Ensure variants are loaded and processed synchronously just once

[rosa]

Motivation / Background

Today, while looking into some 500 errors from GET /rails/active_storage/representations/redirect/ requests, I noticed an unexpected behaviour in our logs. We were computing a variant for a previewable blob (for example, a video or a PDF, whose variants belong to their preview image), and then, after having uploaded it and stored it, we were trying to load it again from the database, failing to find it, then downloading the preview once more and processing the variant again, and finally failing to create the variant record, because it already existed. Turns out, the reason we couldn't find it the second time we tried to load it was that this was happening in the replica, whereas the new variant is created against the primary, by manually connecting to the writer:

rails/activestorage/app/models/active_storage/variant_with_record.rb

Lines 57 to 62 in 17f6e00

	@record =
	ActiveRecord::Base.connected_to(role: ActiveRecord.writing_role) do
	blob.variant_records.create_or_find_by!(variation_digest: variation.digest) do |record|
	record.image.attach(image)
	end
	end

This PR avoids loading the record that was created within the request once more just to get the redirect URL. See below for more details.

Detail

Even though we track and memoize the just-created @record in ActiveStorage::VariantWithRecord when we call processed (which calls process), we always instantiate a new object when we call ActiveStorage::Blob#variant.

rails/activestorage/app/models/active_storage/blob/representable.rb

Lines 100 to 102 in 17f6e00

	def variant(transformations)
	if variable?
	variant_class.new(self, ActiveStorage::Variation.wrap(transformations).default_to(default_variant_transformations))

If we only call this once, that's fine, but when processing the variant via GET /rails/active_storage/representations/redirect, we call this twice: one when setting the representation and processing it:

rails/activestorage/app/controllers/active_storage/representations/base_controller.rb

Lines 13 to 14 in 17f6e00

	def set_representation
	@representation = @blob.representation(params[:variation_key]).processed

and another one when calling @representation.url for the redirect:

rails/activestorage/app/controllers/active_storage/representations/redirect_controller.rb

Line 12 in 17f6e00

redirect_to @representation.url(disposition: params[:disposition]), allow_other_host: true

which calls ActiveStorage::Preview#url, which ultimately calls ActiveStorage::VariantWithRecord.processed again, but on a new instance of ActiveStorage::VariantWithRecord. This checks whether the variant record exists (the one we just created in the previous call to processed), but does so against the replica, which is unlikely to find it if there's any delay, since these calls are very close together.

With this change, we make sure we reuse the variant instance we just calculated for the preview image.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Unrelated changes should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.