test: add test cases for overflow and pricision lost

prometheus/counter.go

@@ -128,7 +128,7 @@ type counter struct {
 //
 // note that if a part of addend is added on base but the left causes a rounding error,
 // we don't respect this case
-func addWithRoundingErrorChecking(base float64, addend float64) (float64, bool) {
+func addWithRoundingErrorChecking(base, addend float64) (float64, bool) {
 	if addend == 0 {
 		return base, false
 	}

prometheus/counter_test.go

@@ -79,6 +79,114 @@ func TestCounterAdd(t *testing.T) {
 	}
 }
 
+func TestCounterOverflowAdd(t *testing.T) {
+	now := time.Now()
+
+	counter := NewCounter(CounterOpts{
+		Name:        "test",
+		Help:        "test help",
+		ConstLabels: Labels{"a": "1", "b": "2"},
+		now:         func() time.Time { return now },
+	}).(*counter)
+	counter.change = math.MaxUint64
+	counter.Inc()
+
+	// this is expected due to the overflow
+	if expected, got := uint64(0), counter.change; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	m := &dto.Metric{}
+	counter.Write(m)
+
+	expected := &dto.Metric{
+		Label: []*dto.LabelPair{
+			{Name: proto.String("a"), Value: proto.String("1")},
+			{Name: proto.String("b"), Value: proto.String("2")},
+		},
+		Counter: &dto.Counter{
+			Value:            proto.Float64(0),
+			CreatedTimestamp: timestamppb.New(now),
+		},
+	}
+	if !proto.Equal(expected, m) {
+		t.Errorf("expected %q, got %q", expected, m)
+	}
+}
+
+func TestCounterPrecisionLosingAdd(t *testing.T) {
+	now := time.Now()
+
+	counter := NewCounter(CounterOpts{
+		Name:        "test",
+		Help:        "test help",
+		ConstLabels: Labels{"a": "1", "b": "2"},
+		now:         func() time.Time { return now },
+	}).(*counter)
+
+	counter.Add(1 << 53)
+	if expected, got := float64(1<<53), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+
+	counter.Inc()
+	if expected, got := uint64(1), counter.change; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	// along with the value 1 inside change, the sum could be added into the float without losing precision
+	counter.Add(1)
+	if expected, got := float64(1<<53+2), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	// the change is reset to 0
+	if expected, got := uint64(0), counter.change; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	// the value will flip with 2
+	counter.Add(1)
+	if expected, got := float64(1<<53+4), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(0), counter.change; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	counter.Add(1)
+	if expected, got := float64(1<<53+4), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(1), counter.change; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	counter.Add(1)
+	if expected, got := float64(1<<53+6), math.Float64frombits(counter.valBits); expected != got {
+		t.Errorf("Expected %f, got %f.", expected, got)
+	}
+	if expected, got := uint64(0), counter.change; expected != got {
+		t.Errorf("Expected %d, got %d.", expected, got)
+	}
+
+	m := &dto.Metric{}
+	counter.Write(m)
+
+	expected := &dto.Metric{
+		Label: []*dto.LabelPair{
+			{Name: proto.String("a"), Value: proto.String("1")},
+			{Name: proto.String("b"), Value: proto.String("2")},
+		},
+		Counter: &dto.Counter{
+			Value:            proto.Float64(1<<53 + 6),
+			CreatedTimestamp: timestamppb.New(now),
+		},
+	}
+	if !proto.Equal(expected, m) {
+		t.Errorf("expected %q, got %q", expected, m)
+	}
+}
+
 func decreaseCounter(c *counter) (err error) {
 	defer func() {
 		if e := recover(); e != nil {
@@ -418,6 +526,21 @@ func Test_hasRoundingError(t *testing.T) {
 			wantRoundingError: true,
 			wantNumber:        1 << 53,
 		},
+		{
+			name:              "no rounding error",
+			base:              1 << 53,
+			delta:             2,
+			wantRoundingError: true,
+			wantNumber:        1<<53 + 2,
+		},
+		{
+			name:              "no rounding error",
+			base:              1<<53 + 2,
+			delta:             1,
+			wantRoundingError: false,
+			// there is a precision losing
+			wantNumber: 1<<53 + 4,
+		},
 		{
 			name:              "rounding error",
 			base:              1 << 54,