!str Logging error instead of failing the keepAlive operator.

stream/src/main/scala/org/apache/pekko/stream/impl/Timers.scala

@@ -238,7 +238,7 @@ import pekko.stream.stage._
     override val shape: FlowShape[I, O] = FlowShape(in, out)
 
     override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
-      new TimerGraphStageLogic(shape) with InHandler with OutHandler {
+      new TimerGraphStageLogic(shape) with StageLogging with InHandler with OutHandler {
         private var nextDeadline: Long = System.nanoTime + timeout.toNanos
         private val contextPropagation = ContextPropagation()
 
@@ -268,33 +268,29 @@ import pekko.stream.stage._
             push(out, grab(in))
             if (isClosed(in)) completeStage()
             else pull(in)
+          } else emitInjectedElementOrReschedule(onTimer = false)
+        }
+
+        private def emitInjectedElementOrReschedule(onTimer: Boolean): Unit = {
+          val now = System.nanoTime()
+          val diff = now - nextDeadline
+          if (diff < 0) {
+            if (onTimer) {
+              // Clock may be non-monotonic, see https://stackoverflow.com/questions/51344787/in-what-cases-clock-monotonic-might-not-be-available
+              log.info(
+                s"Timer should have triggered only after deadline but now is $now and deadline was $nextDeadline diff $diff. (time running backwards?) Reschedule instead of emitting.")
+            }
+            scheduleOnce(GraphStageLogicTimer, FiniteDuration(-diff, TimeUnit.NANOSECONDS))
           } else {
-            val now = System.nanoTime()
+            push(out, inject())
-            // Idle timeout triggered a while ago and we were just waiting for pull.
+            nextDeadline = now + timeout.toNanos
-            // In the case of now == deadline, the deadline has not passed strictly, but scheduling another thunk
-            // for that seems wasteful.
-            if (now - nextDeadline >= 0) {
-              nextDeadline = now + timeout.toNanos
-              push(out, inject())
-            } else
-              scheduleOnce(GraphStageLogicTimer, FiniteDuration(nextDeadline - now, TimeUnit.NANOSECONDS))
           }
         }
 
-        override protected def onTimer(timerKey: Any): Unit = {
+        override protected def onTimer(timerKey: Any): Unit = emitInjectedElementOrReschedule(onTimer = true)
-          val now = System.nanoTime()
-          // Timer is reliably cancelled if a regular element arrives first. Scheduler rather schedules too late
-          // than too early so the deadline must have passed at this time.
-          assert(
-            now - nextDeadline >= 0,
-            s"Timer should have triggered only after deadline but now is $now and deadline was $nextDeadline diff ${now - nextDeadline}.")
-          push(out, inject())
-          nextDeadline = now + timeout.toNanos
-        }
       }
 
-    override def toString = "IdleTimer"
+    override def toString = "IdleInject"
-
   }
 
   case object GraphStageLogicTimer