fix(CMS.Repo): avoid caching errors as []

[thecristen]

Scope

Prompted by investigating the missing homepage content reported in this thread this morning!

Implementation

The missing content on the page is associated with calls to CMS.Repo.whats_happening(). I noticed that its current implementation would cause errors from this call to get cached as empty lists.

How?

To dissect the initial code:

@decorate cacheable(..omitted for brevity)
def whats_happening do
  case @cms_api.view("/cms/whats-happening", []) do
    {:ok, api_data} -> Enum.map(api_data, &WhatsHappeningItem.from_api/1)
    _ -> []
  end
end

This cacheable decorator function has a default value set for its :match function as described here, TLDR the following results are not cached: :error, nil, and {:error, _} tuples. Which means the above setup obscures errors from the @cms_api.view/2 call by turning them into [], which in turn will get cached.

To prevent this from happening, I adjusted this to follow a pattern we use in many places throughout Dotcom:

def function_thats_called do
  case other_function_thats_cached do
    {:ok, value} -> (whatever)
    _ -> (whatever we want to obscure the error with)
  end
end

@decorate cacheable(...etc)
def other_function_thats_cached do
  @cms_api.view(...) # something that can actually return errors!
end

I took a cursory look for similar things happening elsewhere in the homepage, and adjusted those too.