[#1] mclock: Fix possible deadlock

There is a possible call-chain `scheduleRequest()` -> `runAt()` -> `scheduleRequest()`,
so second `scheduleRequest()` may be locked on mutext held by first `scheduleRequest()`.

Signed-off-by: Dmitrii Stepanov <d.stepanov@yadro.com>

scheduling/clock.go

@@ -36,7 +36,10 @@ func (c *systemClock) now() float64 {
 }
 
 func (c *systemClock) runAt(ts float64, f func()) {
-	c.schedule <- scheduleInfo{ts: ts, f: f}
+	select {
+	case c.schedule <- scheduleInfo{ts: ts, f: f}:
+	default: // timer fired, scheduleRequest will call runAt again
+	}
 }
 
 func (c *systemClock) close() {
@@ -48,7 +51,8 @@ func (c *systemClock) start() {
 	c.wg.Add(1)
 	go func() {
 		defer c.wg.Done()
-		t := time.NewTimer(time.Hour)
+		t := time.NewTimer(0)
+		<-t.C
 		var f func()
 		for {
 			select {
@@ -61,11 +65,10 @@ func (c *systemClock) start() {
 				if !ok {
 					return
 				}
+				var d time.Duration
 				now := c.now()
-				if now >= s.ts {
+				if now < s.ts {
-					s.f()
+					d = time.Duration((s.ts - now) * 1e9)
-					f = nil
-					continue
 				}
 				if !t.Stop() {
 					select {
@@ -73,7 +76,7 @@ func (c *systemClock) start() {
 					default:
 					}
 				}
-				t.Reset(time.Duration((s.ts - now) * 1e9))
+				t.Reset(d)
 				f = s.f
 			}
 		}