Implement per-operation options for storage standalone functions

[coryan]

The storage library has a few standalone functions that refactor common workflows making multiple calls to the storage::Client class. For these classes we need a different implementation of the per-operation options. Consider, for example, ComposeMany(). Basically it does something like this:

template <typename Options>
StatusOr<ObjectMetadata> ComposeMany(
      storage::Client client, std::vector<std::string> object_names, Options&&... options) {
  auto c1 = client.Compose(/* some of the objects from `object_names */, std::forward<Options>(options));
  auto c2 = client.Compose(/* some of the objects from `object_names */, std::forward<Options>(options));
....
  auto cN = client.Compose(/* some of the objects from `object_names */, std::forward<Options>(options));

  return client.Compose(c1 .... cN, std::forward<Options>(options));
}

The problem is that any google::cloud::Options hidden in the parameter pack may get moved-copied in the first call creating c1. The remaining calls get the wrong overrides. We need to do something like:

template <typename Options>
StatusOr<ObjectMetadata> ComposeMany(
      storage::Client client, std::vector<std::string> object_names, Options&&... options) {
  auto const grouped = storage::internal::GroupOptions(std::forward<Options>(options)...);
  auto c1 = client.Compose(/* some of the objects from `object_names */, std::forward<Options>(options)..., grouped);
  auto c2 = client.Compose(/* some of the objects from `object_names */, std::forward<Options>(options)..., grouped);
....
  auto cN = client.Compose(/* some of the objects from `object_names */, std::forward<Options>(options)..., grouped);

  return client.Compose(c1 .... cN, std::forward<Options>(options)..., grouped);
}

I think the relevant functions are:

• DeleteByPrefix()
• ComposeMany()
• ParallelUploadFile()

These functions have many helpers in the internal namespace, but I think we can ignore those?

[coryan]

Still want this.

[coryan]

Evidently we don't have time to work on this. To the backlog.