#409

Reviewed-on: #408
Co-authored-by: miteruzo <miteruzo@naver.com>
Co-committed-by: miteruzo <miteruzo@naver.com>

backend/app/controllers/preview_controller.rb

@@ -9,6 +9,8 @@ class PreviewController < ApplicationController
     render_bad_request(e.message)
   rescue Preview::HttpFetcher::FetchTimeout => e
     render_preview_error(e.message, :gateway_timeout)
+  rescue Preview::HttpFetcher::ResponseTooLarge => e
+    render_preview_error(e.message, :payload_too_large)
   rescue Preview::HttpFetcher::FetchFailed => e
     render_preview_error(e.message, :bad_gateway)
   end
@@ -28,6 +30,8 @@ class PreviewController < ApplicationController
     render_bad_request(e.message)
   rescue Preview::HttpFetcher::FetchTimeout => e
     render_preview_error(e.message, :gateway_timeout)
+  rescue Preview::HttpFetcher::ResponseTooLarge => e
+    render_preview_error(e.message, :payload_too_large)
   rescue Preview::HttpFetcher::FetchFailed => e
     render_preview_error(e.message, :bad_gateway)
   rescue Preview::ThumbnailFetcher::GenerationFailed, MiniMagick::Error => e

backend/app/models/post.rb

@@ -57,9 +57,9 @@ class Post < ApplicationRecord
 
   attribute :version_no, :integer, default: 1
 
-  before_validation :normalise_url
+  before_validation :normalise_url, if: :will_save_change_to_url?
 
-  validates :url, presence: true, uniqueness: true
+  validates :url, presence: true, uniqueness: true, length: { maximum: 768 }
   validates :video_ms, numericality: { only_integer: true, greater_than: 0 }, allow_nil: true
 
   validate :validate_original_created_range
@@ -180,7 +180,7 @@ class Post < ApplicationRecord
 
     u.host = u.host.downcase if u.host
     u.path = u.path.sub(/\/\Z/, '') if u.path.present?
-    self.url = u.to_s
+    self.url = PostUrlSanitisationRule.sanitise(u.to_s)
   rescue URI::InvalidURIError
     ;
   end

