uploads: merge tags when a duplicate is uploaded (fix #3130).

Automatically merge tags when uploading a duplicate.

There are two cases:

* You try to upload an image, but it's already on Danbooru. In this case
  you'll be immediately redirected to the original post, before you
  can start tagging the upload.

* You're uploading an image, it wasn't a dupe when you first opened the
  upload page, but you got sniped while tagging it. In this case your tags
  will be merged with the original post, and you will be redirected to the
  original post.

There are a few corner cases:

* If you don't have permission to edit the original post, for example
  because it's banned or has a censored tag, then your tags won't be
  merged and will be silently ignored.

* Only the tags, rating, and parent ID will be merged. The source and
  artist commentary won't be merged. This is so that if an artist uploads
  the exact same file to multiple sites, the new source won't override
  the original source.

* Some tags might be contradictory. For example, the new post might
  be tagged translation_request, but the original post might already be
  translated. It's up to the user to fix these things afterwards.

app/controllers/posts_controller.rb

@@ -67,12 +67,17 @@ class PostsController < ApplicationController
     @post = authorize Post.new_from_upload(permitted_attributes(Post))
     @post.save
 
-    if @post.errors.any?
+    if @post.errors.none?
+      respond_with(@post)
+    elsif @post.errors.of_kind?(:md5, :taken)
+      @original_post = Post.find_by!(md5: @post.md5)
+      @original_post.update(rating: @post.rating, parent_id: @post.parent_id, tag_string: "#{@original_post.tag_string} #{@post.tag_string}")
+      flash[:notice] = "Duplicate of post ##{@original_post.id}; merging tags"
+      redirect_to @original_post
+    else
       @upload = UploadMediaAsset.find(params[:post][:upload_media_asset_id]).upload
       flash[:notice] = @post.errors.full_messages.join("; ")
       respond_with(@post, render: { template: "uploads/show" })
-    else
-      respond_with(@post)
     end
   end
 

app/controllers/uploads_controller.rb

@@ -38,6 +38,12 @@ class UploadsController < ApplicationController
   def show
     @upload = authorize Upload.find(params[:id])
     @post = Post.new(uploader: @upload.uploader, uploader_ip_addr: @upload.uploader_ip_addr, source: @upload.source, rating: nil, **permitted_attributes(Post))
-    respond_with(@upload)
+
+    if request.format.html? && @upload.is_completed? && @upload.media_assets.first&.post.present?
+      flash[:notice] = "Duplicate of post ##{@upload.media_assets.first.post.id}"
+      redirect_to @upload.media_assets.first.post
+    else
+      respond_with(@upload)
+    end
   end
 end

app/models/media_asset.rb

@@ -12,6 +12,7 @@ class MediaAsset < ApplicationRecord
   LARGE_IMAGE_WIDTH = Danbooru.config.large_image_width
   STORAGE_SERVICE = Danbooru.config.storage_manager
 
+  has_one :post, foreign_key: :md5, primary_key: :md5
   has_one :media_metadata, dependent: :destroy
   has_one :pixiv_ugoira_frame_data, class_name: "PixivUgoiraFrameData", foreign_key: :md5, primary_key: :md5
 

test/functional/posts_controller_test.rb

@@ -5,9 +5,9 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
     assert_equal(expected, response.parsed_body.css("link[rel=canonical]").attribute("href").value)
   end
 
-  def create_post!(user: create(:user), rating: "q", tag_string: "tagme", **params)
+  def create_post!(user: create(:user), media_asset: build(:media_asset), rating: "q", tag_string: "tagme", **params)
     upload = build(:upload, uploader: user)
-    asset = create(:upload_media_asset, upload: upload)
+    asset = create(:upload_media_asset, upload: upload, media_asset: media_asset)
     post_auth posts_path, user, params: { post: { upload_media_asset_id: asset.id, rating: rating, tag_string: tag_string, **params }}
 
     Post.last
@@ -655,6 +655,21 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
         assert_equal("test", @post.tag_string)
       end
 
+      should "re-render the upload page if the upload fails" do
+        @post = create_post!(rating: "z", tag_string: "tagme")
+        assert_response :success
+      end
+
+      should "merge the tags and redirect to the original post if the upload is a duplicate of an existing post" do
+        media_asset = create(:media_asset)
+        post1 = create_post!(rating: "s", tag_string: "post1", media_asset: media_asset)
+        post2 = create_post!(rating: "e", tag_string: "post2", media_asset: media_asset)
+
+        assert_redirected_to post1
+        assert_equal("post1 post2", post1.reload.tag_string)
+        assert_equal("e", post1.rating)
+      end
+
       should "apply the rating from the tags" do
         @post = create_post!(rating: nil, tag_string: "rating:s")
 

test/functional/uploads_controller_test.rb

@@ -130,6 +130,14 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
 
         assert_response :success
       end
+
+      should "redirect a completed upload to the original post if it's a duplicate of an existing post" do
+        @upload = create(:completed_file_upload, uploader: @user)
+        @post = create(:post, md5: @upload.media_assets.first.md5, media_asset: @upload.media_assets.first)
+        get_auth upload_path(@upload), @user
+
+        assert_redirected_to @post
+      end
     end
 
     context "create action" do

test/unit/upload_service_test.rb

@@ -311,27 +311,6 @@ class UploadServiceTest < ActiveSupport::TestCase
       CurrentUser.ip_addr = nil
     end
 
-    context "with a preprocessed predecessor" do
-      setup do
-        @predecessor = FactoryBot.create(:source_upload, status: "preprocessed", source: @source, image_height: 0, image_width: 0, file_size: 1, md5: 'd34e4cf0a437a5d65f8e82b7bcd02606', file_ext: "jpg")
-        @tags = 'hello world'
-      end
-
-      context "when the file has already been uploaded" do
-        setup do
-          @asset = MediaAsset.find_by_md5("d34e4cf0a437a5d65f8e82b7bcd02606")
-          @post = create(:post, md5: "d34e4cf0a437a5d65f8e82b7bcd02606", media_asset: @asset)
-          @service = subject.new(source: @source)
-        end
-
-        should "point to the dup post in the upload" do
-          @upload = subject.new(source: @source, tag_string: @tags).start!
-          @predecessor.reload
-          assert_equal("error: ActiveRecord::RecordInvalid - Validation failed: Md5 duplicate: #{@post.id}", @predecessor.status)
-        end
-      end
-    end
-
     context "with a source containing unicode characters" do
       should "normalize unicode characters in the source field" do
         source1 = "poke\u0301mon" # pokémon (nfd form)