lib/streamaggr: prevent rate_sum and rate_avg from producing NaNs (#6482)

### Describe Your Changes

* check if `lastValue` was seen at least twice with different
timestamps. Otherwise, the difference between last timestamp and
previous timestamp could be `0` and will result into `NaN` calculation
* check if there items left in lastValue map after staleness cleanup.
Otherwise, `rate_avg` could have produce `NaN` result.

### Checklist

The following checks are **mandatory**:

- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>

docs/CHANGELOG.md

@@ -40,6 +40,7 @@ See also [LTS releases](https://docs.victoriametrics.com/lts-releases/).
 
 * BUGFIX: all VictoriaMetrics components: prioritize `-configAuthKey` and `-reloadAuthKey` over `-httpAuth.*` settings. This change aligns behavior of mentioned flags with other auth flags like `-metricsAuthKey`, `-flagsAuthKey`, `-pprofAuthKey`. Check [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6329).
 * BUGFIX: [vmctl](https://docs.victoriametrics.com/vmctl/): add `--disable-progress-bar` global command-line flag. It can be used for disabling dynamic progress bar for all migration modes. `--vm-disable-progress-bar`  command-line flag is deprecated and will be removed in the future releases. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6367).
+* BUGFIX: [stream aggregation](https://docs.victoriametrics.com/stream-aggregation/): prevent [rate_sum](https://docs.victoriametrics.com/stream-aggregation/#rate_sum) and [rate_avg](https://docs.victoriametrics.com/stream-aggregation/#rate_avg) producing `NaN` results for stale time series. Before, when series matched for aggregation became stale or weren't updated during aggregation interval, the `rate_sum` or `rate_avg` could produce data point with `NaN` value. During visualization, such aggregation results would be displayed as gaps in time series.
 
 ## [v1.102.0-rc1](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.102.0-rc1)
 

lib/streamaggr/rate.go

@@ -105,8 +105,7 @@ func (as *rateAggrState) pushSamples(samples []pushSample) {
 	}
 }
 
-func (as *rateAggrState) flushState(ctx *flushCtx, resetState bool) {
-	_ = resetState // it isn't used here
+func (as *rateAggrState) flushState(ctx *flushCtx, _ bool) {
 	currentTime := fasttime.UnixTimestamp()
 	currentTimeMsec := int64(currentTime) * 1000
 
@@ -127,23 +126,29 @@ func (as *rateAggrState) flushState(ctx *flushCtx, resetState bool) {
 
 		// Delete outdated entries in sv.lastValues
 		var rate float64
-		m := sv.lastValues
-		for k1, v1 := range m {
+		lvs := sv.lastValues
+		for k1, v1 := range lvs {
 			if currentTime > v1.deleteDeadline {
-				delete(m, k1)
-			} else if v1.prevTimestamp > 0 {
-				rate += v1.total * 1000 / float64(v1.timestamp-v1.prevTimestamp)
+				delete(lvs, k1)
+				continue
+			}
+			rateInterval := v1.timestamp - v1.prevTimestamp
+			if v1.prevTimestamp > 0 && rateInterval > 0 {
+				// calculate rate only if value was seen at least twice with different timestamps
+				rate += v1.total * 1000 / float64(rateInterval)
 				v1.prevTimestamp = v1.timestamp
 				v1.total = 0
-				m[k1] = v1
+				lvs[k1] = v1
 			}
 		}
-		if as.suffix == "rate_avg" {
-			// note: capture m length after deleted items were removed
-			rate /= float64(len(m))
-		}
+		// capture m length after deleted items were removed
+		totalItems := len(lvs)
 		sv.mu.Unlock()
 
+		if as.suffix == "rate_avg" && totalItems > 0 {
+			rate /= float64(totalItems)
+		}
+
 		key := k.(string)
 		ctx.appendSeries(key, as.suffix, currentTimeMsec, rate)
 		return true

lib/streamaggr/streamaggr_test.go

@@ -852,6 +852,18 @@ foo{abc="456", cde="1"} 10 10
 foo:1m_by_cde_rate_sum{cde="1"} 0.65
 `, "1111")
 
+	// rate with duplicated events
+	f(`     
+- interval: 1m
+  by: [cde]
+  outputs: [rate_sum, rate_avg]
+`, `
+foo{abc="123", cde="1"} 4  10
+foo{abc="123", cde="1"} 4  10
+`, `foo:1m_by_cde_rate_avg{cde="1"} 0
+foo:1m_by_cde_rate_sum{cde="1"} 0
+`, "11")
+
 	// keep_metric_names
 	f(`
 - interval: 1m