Handle common escape characters properly. (#2995)

Fixes #2974

Signed-off-by: Goutham Veeramachaneni <cs14btech11014@iith.ac.in>

pkg/textparse/parse.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"io"
 	"sort"
+	"strings"
 	"unsafe"
 
 	"github.com/prometheus/prometheus/pkg/labels"
@@ -123,6 +124,12 @@ func (p *Parser) Metric(l *labels.Labels) string {
 		c := p.l.offsets[i+2] - p.l.mstart
 		d := p.l.offsets[i+3] - p.l.mstart
 
+		// Replacer causes allocations. Replace only when necessary.
+		if strings.IndexByte(s[c:d], byte('\\')) >= 0 {
+			*l = append(*l, labels.Label{Name: s[a:b], Value: replacer.Replace(s[c:d])})
+			continue
+		}
+
 		*l = append(*l, labels.Label{Name: s[a:b], Value: s[c:d]})
 	}
 
@@ -131,6 +138,14 @@ func (p *Parser) Metric(l *labels.Labels) string {
 	return s
 }
 
+var replacer = strings.NewReplacer(
+	`\"`, `"`,
+	`\\`, `\`,
+	`\n`, `
+`,
+	`\t`, `	`,
+)
+
 func yoloString(b []byte) string {
 	return *((*string)(unsafe.Pointer(&b)))
 }

pkg/textparse/parse_test.go

@@ -45,7 +45,8 @@ some:aggregate:rate5m{a_b="c"}	1
 # TYPE go_goroutines gauge
 go_goroutines 33  	123123
 _metric_starting_with_underscore 1
-testmetric{_label_starting_with_underscore="foo"} 1`
+testmetric{_label_starting_with_underscore="foo"} 1
+testmetric{label="\"bar\""} 1`
 	input += "\nnull_byte_metric{a=\"abc\x00\"} 1"
 
 	int64p := func(x int64) *int64 { return &x }
@@ -113,6 +114,10 @@ testmetric{_label_starting_with_underscore="foo"} 1`
 			m:    "testmetric{_label_starting_with_underscore=\"foo\"}",
 			v:    1,
 			lset: labels.FromStrings("__name__", "testmetric", "_label_starting_with_underscore", "foo"),
+		}, {
+			m:    "testmetric{label=\"\\\"bar\\\"\"}",
+			v:    1,
+			lset: labels.FromStrings("__name__", "testmetric", "label", `"bar"`),
 		}, {
 			m:    "null_byte_metric{a=\"abc\x00\"}",
 			v:    1,