pkg/timecache: implement a time cache

pkg/timecache/timecache.go

@@ -0,0 +1,127 @@
+// Package timecache provides a cache for the system clock, to avoid calls to
+// time.Now().
+// The time is stored as one int64 which holds the number of nanoseconds since
+// the Unix Epoch. The value is accessed using atomic primitives, without
+// locking.
+// The package runs a global singleton TimeCache that is is updated every
+// second.
+package timecache
+
+import (
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// t is the global TimeCache.
+var t *TimeCache
+
+func init() {
+	t = &TimeCache{
+		clock:   time.Now().UnixNano(),
+		closed:  make(chan struct{}),
+		running: make(chan struct{}),
+	}
+
+	go t.Run(1 * time.Second)
+}
+
+// A TimeCache is a cache for the current system time.
+// The cached time has nanosecond precision.
+type TimeCache struct {
+	// clock saves the current time's nanoseconds since the Epoch.
+	// Must be accessed atomically.
+	clock int64
+
+	closed  chan struct{}
+	running chan struct{}
+	m       sync.Mutex
+}
+
+// New returns a new TimeCache instance.
+// The TimeCache must be started to update the time.
+func New() *TimeCache {
+	return &TimeCache{
+		clock:   time.Now().UnixNano(),
+		closed:  make(chan struct{}),
+		running: make(chan struct{}),
+	}
+}
+
+// Run runs the TimeCache, updating the cached clock value once every interval
+// and blocks until Stop is called.
+func (t *TimeCache) Run(interval time.Duration) {
+	t.m.Lock()
+	select {
+	case <-t.running:
+		panic("Run called multiple times")
+	default:
+	}
+	close(t.running)
+	t.m.Unlock()
+
+	tick := time.NewTicker(interval)
+	defer tick.Stop()
+	for {
+		select {
+		case <-t.closed:
+			tick.Stop()
+			return
+		case now := <-tick.C:
+			atomic.StoreInt64(&t.clock, now.UnixNano())
+		}
+	}
+}
+
+// Stop stops the TimeCache.
+// The cached time remains valid but will not be updated anymore.
+// A TimeCache can not be restarted. Construct a new one instead.
+// Calling Stop again is a no-op.
+func (t *TimeCache) Stop() {
+	t.m.Lock()
+	defer t.m.Unlock()
+
+	select {
+	case <-t.closed:
+		return
+	default:
+	}
+	close(t.closed)
+}
+
+// Now returns the cached time as a time.Time value.
+func (t *TimeCache) Now() time.Time {
+	return time.Unix(0, atomic.LoadInt64(&t.clock))
+}
+
+// NowUnixNano returns the cached time as nanoseconds since the Unix Epoch.
+func (t *TimeCache) NowUnixNano() int64 {
+	return atomic.LoadInt64(&t.clock)
+}
+
+// NowUnix returns the cached time as seconds since the Unix Epoch.
+func (t *TimeCache) NowUnix() int64 {
+	// Adopted from time.Unix
+	nsec := atomic.LoadInt64(&t.clock)
+	sec := nsec / 1e9
+	nsec -= sec * 1e9
+	if nsec < 0 {
+		sec--
+	}
+	return sec
+}
+
+// Now calls Now on the global TimeCache instance.
+func Now() time.Time {
+	return t.Now()
+}
+
+// NowUnixNano calls NowUnixNano on the global TimeCache instance.
+func NowUnixNano() int64 {
+	return t.NowUnixNano()
+}
+
+// NowUnix calls NowUnix on the global TimeCache instance.
+func NowUnix() int64 {
+	return t.NowUnix()
+}