lib/promscrape: eliminate data race in stream parse mode

Previously `-promscrape.streamParse` mode could result in garbage labels for the scraped metrics because of data race.
See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/825#issuecomment-723198247

CHANGELOG.md

@@ -10,6 +10,8 @@
 * FEATURE: vmagent: add `/ready` HTTP endpoint, which returns 200 OK status code when all the service discovery has been initialized.
   This may be useful during rolling upgrades. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/875
 
+* BUGFIX: vmagent: eliminate data race when `-promscrape.streamParse` command-line is set. Previously this mode could result in scraped metrics with garbage labels.
+  See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/825#issuecomment-723198247 for details.
 * BUGFIX: properly calculate `topk_*` and `bottomk_*` functions from [MetricsQL](https://victoriametrics.github.io/MetricsQL.html) for time series with gaps.
   See https://github.com/VictoriaMetrics/VictoriaMetrics/pull/883
 

lib/promscrape/scrapework.go

@@ -325,18 +325,16 @@ func (sw *scrapeWork) scrapeStream(scrapeTimestamp, realTimestamp int64) error {
 		samplesScraped += len(rows)
 		for i := range rows {
 			sw.addRowToTimeseries(wc, &rows[i], scrapeTimestamp, true)
-			if len(wc.labels) > 40000 {
-				// Limit the maximum size of wc.writeRequest.
-				// This should reduce memory usage when scraping targets with millions of metrics and/or labels.
-				// For example, when scraping /federate handler from Prometheus - see https://prometheus.io/docs/prometheus/latest/federation/
-				samplesPostRelabeling += len(wc.writeRequest.Timeseries)
-				sw.updateSeriesAdded(wc)
-				startTime := time.Now()
-				sw.PushData(&wc.writeRequest)
-				pushDataDuration.UpdateDuration(startTime)
-				wc.resetNoRows()
-			}
 		}
+		// Push the collected rows to sw before returning from the callback, since they cannot be held
+		// after returning from the callback - this will result in data race.
+		// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/825#issuecomment-723198247
+		samplesPostRelabeling += len(wc.writeRequest.Timeseries)
+		sw.updateSeriesAdded(wc)
+		startTime := time.Now()
+		sw.PushData(&wc.writeRequest)
+		pushDataDuration.UpdateDuration(startTime)
+		wc.resetNoRows()
 		return nil
 	})
 	scrapedSamples.Update(float64(samplesScraped))
@@ -352,8 +350,6 @@ func (sw *scrapeWork) scrapeStream(scrapeTimestamp, realTimestamp int64) error {
 		}
 		scrapesFailed.Inc()
 	}
-	samplesPostRelabeling += len(wc.writeRequest.Timeseries)
-	sw.updateSeriesAdded(wc)
 	seriesAdded := sw.finalizeSeriesAdded(samplesPostRelabeling)
 	sw.addAutoTimeseries(wc, "up", float64(up), scrapeTimestamp)
 	sw.addAutoTimeseries(wc, "scrape_duration_seconds", duration, scrapeTimestamp)

lib/uint64set/uint64set.go

@@ -927,9 +927,13 @@ func (b *bucket16) delFromSmallPool(x uint16) bool {
 func (b *bucket16) appendTo(dst []uint64, hi uint32, hi16 uint16) []uint64 {
 	hi64 := uint64(hi)<<32 | uint64(hi16)<<16
 	if b.bits == nil {
+		// Sort a copy of b.smallPool, since b must be readonly in order to prevent from data races
+		// when b.appendTo is called from concurrent goroutines.
+		smallPool := b.smallPool
+
 		// Use uint16Sorter instead of sort.Slice here in order to reduce memory allocations.
 		a := uint16SorterPool.Get().(*uint16Sorter)
-		*a = uint16Sorter(b.smallPool[:b.smallPoolLen])
+		*a = uint16Sorter(smallPool[:b.smallPoolLen])
 		if len(*a) > 1 && !sort.IsSorted(a) {
 			sort.Sort(a)
 		}