expfmt: throw error when metric's name not provided instead panicing

expfmt/text_parse.go

@@ -109,7 +109,7 @@ func NewTextParser(nameValidationScheme model.ValidationScheme) TextParser {
 // Summaries and histograms are rather special beasts. You would probably not
 // use them in the simple text format anyway. This method can deal with
 // summaries and histograms if they are presented in exactly the way the
-// text.Create function creates them.
+// text. Create function creates them.
 //
 // This method must not be called concurrently. If you want to parse different
 // input concurrently, instantiate a separate Parser for each goroutine.
@@ -188,6 +188,7 @@ func (p *TextParser) reset(in io.Reader) {
 	p.currentQuantile = math.NaN()
 	p.currentBucket = math.NaN()
 	p.currentMF = nil
+	p.currentMetric = nil
 }
 
 // startOfLine represents the state where the next byte read from p.buf is the
@@ -485,7 +486,7 @@ func (p *TextParser) startLabelValue() stateFn {
 		return p.startLabelName
 
 	case '}':
-		if p.currentMF == nil {
+		if p.currentMF == nil || p.currentMetric == nil {
 			p.parseError("invalid metric name")
 			return nil
 		}

expfmt/text_parse_test.go

@@ -1332,6 +1332,16 @@ request_duration_microseconds_count 2693
 `,
 			errUTF8: `text format parsing error in line 5: negative bucket population for histogram "request_duration_microseconds"`,
 		},
+		// 40: Metric's name missing.
+		{
+			in: `
+# HELP backupmonitor_size The size of the given backup.
+# TYPE backupmonitor_size counter
+{host="local", dir="alpha"} 1834194837
+{host="remote", dir="beta"} 133638016
+`,
+			errUTF8: `text format parsing error in line 4: invalid metric name`,
+		},
 	}
 	for i, scenario := range scenarios {
 		parser.scheme = model.UTF8Validation