metricsql: fix label_join() when dst_label is equal to one of the `… (#5886)

* metricsql: fix label_join() when `dst_label` is equal to one of the `src_label`

* Update app/vmselect/promql/transform.go

* Update docs/CHANGELOG.md

---------

Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>

app/vmselect/promql/exec_test.go

@@ -2317,6 +2317,21 @@ func TestExecSuccess(t *testing.T) {
 		resultExpected := []netstorage.Result{r}
 		f(q, resultExpected)
 	})
+	t.Run(`label_join dst_label is equal to src_label`, func(t *testing.T) {
+		t.Parallel()
+		q := `label_join(label_join(time(), "bar", "sep1", "a", "b"), "bar", "sep2", "a", "bar")`
+		r := netstorage.Result{
+			MetricName: metricNameExpected,
+			Values:     []float64{1000, 1200, 1400, 1600, 1800, 2000},
+			Timestamps: timestampsExpected,
+		}
+		r.MetricName.Tags = []storage.Tag{{
+			Key:   []byte("bar"),
+			Value: []byte("sep2sep1"),
+		}}
+		resultExpected := []netstorage.Result{r}
+		f(q, resultExpected)
+	})
 	t.Run(`label_value()`, func(t *testing.T) {
 		t.Parallel()
 		q := `with (

app/vmselect/promql/transform.go

@@ -1940,8 +1940,7 @@ func transformLabelJoin(tfa *transformFuncArg) ([]*timeseries, error) {
 	for _, ts := range rvs {
 		mn := &ts.MetricName
 		dstValue := getDstValue(mn, dstLabel)
-		b := *dstValue
+		var b []byte
-		b = b[:0]
 		for j, srcLabel := range srcLabels {
 			srcValue := mn.GetTagValue(srcLabel)
 			b = append(b, srcValue...)

docs/CHANGELOG.md

@@ -54,7 +54,7 @@ See also [LTS releases](https://docs.victoriametrics.com/LTS-releases.html).
 * BUGFIX: [vmalert](https://docs.victoriametrics.com/#vmalert): consistently sort groups by name and filename on `/groups` page in UI. This should prevent non-deterministic sorting for groups with identical names.
 * BUGFIX: [vmui](https://docs.victoriametrics.com/#vmui): fix Popper display issues. See [this pull request](https://github.com/VictoriaMetrics/VictoriaMetrics/pull/5842).
 * BUGFIX: [vmselect](https://docs.victoriametrics.com/): format time with milliseconds precision when `__timestamp__:rfc3339` is specified for [/api/v1/export/csv](https://docs.victoriametrics.com/#how-to-export-csv-data) API response. See this [issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5837) for details.
-
+* BUGFIX: [MetricsQL](https://docs.victoriametrics.com/MetricsQL.html): properly construct the destination label in [label_join](https://docs.victoriametrics.com/metricsql/#label_join) if it is used as source label. See this [issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5881) for details.
 
 ## [v1.98.0](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.98.0)