app/vmagent: fix rare hash collision in getLabelsHash (#10937)

Add '=' separator between label name and value when computing the hash
to prevent false collisions, like {a="bc"} and {ab="c"} hashing to the
same value.

getLabelsHashForShard is added to avoid sharding disruptions in vmagent
(-remoteWrite.shardByURL=true mode). The function preserves previous
behavior, without '=' between name and value.

PR https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10937

app/vmagent/remotewrite/remotewrite.go

@@ -699,7 +699,7 @@ func shardAmountRemoteWriteCtx(tssBlock []prompb.TimeSeries, shards [][]prompb.T
 			}
 			tmpLabels.Labels = hashLabels
 		}
-		h := getLabelsHash(hashLabels)
+		h := getLabelsHashForShard(hashLabels)
 
 		// Get the rwctxIdx through consistent hashing and then map it to the index in shards.
 		// The rwctxIdx is not always equal to the shardIdx, for example, when some rwctx are not available.
@@ -790,11 +790,28 @@ var (
 	dailySeriesLimitRowsDropped  = metrics.NewCounter(`vmagent_daily_series_limit_rows_dropped_total`)
 )
 
+// getLabelsHashForShard is a separate function from getLabelsHash because
+// it omits the '=' separator between label name and value for backward compatibility.
+// Changing it would re-shard all series across remoteWrite targets.
+func getLabelsHashForShard(labels []prompb.Label) uint64 {
+	bb := labelsHashBufPool.Get()
+	b := bb.B[:0]
+	for _, label := range labels {
+		b = append(b, label.Name...)
+		b = append(b, label.Value...)
+	}
+	h := xxhash.Sum64(b)
+	bb.B = b
+	labelsHashBufPool.Put(bb)
+	return h
+}
+
 func getLabelsHash(labels []prompb.Label) uint64 {
 	bb := labelsHashBufPool.Get()
 	b := bb.B[:0]
 	for _, label := range labels {
 		b = append(b, label.Name...)
+		b = append(b, '=')
 		b = append(b, label.Value...)
 	}
 	h := xxhash.Sum64(b)

app/vmagent/remotewrite/remotewrite_test.go

@@ -25,7 +25,7 @@ func TestGetLabelsHash_Distribution(t *testing.T) {
 		t.Helper()
 
 		// Distribute itemsCount hashes returned by getLabelsHash() across bucketsCount buckets.
-		itemsCount := 1_000 * bucketsCount
+		itemsCount := 10_000 * bucketsCount
 		m := make([]int, bucketsCount)
 		var labels []prompb.Label
 		for i := range itemsCount {
@@ -44,10 +44,12 @@ func TestGetLabelsHash_Distribution(t *testing.T) {
 		}
 
 		// Verify that the distribution is even
-		expectedItemsPerBucket := itemsCount / bucketsCount
+		expectedItemsPerBucket := float64(itemsCount / bucketsCount)
+		allowedDeviation := math.Round(float64(expectedItemsPerBucket) * 0.04)
 		for _, n := range m {
-			if math.Abs(1-float64(n)/float64(expectedItemsPerBucket)) > 0.04 {
-				t.Fatalf("unexpected items in the bucket for %d buckets; got %d; want around %d", bucketsCount, n, expectedItemsPerBucket)
+			if math.Abs(expectedItemsPerBucket-float64(n)) > allowedDeviation {
+				t.Fatalf("unexpected items in the bucket for %d buckets; got %d; want in range [%.0f, %.0f]",
+					bucketsCount, n, expectedItemsPerBucket-allowedDeviation, expectedItemsPerBucket+allowedDeviation)
 			}
 		}
 	}

docs/victoriametrics/changelog/CHANGELOG.md

@@ -30,6 +30,7 @@ See also [LTS releases](https://docs.victoriametrics.com/victoriametrics/lts-rel
 
 * BUGFIX: [stream aggregation](https://docs.victoriametrics.com/victoriametrics/stream-aggregation/): stop emitting stale values for `quantiles(...)` outputs when a time series has no samples during the current aggregation interval. Thanks to @alexei38 for the [pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10918).
 * BUGFIX: [stream aggregation](https://docs.victoriametrics.com/victoriametrics/stream-aggregation/): extend delay on aggregation windows flush by the biggest lag among pushed samples. Before, the delay was calculated as 95th percentile across samples, which could underrepresent outliers and reject them from aggregation as "too old". See [#10402](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/10402).
+* BUGFIX: [vmagent](https://docs.victoriametrics.com/victoriametrics/vmagent/): fix a bug in [cardinality limiters](https://docs.victoriametrics.com/victoriametrics/vmagent/#cardinality-limiter) where series with different labels, like `{a="bc"}` and `{ab="c"}`, could be incorrectly treated as identical and dropped. See [#10937](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10937).
 
 ## [v1.143.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.143.0)
 

lib/promscrape/scrapework.go

@@ -961,6 +961,7 @@ func getLabelsHash(labels []prompb.Label) uint64 {
 
 	for _, label := range labels {
 		b = append(b, label.Name...)
+		b = append(b, '=')
 		b = append(b, label.Value...)
 	}
 	h := xxhash.Sum64(b)

lib/promscrape/scrapework_test.go

@@ -700,7 +700,7 @@ func TestScrapeWorkScrapeInternalStreamConcurrency(t *testing.T) {
 		StreamParse:   true,
 		ScrapeTimeout: time.Second * 42,
 		SeriesLimit:   4000,
-	}, 3, 4015, 2, 50)
+	}, 3, 4012, 2, 50)
 }
 
 func TestScrapeWorkScrapeInternalWithMaxScrapeSize(t *testing.T) {