lib/protoparser/prometheus: optimize GetRowsDiff() function

This should help https://github.com/VictoriaMetrics/VictoriaMetrics/issues/1745 ,
since the provided profile shows that the majority of CPU and memory is spent in this function
during `streamParse` when `-promscrape.noStaleMarkers` wasn't set.

lib/protoparser/prometheus/parser.go

@@ -1,9 +1,11 @@
 package prometheus
 
 import (
+	"bytes"
 	"fmt"
-	"strconv"
+	"sort"
 	"strings"
+	"sync"
 
 	"github.com/VictoriaMetrics/VictoriaMetrics/lib/logger"
 	"github.com/VictoriaMetrics/metrics"
@@ -357,6 +359,29 @@ func unescapeValue(s string) string {
 	return string(b)
 }
 
+func appendEscapedValue(dst []byte, s string) []byte {
+	// label_value can be any sequence of UTF-8 characters, but the backslash (\), double-quote ("),
+	// and line feed (\n) characters have to be escaped as \\, \", and \n, respectively.
+	// See https://github.com/prometheus/docs/blob/master/content/docs/instrumenting/exposition_formats.md
+	for {
+		n := strings.IndexAny(s, "\\\"\n")
+		if n < 0 {
+			return append(dst, s...)
+		}
+		dst = append(dst, s[:n]...)
+		switch s[n] {
+		case '\\':
+			dst = append(dst, "\\\\"...)
+		case '"':
+			dst = append(dst, "\\\""...)
+		case '\n':
+			dst = append(dst, "\\n"...)
+		}
+		s = s[n+1:]
+	}
+
+}
+
 func prevBackslashesCount(s string) int {
 	n := 0
 	for len(s) > 0 && s[len(s)-1] == '\\' {
@@ -370,46 +395,125 @@ func prevBackslashesCount(s string) int {
 //
 // The returned rows have default value 0 and have no timestamps.
 func GetRowsDiff(s1, s2 string) string {
-	var r1, r2 Rows
-	r1.Unmarshal(s1)
-	r2.Unmarshal(s2)
-	rows1 := r1.Rows
-	rows2 := r2.Rows
-	m := make(map[string]bool, len(rows2))
-	for i := range rows2 {
-		r := &rows2[i]
-		key := marshalMetricNameWithTags(r)
-		m[key] = true
+	li1 := getLinesIterator()
+	li2 := getLinesIterator()
+	defer func() {
+		putLinesIterator(li1)
+		putLinesIterator(li2)
+	}()
+	li1.Init(s1)
+	li2.Init(s2)
+	if !li1.NextKey() {
+		return ""
 	}
 	var diff []byte
-	for i := range rows1 {
-		r := &rows1[i]
-		key := marshalMetricNameWithTags(r)
-		if !m[key] {
-			diff = append(diff, key...)
-			diff = append(diff, " 0\n"...)
+	if !li2.NextKey() {
+		diff = appendKeys(diff, li1)
+		return string(diff)
+	}
+	for {
+		switch bytes.Compare(li1.Key, li2.Key) {
+		case -1:
+			diff = appendKey(diff, li1.Key)
+			if !li1.NextKey() {
+				return string(diff)
+			}
+		case 0:
+			if !li1.NextKey() {
+				return string(diff)
+			}
+			if !li2.NextKey() {
+				diff = appendKeys(diff, li1)
+				return string(diff)
+			}
+		case 1:
+			if !li2.NextKey() {
+				diff = appendKeys(diff, li1)
+				return string(diff)
+			}
 		}
 	}
-	return string(diff)
 }
 
-func marshalMetricNameWithTags(r *Row) string {
-	if len(r.Tags) == 0 {
-		return r.Metric
+type linesIterator struct {
+	rows     []Row
+	a        []string
+	tagsPool []Tag
+
+	// Key contains the next key after NextKey call
+	Key []byte
+}
+
+var linesIteratorPool sync.Pool
+
+func getLinesIterator() *linesIterator {
+	v := linesIteratorPool.Get()
+	if v == nil {
+		return &linesIterator{}
 	}
-	var b []byte
-	b = append(b, r.Metric...)
-	b = append(b, '{')
-	for i, t := range r.Tags {
-		b = append(b, t.Key...)
-		b = append(b, '=')
-		b = strconv.AppendQuote(b, t.Value)
-		if i+1 < len(r.Tags) {
-			b = append(b, ',')
+	return v.(*linesIterator)
+}
+
+func putLinesIterator(li *linesIterator) {
+	li.a = nil
+	linesIteratorPool.Put(li)
+}
+
+func (li *linesIterator) Init(s string) {
+	a := strings.Split(s, "\n")
+	sort.Strings(a)
+	li.a = a
+}
+
+// NextKey advances to the next key in li.
+//
+// It returns true if the next key is found and Key is successcully updated.
+func (li *linesIterator) NextKey() bool {
+	for {
+		if len(li.a) == 0 {
+			return false
+		}
+		li.rows, li.tagsPool = unmarshalRow(li.rows[:0], li.a[0], li.tagsPool[:0], false, stdErrLogger)
+		li.a = li.a[1:]
+		if len(li.rows) > 0 {
+			li.Key = marshalMetricNameWithTags(li.Key[:0], &li.rows[0])
+			return true
 		}
 	}
-	b = append(b, '}')
-	return string(b)
+}
+
+func appendKey(dst, key []byte) []byte {
+	dst = append(dst, key...)
+	dst = append(dst, " 0\n"...)
+	return dst
+}
+
+func appendKeys(dst []byte, li *linesIterator) []byte {
+	for {
+		dst = appendKey(dst, li.Key)
+		if !li.NextKey() {
+			return dst
+		}
+	}
+}
+
+func marshalMetricNameWithTags(dst []byte, r *Row) []byte {
+	dst = append(dst, r.Metric...)
+	if len(r.Tags) == 0 {
+		return dst
+	}
+	dst = append(dst, '{')
+	for i, t := range r.Tags {
+		dst = append(dst, t.Key...)
+		dst = append(dst, `="`...)
+		dst = appendEscapedValue(dst, t.Value)
+		dst = append(dst, '"')
+		if i+1 < len(r.Tags) {
+			dst = append(dst, ',')
+		}
+	}
+	dst = append(dst, '}')
+	return dst
 }
 
 // AreIdenticalSeriesFast returns true if s1 and s2 contains identical Prometheus series with possible different values.

lib/protoparser/prometheus/parser_test.go

@@ -164,6 +164,20 @@ func TestUnescapeValue(t *testing.T) {
 	f(`foo\`, "foo\\")
 }
 
+func TestAppendEscapedValue(t *testing.T) {
+	f := func(s, resultExpected string) {
+		t.Helper()
+		result := appendEscapedValue(nil, s)
+		if string(result) != resultExpected {
+			t.Fatalf("unexpected result; got %q; want %q", result, resultExpected)
+		}
+	}
+	f(``, ``)
+	f(`f`, `f`)
+	f(`foobar`, `foobar`)
+	f("\"\n\t\\xyz", "\\\"\\n\t\\\\xyz")
+}
+
 func TestRowsUnmarshalFailure(t *testing.T) {
 	f := func(s string) {
 		t.Helper()