git/githistory/log: panic() when *PercentageTask is over-counted

git/githistory/log/percentage_task.go

@@ -38,7 +38,9 @@ func NewPercentageTask(msg string, total uint64) *PercentageTask {
 // Count returns the new total number of (atomically managed) elements that have
 // been completed.
 func (c *PercentageTask) Count(n uint64) (new uint64) {
-	new = atomic.AddUint64(&c.n, n)
+	if new = atomic.AddUint64(&c.n, n); new > c.total {
+		panic("git/githistory/log: counted too many items")
+	}
 
 	var percentage float64
 	if c.total == 0 {

git/githistory/log/percentage_task_test.go

@@ -60,3 +60,12 @@ func TestPercentageTaskIsThrottled(t *testing.T) {
 	assert.True(t, throttled,
 		"git/githistory/log: expected *PercentageTask to be Throttle()-d")
 }
+
+func TestPercentageTaskPanicsWhenOvercounted(t *testing.T) {
+	task := NewPercentageTask("example", 0)
+	defer func() {
+		assert.Equal(t, "git/githistory/log: counted too many items", recover())
+	}()
+
+	task.Count(1)
+}