+
+import "time"
+
+// wakeDetector notices when the machine has returned from system sleep and measures how long it was suspended.
+//
+// It samples the spread between two kernel clocks: one that pauses across a suspend and one that keeps counting
+// (suspendClockDelta, per platform). While the machine is awake the spread is constant no matter how starved,
+// stopped, or stepped this process is — SIGSTOP, debugger pauses, scheduler starvation, and NTP adjustments move
+// both clocks together or neither, so none of them can fake a wake. A true suspend is the only thing that grows
+// the spread, and it grows by exactly the time spent suspended.
+//
+// Sample is intended to piggyback on a ticker the caller already runs; it costs two clock reads. It is not safe
+// for concurrent use.
+type wakeDetector struct {
+	// read returns the current spread between the two clocks, false if this platform can't provide one.
+	read   func() (time.Duration, bool)
+	last   time.Duration
+	primed bool
+}
+
+func newWakeDetector() *wakeDetector {
+	return &wakeDetector{read: suspendClockDelta}
+}
+
+// Sample returns how long the machine was suspended since the previous call, 0 if it wasn't, and false if the
+// platform has no way to tell. The first call primes the baseline and always reports 0.
+func (w *wakeDetector) Sample() (time.Duration, bool) {
+	delta, ok := w.read()
+	if !ok {
+		return 0, false
+	}
+
+	if !w.primed {
+		w.primed = true
+		w.last = delta
+		return 0, true
+	}
+
+	slept := delta - w.last
+	w.last = delta
+	if slept < 0 {
+		// The clock pair is read non-atomically so tiny negative jitter is possible; it is never a wake.
+		slept = 0
+	}
+	return slept, true
+}