When accessing uncached media attachment, redownload it (#4955)

* When accessing uncached media attachment, redownload it

* Prevent re-download of rejected media

app/controllers/media_proxy_controller.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+class MediaProxyController < ApplicationController
+  include RoutingHelper
+
+  def show
+    RedisLock.acquire(lock_options) do |lock|
+      if lock.acquired?
+        @media_attachment = MediaAttachment.remote.find(params[:id])
+        redownload! if @media_attachment.needs_redownload? && !reject_media?
+      end
+    end
+
+    redirect_to full_asset_url(@media_attachment.file.url(version))
+  end
+
+  private
+
+  def redownload!
+    @media_attachment.file_remote_url = @media_attachment.remote_url
+    @media_attachment.touch(:created_at)
+    @media_attachment.save!
+  end
+
+  def version
+    if request.path.ends_with?('/small')
+      :small
+    else
+      :original
+    end
+  end
+
+  def lock_options
+    { redis: Redis.current, key: "media_download:#{params[:id]}" }
+  end
+
+  def reject_media?
+    DomainBlock.find_by(domain: @media_attachment.account.domain)&.reject_media?
+  end
+end