[#2180] Low-level Carrierwave to ActiveStorage migration

app/services/carrierwave_active_storage_migration_service.rb

@@ -0,0 +1,143 @@
+class CarrierwaveActiveStorageMigrationService
+  def ensure_openstack_copy_possible!(uploader)
+    ensure_active_storage_openstack!
+    ensure_carrierwave_openstack!(uploader)
+    ensure_active_storage_and_carrierwave_credetials_match(uploader)
+  end
+
+  def ensure_active_storage_openstack!
+    # If we manage to get the client, it means that ActiveStorage is on OpenStack
+    openstack_client!
+  end
+
+  def openstack_client!
+    @openstack_client ||= active_storage_openstack_client!
+  end
+
+  def active_storage_openstack_client!
+    service = ActiveStorage::Blob.service
+
+    if !defined?(ActiveStorage::Service::OpenStackService) ||
+        !service.is_a?(ActiveStorage::Service::OpenStackService)
+      raise StandardError, 'ActiveStorage must be backed by OpenStack'
+    end
+
+    service.client
+  end
+
+  def ensure_carrierwave_openstack!(uploader)
+    storage = fog_client!(uploader)
+
+    if !defined?(Fog::OpenStack::Storage::Real) ||
+        !storage.is_a?(Fog::OpenStack::Storage::Real)
+      raise StandardError, 'Carrierwave must be backed by OpenStack'
+    end
+  end
+
+  def fog_client!(uploader)
+    storage = uploader.new.send(:storage)
+
+    if !defined?(CarrierWave::Storage::Fog) ||
+        !storage.is_a?(CarrierWave::Storage::Fog)
+      raise StandardError, 'Carrierwave must be backed by a Fog provider'
+    end
+
+    storage.connection
+  end
+
+  # OpenStack Swift's COPY object command works across different buckets, but they still need
+  # to be on the same object store. This method tries to ensure that Carrierwave and ActiveStorage
+  # are indeed pointing to the same Swift store.
+  def ensure_active_storage_and_carrierwave_credetials_match(uploader)
+    auth_keys = [
+      :openstack_tenant,
+      :openstack_api_key,
+      :openstack_username,
+      :openstack_region,
+      :openstack_management_url
+    ]
+
+    active_storage_creds = openstack_client!.credentials.slice(*auth_keys)
+    carrierwave_creds = fog_client!(uploader).credentials.slice(*auth_keys)
+
+    if active_storage_creds != carrierwave_creds
+      raise StandardError, "Active Storage and Carrierwave credentials must match"
+    end
+  end
+
+  # If identify is true, force ActiveStorage to examine the beginning of the file
+  # to determine its MIME type. This identification does not happen immediately,
+  # but when the first attachment that references this blob is created.
+  def make_blob(uploader, created_at, filename: nil, identify: false)
+    content_type = uploader.content_type
+
+    ActiveStorage::Blob.create(
+      filename: filename || uploader.filename,
+      content_type: uploader.content_type,
+      identified: content_type.present? && !identify,
+      byte_size: uploader.size,
+      checksum: checksum(uploader),
+      created_at: created_at
+    )
+  end
+
+  def checksum(uploader)
+    hex_to_base64(uploader.file.send(:file).etag)
+  end
+
+  def hex_to_base64(hexdigest)
+    [[hexdigest].pack("H*")].pack("m0")
+  end
+
+  def copy_from_carrierwave_to_active_storage!(source_name, blob)
+    openstack_client!.copy_object(
+      carrierwave_container_name,
+      source_name,
+      active_storage_container_name,
+      blob.key
+    )
+
+    fix_content_type(blob)
+  end
+
+  def carrierwave_container_name
+    Rails.application.secrets.fog[:directory]
+  end
+
+  def active_storage_container_name
+    ENV['FOG_ACTIVESTORAGE_DIRECTORY']
+  end
+
+  def delete_from_active_storage!(blob)
+    openstack_client!.delete_object(
+      active_storage_container_name,
+      blob.key
+    )
+  end
+
+  # Before calling this method, you must make sure the file has been uploaded for the blob.
+  # Otherwise, this method might fail if it needs to read the beginning of the file to
+  # update the blob’s MIME type.
+  def make_attachment(model, attachment_name, blob)
+    attachment = ActiveStorage::Attachment.create(
+      name: attachment_name,
+      record_type: model.class.base_class.name,
+      record_id: model.id,
+      blob: blob,
+      created_at: model.updated_at.iso8601
+    )
+
+    # Making the attachment may have triggerred MIME type auto detection on the blob,
+    # so we make sure to sync that potentially new MIME type to the object in OpenStack
+    fix_content_type(blob)
+
+    attachment
+  end
+
+  def fix_content_type(blob)
+    # In OpenStack, ActiveStorage cannot inject the MIME type on the fly during direct
+    # download. Instead, the MIME type needs to be stored statically on the file object
+    # in OpenStack. This is what this call does.
+    blob.service.change_content_type(blob.key, blob.content_type)
+  end
+end

spec/services/carrierwave_active_storage_migration_service_spec.rb

@@ -0,0 +1,9 @@
+require 'spec_helper'
+
+describe CarrierwaveActiveStorageMigrationService do
+  describe '#hex_to_base64' do
+    let(:service) { CarrierwaveActiveStorageMigrationService.new }
+
+    it { expect(service.hex_to_base64('deadbeef')).to eq('3q2+7w==') }
+  end
+end