Fix pointer receiver in COWMetric.Set().

model/metric.go

@@ -101,7 +101,7 @@ type COWMetric struct {
 
 // Set sets a label name in the wrapped Metric to a given value and copies the
 // Metric initially, if it is not already a copy.
-func (m COWMetric) Set(ln LabelName, lv LabelValue) {
+func (m *COWMetric) Set(ln LabelName, lv LabelValue) {
 	m.doCOW()
 	m.Metric[ln] = lv
 }

model/metric_test.go

@@ -70,3 +70,52 @@ func BenchmarkMetric(b *testing.B) {
 		testMetric(b)
 	}
 }
+
+func TestCOWMetric(t *testing.T) {
+	testMetric := Metric{
+		"to_delete": "test1",
+		"to_change": "test2",
+	}
+
+	scenarios := []struct {
+		fn  func(*COWMetric)
+		out Metric
+	}{
+		{
+			fn: func(cm *COWMetric) {
+				cm.Delete("to_delete")
+			},
+			out: Metric{
+				"to_change": "test2",
+			},
+		},
+		{
+			fn: func(cm *COWMetric) {
+				cm.Set("to_change", "changed")
+			},
+			out: Metric{
+				"to_delete": "test1",
+				"to_change": "changed",
+			},
+		},
+	}
+
+	for i, s := range scenarios {
+		orig := testMetric.Clone()
+		cm := &COWMetric{
+			Metric: orig,
+		}
+
+		s.fn(cm)
+
+		// Test that the original metric was not modified.
+		if !orig.Equal(testMetric) {
+			t.Fatalf("%d. original metric changed; expected %v, got %v", i, testMetric, orig)
+		}
+
+		// Test that the new metric has the right changes.
+		if !cm.Metric.Equal(s.out) {
+			t.Fatalf("%d. copied metric doesn't contain expected changes; expected %v, got %v", i, s.out, cm.Metric)
+		}
+	}
+}