app/vmselect/prometheus: properly encode Prometheus label values at /federate endpoint

Prometheus spec says that only \, \n and " must be escaped inside label values.
See 995743836e/content/docs/instrumenting/exposition_formats.md (L90)

See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5431

app/vmselect/prometheus/export.qtpl

@@ -149,11 +149,11 @@
 	{% if len(mn.Tags) > 0 %}
 	{
 		{% code tags := mn.Tags %}
-		{%z= tags[0].Key %}={%qz= tags[0].Value %}
+		{%z= tags[0].Key %}={%= escapePrometheusLabel(tags[0].Value) %}
 		{% code tags = tags[1:] %}
 		{% for i := range tags %}
 			{% code tag := &tags[i] %}
-			,{%z= tag.Key %}={%qz= tag.Value %}
+			,{%z= tag.Key %}={%= escapePrometheusLabel(tag.Value) %}
 		{% endfor %}
 	}
 	{% endif %}
@@ -173,4 +173,26 @@
 	{% endif %}
 {% endfunc %}
 
+{% func escapePrometheusLabel(b []byte) %}
+	"
+	{% for len(b) > 0 %}
+		{% code n := bytes.IndexAny(b, "\\\n\"") %}
+		{% if n < 0 %}
+			{%z= b %}
+			{% break %}
+		{% endif %}
+		{%z= b[:n] %}
+		{% switch b[n] %}
+		{% case '\\' %}
+			\\
+		{% case '\n' %}
+			\n
+		{% case '"' %}
+			\"
+		{% endswitch %}
+		{% code b = b[n+1:] %}
+	{% endfor %}
+	"
+{% endfunc %}
+
 {% endstripspace %}

app/vmselect/prometheus/export.qtpl.go

@@ -495,7 +495,7 @@ func streamprometheusMetricName(qw422016 *qt422016.Writer, mn *storage.MetricNam
 //line app/vmselect/prometheus/export.qtpl:152
 		qw422016.N().S(`=`)
 //line app/vmselect/prometheus/export.qtpl:152
-		qw422016.N().QZ(tags[0].Value)
+		streamescapePrometheusLabel(qw422016, tags[0].Value)
 //line app/vmselect/prometheus/export.qtpl:153
 		tags = tags[1:]
 
@@ -511,7 +511,7 @@ func streamprometheusMetricName(qw422016 *qt422016.Writer, mn *storage.MetricNam
 //line app/vmselect/prometheus/export.qtpl:156
 			qw422016.N().S(`=`)
 //line app/vmselect/prometheus/export.qtpl:156
-			qw422016.N().QZ(tag.Value)
+			streamescapePrometheusLabel(qw422016, tag.Value)
 //line app/vmselect/prometheus/export.qtpl:157
 		}
 //line app/vmselect/prometheus/export.qtpl:157
@@ -599,3 +599,74 @@ func convertValueToSpecialJSON(v float64) string {
 	return qs422016
 //line app/vmselect/prometheus/export.qtpl:174
 }
+
+//line app/vmselect/prometheus/export.qtpl:176
+func streamescapePrometheusLabel(qw422016 *qt422016.Writer, b []byte) {
+//line app/vmselect/prometheus/export.qtpl:176
+	qw422016.N().S(`"`)
+//line app/vmselect/prometheus/export.qtpl:178
+	for len(b) > 0 {
+//line app/vmselect/prometheus/export.qtpl:179
+		n := bytes.IndexAny(b, "\\\n\"")
+
+//line app/vmselect/prometheus/export.qtpl:180
+		if n < 0 {
+//line app/vmselect/prometheus/export.qtpl:181
+			qw422016.N().Z(b)
+//line app/vmselect/prometheus/export.qtpl:182
+			break
+//line app/vmselect/prometheus/export.qtpl:183
+		}
+//line app/vmselect/prometheus/export.qtpl:184
+		qw422016.N().Z(b[:n])
+//line app/vmselect/prometheus/export.qtpl:185
+		switch b[n] {
+//line app/vmselect/prometheus/export.qtpl:186
+		case '\\':
+//line app/vmselect/prometheus/export.qtpl:186
+			qw422016.N().S(`\\`)
+//line app/vmselect/prometheus/export.qtpl:188
+		case '\n':
+//line app/vmselect/prometheus/export.qtpl:188
+			qw422016.N().S(`\n`)
+//line app/vmselect/prometheus/export.qtpl:190
+		case '"':
+//line app/vmselect/prometheus/export.qtpl:190
+			qw422016.N().S(`\"`)
+//line app/vmselect/prometheus/export.qtpl:192
+		}
+//line app/vmselect/prometheus/export.qtpl:193
+		b = b[n+1:]
+
+//line app/vmselect/prometheus/export.qtpl:194
+	}
+//line app/vmselect/prometheus/export.qtpl:194
+	qw422016.N().S(`"`)
+//line app/vmselect/prometheus/export.qtpl:196
+}
+
+//line app/vmselect/prometheus/export.qtpl:196
+func writeescapePrometheusLabel(qq422016 qtio422016.Writer, b []byte) {
+//line app/vmselect/prometheus/export.qtpl:196
+	qw422016 := qt422016.AcquireWriter(qq422016)
+//line app/vmselect/prometheus/export.qtpl:196
+	streamescapePrometheusLabel(qw422016, b)
+//line app/vmselect/prometheus/export.qtpl:196
+	qt422016.ReleaseWriter(qw422016)
+//line app/vmselect/prometheus/export.qtpl:196
+}
+
+//line app/vmselect/prometheus/export.qtpl:196
+func escapePrometheusLabel(b []byte) string {
+//line app/vmselect/prometheus/export.qtpl:196
+	qb422016 := qt422016.AcquireByteBuffer()
+//line app/vmselect/prometheus/export.qtpl:196
+	writeescapePrometheusLabel(qb422016, b)
+//line app/vmselect/prometheus/export.qtpl:196
+	qs422016 := string(qb422016.B)
+//line app/vmselect/prometheus/export.qtpl:196
+	qt422016.ReleaseByteBuffer(qb422016)
+//line app/vmselect/prometheus/export.qtpl:196
+	return qs422016
+//line app/vmselect/prometheus/export.qtpl:196
+}

