Fix escaping for `Matcher.String()`

Now the string created will correctly parse back.

Signed-off-by: beorn7 <beorn@grafana.com>

pkg/labels/matcher.go

@@ -16,6 +16,7 @@ package labels
 import (
 	"fmt"
 	"regexp"
+	"strings"
 )
 
 // MatchType is an enum for label matching types.
@@ -69,7 +70,7 @@ func NewMatcher(t MatchType, n, v string) (*Matcher, error) {
 }
 
 func (m *Matcher) String() string {
-	return fmt.Sprintf("%s%s%q", m.Name, m.Type, m.Value)
+	return fmt.Sprintf(`%s%s"%s"`, m.Name, m.Type, openMetricsEscape(m.Value))
 }
 
 // Matches returns whether the matcher matches the given string value.
@@ -86,3 +87,16 @@ func (m *Matcher) Matches(s string) bool {
 	}
 	panic("labels.Matcher.Matches: invalid match type")
 }
+
+// openMetricsEscape is similar to the usual string escaping, but more
+// restricted. It merely replaces a new-line character with '\n', a double-quote
+// character with '\"', and a backslack with '\\', which is the escaping used by
+// OpenMetrics.
+func openMetricsEscape(s string) string {
+	r := strings.NewReplacer(
+		`\`, `\\`,
+		"\n", `\n`,
+		`"`, `\"`,
+	)
+	return r.Replace(s)
+}

pkg/labels/matcher_test.go

@@ -122,3 +122,72 @@ func TestMatcher(t *testing.T) {
 		}
 	}
 }
+
+func TestMatcherString(t *testing.T) {
+	tests := []struct {
+		name  string
+		op    MatchType
+		value string
+		want  string
+	}{
+		{
+			name:  `foo`,
+			op:    MatchEqual,
+			value: `bar`,
+			want:  `foo="bar"`,
+		},
+		{
+			name:  `foo`,
+			op:    MatchNotEqual,
+			value: `bar`,
+			want:  `foo!="bar"`,
+		},
+		{
+			name:  `foo`,
+			op:    MatchRegexp,
+			value: `bar`,
+			want:  `foo=~"bar"`,
+		},
+		{
+			name:  `foo`,
+			op:    MatchNotRegexp,
+			value: `bar`,
+			want:  `foo!~"bar"`,
+		},
+		{
+			name:  `foo`,
+			op:    MatchEqual,
+			value: `back\slash`,
+			want:  `foo="back\\slash"`,
+		},
+		{
+			name:  `foo`,
+			op:    MatchEqual,
+			value: `double"quote`,
+			want:  `foo="double\"quote"`,
+		},
+		{
+			name: `foo`,
+			op:   MatchEqual,
+			value: `new
+line`,
+			want: `foo="new\nline"`,
+		},
+		{
+			name: `foo`,
+			op:   MatchEqual,
+			value: `tab	stop`,
+			want: `foo="tab	stop"`,
+		},
+	}
+
+	for _, test := range tests {
+		m, err := NewMatcher(test.op, test.name, test.value)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if got := m.String(); got != test.want {
+			t.Errorf("Unexpected string representation of matcher; want %v, got %v", test.want, got)
+		}
+	}
+}