Extract ActiveStorage::Streaming so your own controllers can use it

[dhh]

If you want to build your own controller that streams from active storage, it's helpful to have a method that does the streaming correctly. One such already existed unextracted in ActiveStorage::BaseController.

[rafaelfranca]

Isn't this code supposed to stream the representation instead of the image? I think that is the reason why the tests are broken.

[kaspth]

They were broken differently before. Think we need the image for the content type headers, but the representation for the streaming.

[dhh]

Did found this weird. That you can't just stream the representation directly. Maybe @georgeclaghorn knows why that is.

[dhh]

All representations are supposed to produce an image, so I think this is correct?

[santib]

is this module included in api_only mode? Asking because otherwise they won't be able to use ActiveStorage anymore

[dhh]

api mode does not support streaming? That sounds like something we should fix.

[santib]

it was a legit question, I don't really know 😇 maybe everything is fine

[dhh]

Please do verify! It's a good question.

[santib]

After checking #32208 , #32238 and actionpack/lib/action_controller/metal/live.rb seems that the issue with ActionController::Base is only present on direct uploads because of some missing middlewares for api_only apps. So ActionController::Live won't cause any issues.

[Bialogs]

Really appreciate this change.

I am wondering if this changes behavior I noticed with the previous implementation of stream. From observation it seemed that our blob would be downloaded to the Rails server entirely, then streamed to the client. Does the ActiveStorage::BaseController stream implementation stream the blob both from S3 to Rails and from Rails to client?

[dhh]

        blob.download do |chunk|
           stream.write chunk
         end

We're streaming the download and streaming the writing. This is happening in chunks. So it doesn't download the whole thing first.