lib/logstorage: quote logfmt strings only if they contain special chars, which could break logfmt parsing and/or reading

(cherry picked from commit 462b7cd597)

lib/logstorage/pipe_pack_logfmt_test.go

@@ -45,7 +45,7 @@ func TestPipePackLogfmt(t *testing.T) {
 		},
 	}, [][]Field{
 		{
-			{"_msg", `_msg=x foo=abc bar="cde=ab"`},
+			{"_msg", `_msg=x foo=abc bar=cde=ab`},
 			{"foo", `abc`},
 			{"bar", `cde=ab`},
 		},

lib/logstorage/rows.go

@@ -91,13 +91,25 @@ func getFieldValue(fields []Field, name string) string {
 
 func needLogfmtQuoting(s string) bool {
 	for _, c := range s {
-		if !isTokenRune(c) {
+		if isLogfmtSpecialChar(c) {
 			return true
 		}
 	}
 	return false
 }
 
+func isLogfmtSpecialChar(c rune) bool {
+	if c <= 0x20 {
+		return true
+	}
+	switch c {
+	case '"', '\\':
+		return true
+	default:
+		return false
+	}
+}
+
 // RenameField renames field with the oldName to newName in Fields
 func RenameField(fields []Field, oldName, newName string) {
 	if oldName == "" {