backend/app/models/post_url_sanitisation_rule.rb

@@ -0,0 +1,163 @@
+class PostUrlSanitisationRule < ApplicationRecord
+  include Discard::Model
+
+  class InvalidUrlError < StandardError
+    attr_reader :invalid_rows
+
+    def initialize(invalid_rows)
+      @invalid_rows = invalid_rows
+      ids = invalid_rows.map { _1.fetch(:post_id) }.join(', ')
+      super("post URL sanitisation produced invalid URLs for posts #{ ids }")
+    end
+  end
+
+  class UrlConflictError < StandardError
+    attr_reader :conflicts
+
+    def initialize(conflicts)
+      @conflicts = conflicts
+      urls = conflicts.map { _1.fetch(:url) }.uniq.join(', ')
+      super("post URL sanitisation conflicts detected for #{ urls }")
+    end
+  end
+
+  self.primary_key = :priority
+
+  default_scope -> { kept }
+
+  validates :source_pattern, presence: true, uniqueness: true
+
+  validate :source_pattern_must_be_regexp
+
+  class << self
+    def sanitise(url) = sanitise_with_rules(url, rules)
+
+    def apply!
+      rewrites = nil
+
+      Post.transaction do
+        compiled_rules = rules
+
+        rewrites = Post.order(:id)
+                       .lock('FOR UPDATE')
+                       .pluck(:id, :url)
+                       .map do |post_id, original_url|
+          { post_id:,
+            original_url:,
+            sanitised_url: sanitise_with_rules(original_url, compiled_rules) }
+        end
+
+        invalid_rows = rewrites.filter { invalid_sanitised_url?(_1.fetch(:sanitised_url)) }
+                               .map { { post_id: _1.fetch(:post_id),
+                                        original_url: _1.fetch(:original_url),
+                                        sanitised_url: _1.fetch(:sanitised_url) } }
+        raise InvalidUrlError.new(invalid_rows) if invalid_rows.present?
+
+        conflicts = build_conflicts(rewrites)
+        raise UrlConflictError.new(conflicts) if conflicts.present?
+
+        changed = rewrites.filter { _1.fetch(:original_url) != _1.fetch(:sanitised_url) }
+        return if changed.empty?
+
+        token = SecureRandom.hex(6)
+
+        changed.each do |row|
+          Post.where(id: row.fetch(:post_id))
+              .update_all(url: temporary_url_for(row.fetch(:post_id), token))
+        end
+
+        changed.each do |row|
+          Post.where(id: row.fetch(:post_id))
+              .update_all(url: row.fetch(:sanitised_url))
+        end
+      end
+    rescue ActiveRecord::RecordNotUnique => error
+      conflicts = build_persisted_conflicts(rewrites)
+      raise error if conflicts.empty?
+
+      raise UrlConflictError.new(conflicts), cause: error
+    end
+
+    private
+
+    def rules = kept.order(:priority).map { |r| [Regexp.new(r.source_pattern), r.replacement] }
+
+    def sanitise_with_rules(url, compiled_rules)
+      compiled_rules.reduce(url.dup) do |value, (pattern, replacement)|
+        value.sub(pattern, replacement)
+      end
+    end
+
+    def temporary_url_for(post_id, token) =
+        "https://post-url-sanitising.invalid/#{ token }/#{ post_id }"
+
+    def invalid_sanitised_url?(url)
+      return true if url.blank?
+      return true if url.length > 768
+
+      parsed = URI.parse(url)
+      return true if !(parsed in URI::HTTP)
+      return true if parsed.host.blank?
+
+      false
+    rescue URI::InvalidURIError
+      true
+    end
+
+    def build_conflicts(rewrites)
+      rewrites
+          .group_by { _1.fetch(:sanitised_url).downcase }
+          .values
+          .filter { _1.size > 1 }
+          .flatten
+          .map { { url: _1.fetch(:sanitised_url),
+                   post_id: _1.fetch(:post_id),
+                   original_url: _1.fetch(:original_url) } }
+    end
+
+    def build_persisted_conflicts(rewrites)
+      return [] if rewrites.blank?
+
+      target_rows = rewrites.filter { _1.fetch(:original_url) != _1.fetch(:sanitised_url) }
+      target_keys = target_rows.map { _1.fetch(:sanitised_url).downcase }.uniq
+      return [] if target_keys.empty?
+
+      target_pairs = target_rows.to_h do |row|
+        [row.fetch(:post_id), row.fetch(:sanitised_url).downcase]
+      end
+
+      persisted_rows = Post.order(:id)
+                           .where('LOWER(url) IN (?)', target_keys)
+                           .pluck(:id, :url)
+                           .reject { |post_id, original_url| target_pairs[post_id] == original_url.downcase }
+                           .map { |post_id, original_url|
+        { url: original_url,
+          post_id:,
+          original_url:,
+          conflict_key: original_url.downcase }
+      }
+
+      target_conflicts = target_rows.map { { url: _1.fetch(:sanitised_url),
+                                             post_id: _1.fetch(:post_id),
+                                             original_url: _1.fetch(:original_url),
+                                             conflict_key: _1.fetch(:sanitised_url).downcase } }
+
+      (persisted_rows + target_conflicts)
+          .group_by { _1.fetch(:conflict_key) }
+          .values
+          .filter { _1.size > 1 }
+          .flatten
+          .map { _1.except(:conflict_key) }
+    end
+  end
+
+  private
+
+  def source_pattern_must_be_regexp
+    return if source_pattern.blank?
+
+    Regexp.new(source_pattern)
+  rescue RegexpError
+    errors.add :source_pattern, '変な正規表現だね〜（笑）'
+  end
+end

backend/app/services/preview/http_fetcher.rb

@@ -1,4 +1,5 @@
 require 'net/http'
+require 'json'
 
 module Preview
   class HttpFetcher
@@ -16,24 +17,57 @@ module Preview
       response = request(uri, addresses.first, max_bytes)
 
       if response.is_a?(Net::HTTPRedirection)
-        raise FetchFailed, 'redirect が多すぎます．' if redirects.zero?
-
         location = response['location']
-        raise FetchFailed, 'redirect 先が不正です．' if location.blank?
+        if redirects.zero?
+          log_failure(:redirect_limit,
+                      url: uri.to_s,
+                      redirects:,
+                      location:,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'])
+          raise FetchFailed, 'redirect が多すぎます．'
+        end
 
