Add unique label validation (#263)

* Add unique label validation

Originally created as a PR in the
[client_golang](prometheus/client_golang#812)
repo however was suggested to move it here.

Signed-off-by: Jarod Watkins <jarod@42lines.net>

expfmt/text_parse.go

@@ -299,6 +299,17 @@ func (p *TextParser) startLabelName() stateFn {
 		p.parseError(fmt.Sprintf("expected '=' after label name, found %q", p.currentByte))
 		return nil
 	}
+	// Check for duplicate label names.
+	labels := make(map[string]struct{})
+	for _, l := range p.currentMetric.Label {
+		lName := l.GetName()
+		if _, exists := labels[lName]; !exists {
+			labels[lName] = struct{}{}
+		} else {
+			p.parseError(fmt.Sprintf("duplicate label names for metric %q", p.currentMF.GetName()))
+			return nil
+		}
+	}
 	return p.startLabelValue
 }
 

expfmt/text_parse_test.go

@@ -635,6 +635,11 @@ metric{quantile="0x1p-3"} 3.14
 `,
 			err: "text format parsing error in line 3: expected float as value for 'quantile' label",
 		},
+		// 33: Check duplicate label.
+		{
+			in:  `metric{label="bla",label="bla"} 3.14`,
+			err: "text format parsing error in line 1: duplicate label names for metric",
+		},
 	}
 
 	for i, scenario := range scenarios {