Log an error if the parsers produce different parsings

This commit updates the fallback parser to compare the parsings
of both the UTF-8 matchers parser and the classic parser, and
log an error if the parsers produce different parsings.

This can happen at present if ecsape sequences such as \n and \t
are used outside of double quotes, as this is not supported in
the UTF-8 matchers parser.

Signed-off-by: George Robinson <george.robinson@grafana.com>

matchers/compat/parse.go

@@ -107,28 +107,38 @@ func utf8MatchersParser(l log.Logger) matchersParser {
 func fallbackMatcherParser(l log.Logger) matcherParser {
 	return func(s string) (*labels.Matcher, error) {
 		var (
-			m          *labels.Matcher
-			err        error
-			invalidErr error
+			classicMatcher *labels.Matcher
+			utf8Matcher    *labels.Matcher
+			invalidErr     error
+			err            error
 		)
 		level.Debug(l).Log("msg", "Parsing with UTF-8 matchers parser, with fallback to classic matchers parser", "input", s)
 		if strings.HasPrefix(s, "{") || strings.HasSuffix(s, "}") {
 			return nil, fmt.Errorf("unexpected open or close brace: %s", s)
 		}
-		m, err = parse.Matcher(s)
+		classicMatcher, invalidErr = labels.ParseMatcher(s)
+		utf8Matcher, err = parse.Matcher(s)
 		if err != nil {
-			m, invalidErr = labels.ParseMatcher(s)
 			if invalidErr != nil {
 				// The input is not valid in the old pkg/labels parser either,
 				// it cannot be valid input.
 				return nil, invalidErr
 			}
 			// The input is valid in the old pkg/labels parser, but not the
 			// new matchers/parse parser.
-			suggestion := m.String()
+			suggestion := classicMatcher.String()
 			level.Warn(l).Log("msg", "Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the old matchers parser as a fallback. To make this input compatible with the new parser please make sure all regular expressions and values are double-quoted. If you are still seeing this message please open an issue.", "input", s, "err", err, "suggestion", suggestion)
+			return classicMatcher, nil
 		}
-		return m, nil
+		// The input is valid in both parsers, so check it parses the same.
+		if utf8Matcher != nil && classicMatcher != nil && *utf8Matcher != *classicMatcher {
+			// The input is valid in both parsers but is producing different
+			// parsing. This should not happen and is a bug that needs to be
+			// reported.
+			level.Error(l).Log("msg", "The UTF-8 matchers parser and the classic matchers parser have produced different parsings. Please report this issue on GitHub.", "input", s)
+			return classicMatcher, nil
+		}
+		return utf8Matcher, nil
 	}
 }
 
@@ -138,33 +148,45 @@ func fallbackMatcherParser(l log.Logger) matcherParser {
 func fallbackMatchersParser(l log.Logger) matchersParser {
 	return func(s string) (labels.Matchers, error) {
 		var (
-			m          []*labels.Matcher
-			err        error
-			invalidErr error
+			classicMatchers labels.Matchers
+			utf8Matchers    labels.Matchers
+			invalidErr      error
+			err             error
 		)
 		level.Debug(l).Log("msg", "Parsing with UTF-8 matchers parser, with fallback to classic matchers parser", "input", s)
-		m, err = parse.Matchers(s)
+		classicMatchers, invalidErr = labels.ParseMatchers(s)
+		utf8Matchers, err = parse.Matchers(s)
 		if err != nil {
-			m, invalidErr = labels.ParseMatchers(s)
 			if invalidErr != nil {
 				// The input is not valid in the old pkg/labels parser either,
 				// it cannot be valid input.
 				return nil, invalidErr
 			}
 			var sb strings.Builder
-			sb.WriteRune('{')
-			for i, n := range m {
+			for i, n := range classicMatchers {
 				sb.WriteString(n.String())
-				if i < len(m)-1 {
+				if i < len(classicMatchers)-1 {
 					sb.WriteRune(',')
 				}
 			}
-			sb.WriteRune('}')
 			suggestion := sb.String()
 			// The input is valid in the old pkg/labels parser, but not the
 			// new matchers/parse parser.
 			level.Warn(l).Log("msg", "Alertmanager is moving to a new parser for labels and matchers, and this input is incompatible. Alertmanager has instead parsed the input using the old matchers parser as a fallback. To make this input compatible with the new parser please make sure all regular expressions and values are double-quoted. If you are still seeing this message please open an issue.", "input", s, "err", err, "suggestion", suggestion)
+			return classicMatchers, nil
+		}
+		// The input is valid in both parsers, so check it parses the same.
+		if len(utf8Matchers) == len(classicMatchers) {
+			for i := 0; i < len(utf8Matchers); i++ {
+				if utf8Matchers[i] != classicMatchers[i] {
+					// The input is valid in both parsers but is producing different
+					// parsing. This should not happen and is a bug that needs to be
+					// reported.
+					level.Error(l).Log("msg", "The UTF-8 matchers parser and the classic matchers parser have produced different parsings. Please report this issue on GitHub.", "input", s)
+					return classicMatchers, nil
+				}
+			}
 		}
-		return m, nil
+		return utf8Matchers, nil
 	}
 }