uploads: enable multi-file uploads when uploading from source.

Make the upload page automatically detect when a source URL has multiple images
and let the user choose which images to post.

For example, when uploading a Twitter or Pixiv post with more than one image, we
direct the user to a page showing a thumbnail for each image and letting
them choose which ones to post.

This is similar to the batch upload page, except we actually download each image
in the background, instead of just hotlinking or proxying the thumbnails through
our servers. This avoids various problems with proxying and makes new features
possible, like showing which images in the batch have already been posted.

app/javascript/src/javascripts/file_upload_component.js

@@ -96,12 +96,12 @@ export default class FileUploadComponent {
     this.$component.find("progress").removeClass("hidden");
     this.$component.find("input").attr("disabled", "disabled");
 
-    while (upload.status === "pending" || upload.status === "processing") {
+    while (upload.media_asset_count <= 1 && upload.status !== "completed" && upload.status !== "error") {
       await Utility.delay(500);
       upload = await $.get(`/uploads/${upload.id}.json`);
     }
 
-    if (upload.status === "completed") {
+    if (upload.media_asset_count > 0) {
       let params = new URLSearchParams(window.location.search);
       let isBookmarklet = params.has("url");
       params.delete("url");

app/jobs/process_upload_media_asset_job.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class ProcessUploadMediaAssetJob < ApplicationJob
+  queue_with_priority -1
+
+  def perform(upload_media_asset)
+    upload_media_asset.process_upload!
+  end
+end

app/models/media_asset.rb

@@ -205,10 +205,11 @@ class MediaAsset < ApplicationRecord
       #
       # This can't be called inside a transaction because the transaction will
       # fail if there's a RecordNotUnique error when the asset already exists.
-      def upload!(media_file)
+      def upload!(media_file, &block)
         raise Error, "File is corrupt" if media_file.is_corrupt?
 
         media_asset = create!(file: media_file, status: :processing)
+        yield media_asset if block_given?
         media_asset.distribute_files!(media_file)
         media_asset.update!(status: :active)
         media_asset
@@ -218,6 +219,7 @@ class MediaAsset < ApplicationRecord
         raise if e.is_a?(ActiveRecord::RecordInvalid) && !e.record.errors.of_kind?(:md5, :taken)
 
         media_asset = find_by!(md5: media_file.md5, status: [:processing, :active])
+        yield media_asset if block_given?
 
         # XXX If the asset is still being processed by another thread, wait up
         # to 30 seconds for it to finish.

app/models/upload.rb

@@ -95,14 +95,13 @@ class Upload < ApplicationRecord
       update!(upload_media_assets: [upload_media_asset], status: "completed", media_asset_count: 1)
     elsif source.present?
       strategy = Sources::Strategies.find(source, referer_url)
-      image_url = strategy.image_url
       page_url = strategy.page_url
 
-      media_file = strategy.download_file!(strategy.image_url)
-      media_asset = MediaAsset.upload!(media_file)
-      upload_media_asset = UploadMediaAsset.new(media_asset: media_asset, source_url: image_url, page_url: page_url, status: "active")
+      upload_media_assets = strategy.image_urls.map do |image_url|
+        UploadMediaAsset.new(source_url: image_url, page_url: page_url, media_asset: nil)
+      end
 
-      update!(upload_media_assets: [upload_media_asset], status: "completed", media_asset_count: 1)
+      update!(upload_media_assets: upload_media_assets, media_asset_count: upload_media_assets.size)
     else
       raise "No file or source given" # Should never happen
     end

app/models/upload_media_asset.rb

@@ -6,6 +6,9 @@ class UploadMediaAsset < ApplicationRecord
   belongs_to :upload
   belongs_to :media_asset, optional: true
 
+  after_create :async_process_upload!
+  after_save :update_upload_status, if: :saved_change_to_status?
+
   enum status: {
     pending: 0,
     processing: 100,
@@ -35,5 +38,29 @@ class UploadMediaAsset < ApplicationRecord
     Sources::Strategies.find(source_url, page_url)
   end
 
+  def async_process_upload!
+    ProcessUploadMediaAssetJob.perform_later(self)
+  end
+
+  def process_upload!
+    update!(status: :processing)
+
+    strategy = Sources::Strategies.find(source_url)
+    media_file = strategy.download_file!(source_url)
+    MediaAsset.upload!(media_file) do |media_asset|
+      update!(media_asset: media_asset)
+    end
+
+    update!(status: :active)
+  rescue Exception => e
+    update!(status: :failed, error: e.message)
+  end
+
+  def update_upload_status
+    upload.with_lock do
+      upload.update!(status: "completed") if upload.upload_media_assets.all?(&:finished?)
+    end
+  end
+
   memoize :source_strategy
 end