lib/promscrape: follow-up for a7e29c38bc

- Document the bugfix at docs/CHANGELOG.md
- Make the fix more durable against future changes when droppedTargetsMap.Register may be called from other places.

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3580

docs/CHANGELOG.md

@@ -27,6 +27,7 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): properly update tooltip when quickly hovering multiple lines on the graph. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3530).
 * BUGFIX: properly parse floating-point numbers without integer or fractional parts such as `.123` and `20.` during [data import](https://docs.victoriametrics.com/#how-to-import-time-series-data). See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3544).
 * BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly parse durations with uppercase suffixes such as `10S`, `5MS`, `1W`, etc. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3589).
+* BUGFIX: [vmagent](https://docs.victoriametrics.com/vmagent.html): fix a panic during target discovery when `vmagent` runs with `-promscrape.dropOriginalLabels` command-line flag. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/3580). The bug has been introduced in [v1.85.0](https://docs.victoriametrics.com/CHANGELOG.html#v1850).
 
 
 ## [v1.85.3](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.85.3)

lib/promscrape/config.go

@@ -1186,9 +1186,7 @@ func (swc *scrapeWorkConfig) getScrapeWork(target string, extraLabels, metaLabel
 	}
 	if labels.Len() == 0 {
 		// Drop target without labels.
-		if !*dropOriginalLabels {
+		droppedTargetsMap.Register(originalLabels, swc.relabelConfigs)
-			droppedTargetsMap.Register(originalLabels, swc.relabelConfigs)
-		}
 		return nil, nil
 	}
 	// See https://www.robustperception.io/life-of-a-label
@@ -1203,9 +1201,7 @@ func (swc *scrapeWorkConfig) getScrapeWork(target string, extraLabels, metaLabel
 	address := labels.Get("__address__")
 	if len(address) == 0 {
 		// Drop target without scrape address.
-		if !*dropOriginalLabels {
+		droppedTargetsMap.Register(originalLabels, swc.relabelConfigs)
-			droppedTargetsMap.Register(originalLabels, swc.relabelConfigs)
-		}
 		return nil, nil
 	}
 	// Usability extension to Prometheus behavior: extract optional scheme and metricsPath from __address__.

lib/promscrape/targetstatus.go

@@ -276,6 +276,10 @@ func (dt *droppedTargets) getTargetsList() []droppedTarget {
 }
 
 func (dt *droppedTargets) Register(originalLabels *promutils.Labels, relabelConfigs *promrelabel.ParsedConfigs) {
+	if *dropOriginalLabels {
+		// The originalLabels must be dropped, so do not register it.
+		return
+	}
 	// It is better to have hash collisions instead of spending additional CPU on originalLabels.String() call.
 	key := labelsHash(originalLabels)
 	currentTime := fasttime.UnixTimestamp()
@@ -301,7 +305,7 @@ func (dt *droppedTargets) Register(originalLabels *promutils.Labels, relabelConf
 
 func labelsHash(labels *promutils.Labels) uint64 {
 	d := xxhashPool.Get().(*xxhash.Digest)
-	for _, label := range labels.Labels {
+	for _, label := range labels.GetLabels() {
 		_, _ = d.WriteString(label.Name)
 		_, _ = d.WriteString(label.Value)
 	}