Chunk taskArns into groups of 100

If the ECS cluster has > 100 tasks, passing them to
ecs.DescribeTasksRequest() will result in the AWS API returning
errors.

This patch breaks them into chunks of at most 100, and calls
DescribeTasks for each chunk.

We also return early in case ListTasks returns no values; this
prevents DescribeTasks from throwing HTTP errors.

provider/ecs_test.go

@@ -308,3 +308,42 @@ func TestFilterInstance(t *testing.T) {
 		}
 	}
 }
+
+func TestTaskChunking(t *testing.T) {
+	provider := &ECS{}
+
+	testval := "a"
+	cases := []struct {
+		count           int
+		expectedLengths []int
+	}{
+		{0, []int(nil)},
+		{1, []int{1}},
+		{99, []int{99}},
+		{100, []int{100}},
+		{101, []int{100, 1}},
+		{199, []int{100, 99}},
+		{200, []int{100, 100}},
+		{201, []int{100, 100, 1}},
+		{555, []int{100, 100, 100, 100, 100, 55}},
+		{1001, []int{100, 100, 100, 100, 100, 100, 100, 100, 100, 100, 1}},
+	}
+
+	for _, c := range cases {
+		var tasks []*string
+		for v := 0; v < c.count; v++ {
+			tasks = append(tasks, &testval)
+		}
+
+		out := provider.chunkedTaskArns(tasks)
+		var outCount []int
+
+		for _, el := range out {
+			outCount = append(outCount, len(el))
+		}
+
+		if !reflect.DeepEqual(outCount, c.expectedLengths) {
+			t.Errorf("Chunking %d elements, expected %#v, got %#v", c.count, c.expectedLengths, outCount)
+		}
+	}
+}