expfmt: reject nameless samples after metric metadata

expfmt/text_parse.go

@@ -194,6 +194,7 @@ func (p *TextParser) reset(in io.Reader) {
 // start of a line (or whitespace leading up to it).
 func (p *TextParser) startOfLine() stateFn {
 	p.lineCount++
+	p.currentMetric = nil
 	p.currentMetricIsInsideBraces = false
 	p.currentMetricInsideBracesIsPresent = false
 	if p.skipBlankTab(); p.err != nil {
@@ -339,12 +340,11 @@ func (p *TextParser) startLabelName() stateFn {
 		return nil // Unexpected end of input.
 	}
 	if p.currentByte == '}' {
-		if p.currentMF == nil {
+		if p.currentMetric == nil {
 			// The closing brace was reached before any metric name was read,
 			// e.g. for the input "{}". There is no metric to attach labels to,
 			// so this is a malformed exposition. This mirrors the guard in
-			// startLabelValue. currentMF (not currentMetric) is checked because
-			// reset only clears currentMF between parses.
+			// startLabelValue. currentMetric is cleared at the start of each line.
 			p.parseError("invalid metric name")
 			p.currentLabelPairs = nil
 			return nil
@@ -495,7 +495,7 @@ func (p *TextParser) startLabelValue() stateFn {
 		return p.startLabelName
 
 	case '}':
-		if p.currentMF == nil {
+		if p.currentMetric == nil {
 			p.parseError("invalid metric name")
 			return nil
 		}

expfmt/text_parse_test.go

@@ -1378,6 +1378,38 @@ func TestTextParseError(t *testing.T) {
 	testTextParseError(t)
 }
 
+func TestTextParseMissingMetricName(t *testing.T) {
+	for _, scheme := range []model.ValidationScheme{model.LegacyValidation, model.UTF8Validation} {
+		t.Run(scheme.String(), func(t *testing.T) {
+			for _, prefix := range []string{
+				"",
+				"#TYPE A000000 summArY\n",
+				"# TYPE metric histogram\n",
+				"# HELP metric Help text.\n",
+				"metric 1\n",
+				"{\"metric\"} 1\n",
+			} {
+				for _, sample := range []string{"{}", "{} 2\n", "{label=\"value\"} 2\n", "{label=\"value\",} 2\n"} {
+					t.Run(prefix+sample, func(t *testing.T) {
+						p := NewTextParser(scheme)
+						for range 2 {
+							_, err := p.TextToMetricFamilies(strings.NewReader(prefix + sample))
+							var parseErr ParseError
+							if !errors.As(err, &parseErr) || parseErr.Msg != "invalid metric name" || parseErr.Line != strings.Count(prefix, "\n")+1 {
+								t.Fatalf("expected invalid metric name on the sample line, got %v", err)
+							}
+							// Exercise reuse after both malformed input and a valid sample.
+							if _, err := p.TextToMetricFamilies(strings.NewReader("valid 1\n")); err != nil {
+								t.Fatalf("parsing valid sample after malformed input: %v", err)
+							}
+						}
+					})
+				}
+			}
+		})
+	}
+}
+
 func BenchmarkParseError(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		testTextParseError(b)