app/vmselect/prometheus/federate_test.go

@@ -0,0 +1,43 @@
+package prometheus
+
+import (
+	"testing"
+
+	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmselect/netstorage"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/storage"
+)
+
+func TestFederate(t *testing.T) {
+	f := func(rs *netstorage.Result, expectedResult string) {
+		t.Helper()
+		result := Federate(rs)
+		if result != expectedResult {
+			t.Fatalf("unexpected result; got\n%s\nwant\n%s", result, expectedResult)
+		}
+	}
+
+	f(&netstorage.Result{}, ``)
+
+	f(&netstorage.Result{
+		MetricName: storage.MetricName{
+			MetricGroup: []byte("foo"),
+			Tags: []storage.Tag{
+				{
+					Key:   []byte("a"),
+					Value: []byte("b"),
+				},
+				{
+					Key:   []byte("qqq"),
+					Value: []byte("\\"),
+				},
+				{
+					Key: []byte("abc"),
+					// Verify that < isn't encoded. See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5431
+					Value: []byte("a<b\"\\c"),
+				},
+			},
+		},
+		Values:     []float64{1.23},
+		Timestamps: []int64{123},
+	}, `foo{a="b",qqq="\\",abc="a<b\"\\c"} 1.23 123`+"\n")
+}

app/vmselect/prometheus/federate_timing_test.go

@@ -0,0 +1,38 @@
+package prometheus
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/VictoriaMetrics/VictoriaMetrics/app/vmselect/netstorage"
+	"github.com/VictoriaMetrics/VictoriaMetrics/lib/storage"
+)
+
+func BenchmarkFederate(b *testing.B) {
+	rs := &netstorage.Result{
+		MetricName: storage.MetricName{
+			MetricGroup: []byte("foo_bar_bazaaaa_total"),
+			Tags: []storage.Tag{
+				{
+					Key:   []byte("instance"),
+					Value: []byte("foobarbaz:2344"),
+				},
+				{
+					Key:   []byte("job"),
+					Value: []byte("aaabbbccc"),
+				},
+			},
+		},
+		Values:     []float64{112.23},
+		Timestamps: []int64{1234567890},
+	}
+
+	b.ReportAllocs()
+	b.RunParallel(func(pb *testing.PB) {
+		var bb bytes.Buffer
+		for pb.Next() {
+			bb.Reset()
+			WriteFederate(&bb, rs)
+		}
+	})
+}

docs/CHANGELOG.md

@@ -18,6 +18,8 @@ The following tip changes can be tested by building VictoriaMetrics components f
 * SECURITY: upgrade base docker image (Alpine) from 3.18.4 to 3.18.5. See [alpine 3.18.5 release notes](https://www.alpinelinux.org/posts/Alpine-3.15.11-3.16.8-3.17.6-3.18.5-released.html).
 * SECURITY: upgrade Go builder from Go1.21.4 to Go1.21.5. See [the list of issues addressed in Go1.21.5](https://github.com/golang/go/issues?q=milestone%3AGo1.21.5+label%3ACherryPickApproved).
 
+* BUGFIX: properly escape `<` character in responses returned via [`/federate`](https://docs.victoriametrics.com/#federation) endpoint. See [this issue](https://github.com/VictoriaMetrics/VictoriaMetrics/issues/5431).
+
 ## [v1.87.11](https://github.com/VictoriaMetrics/VictoriaMetrics/releases/tag/v1.87.11)
 
 Released at 2023-11-14