-        return fetch(URI.join(uri, location).to_s,
-                     max_bytes:,
-                     redirects: redirects - 1)
+        if location.blank?
+          log_failure(:blank_redirect_location,
+                      url: uri.to_s,
+                      redirects:,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'])
+          raise FetchFailed, 'redirect 先が不正です．'
+        end
+
+        redirect_url =
+            begin
+              URI.join(uri, location).to_s
+            rescue URI::InvalidURIError => e
+              log_failure(:invalid_redirect_location,
+                          url: uri.to_s,
+                          redirects:,
+                          location:,
+                          error: e.class.name,
+                          message: e.message)
+              raise FetchFailed, 'redirect 先が不正です．'
+            end
+
+        return fetch(redirect_url, max_bytes:, redirects: redirects - 1)
       end
 
       unless response.is_a?(Net::HTTPSuccess)
+        log_failure(:http_status,
+                    url: uri.to_s,
+                    code: response.code,
+                    content_type: response['content-type'],
+                    content_length: response['content-length'])
         raise FetchFailed, "外部サーバーが HTTP #{ response.code } を返しました．"
       end
 
       Response.new(response.body, response['content-type'].to_s, uri.to_s)
     rescue Net::OpenTimeout, Net::ReadTimeout, Timeout::Error => e
+      log_failure(:timeout, url: uri&.to_s || raw_url, error: e.class.name, message: e.message)
       raise FetchTimeout, e.message
     rescue SocketError, SystemCallError, OpenSSL::SSL::SSLError, EOFError => e
+      log_failure(:network_error, url: uri&.to_s || raw_url, error: e.class.name, message: e.message)
       raise FetchFailed, e.message
     end
 
@@ -51,12 +85,27 @@ module Preview
 
       http.request(request) do |response|
         length = response['content-length'].to_i
-        raise ResponseTooLarge, '外部データが大きすぎます．' if length > max_bytes
+        if length > max_bytes
+          log_failure(:response_too_large,
+                      url: uri.to_s,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'],
+                      max_bytes:)
+          raise ResponseTooLarge, '外部データが大きすぎます．'
+        end
 
         body = +''
         response.read_body do |chunk|
           body << chunk
-          raise ResponseTooLarge, '外部データが大きすぎます．' if body.bytesize > max_bytes
+          next unless body.bytesize > max_bytes
+
+          log_failure(:response_too_large,
+                      url: uri.to_s,
+                      content_type: response['content-type'],
+                      content_length: response['content-length'],
+                      bytes_read: body.bytesize,
+                      max_bytes:)
+          raise ResponseTooLarge, '外部データが大きすぎます．'
         end
         response.instance_variable_set(:@body, body)
         response.instance_variable_set(:@read, true)
@@ -64,6 +113,10 @@ module Preview
       end
     end
 
-    private_class_method :request
+    def self.log_failure(reason, **payload)
+      Rails.logger.warn("preview_http_fetcher_failure #{ { reason:, **payload }.to_json }")
+    end
+
+    private_class_method :request, :log_failure
   end
 end

backend/app/services/preview/known_site_extractor.rb

@@ -1,7 +1,16 @@
 module Preview
   class KnownSiteExtractor
     def self.thumbnail_url(uri)
-      youtube_thumbnail(uri) || niconico_thumbnail(uri)
+      youtube_thumbnail(uri)
+    end
+
+    def self.niconico_video_id(uri)
+      case uri.host&.downcase
+      when 'www.nicovideo.jp', 'nicovideo.jp'
+        uri.path[%r{\A/watch/(sm\d+)\z}, 1]
+      when 'nico.ms'
+        uri.path[%r{\A/(sm\d+)\z}, 1]
+      end
     end
 
     def self.youtube_thumbnail(uri)
@@ -17,16 +26,6 @@ module Preview
       "https://i.ytimg.com/vi/#{ id }/hqdefault.jpg"
     end
 
-    def self.niconico_thumbnail(uri)
-      return unless ['www.nicovideo.jp', 'nicovideo.jp', 'nico.ms'].include?(uri.host&.downcase)
-
-      id = uri.path[%r{/(?:watch/)?(sm\d+)\z}, 1]
-      return unless id
-
-      numeric_id = id.delete_prefix('sm')
-      "https://nicovideo.cdn.nimg.jp/thumbnails/#{ numeric_id }/#{ numeric_id }.L"
-    end
-
-    private_class_method :youtube_thumbnail, :niconico_thumbnail
+    private_class_method :youtube_thumbnail
   end
 end

backend/app/services/preview/thumbnail_fetcher.rb

@@ -1,38 +1,95 @@
 module Preview
   class ThumbnailFetcher
     class GenerationFailed < StandardError; end
+    ALLOWED_IMAGE_CONTENT_TYPES = [
+      'image/jpeg', 'image/png', 'image/gif', 'image/webp'
+    ].freeze
+    HTML_MAX_BYTES = 1.megabyte
+    NICONICO_XML_MAX_BYTES = 256.kilobytes
 
     def self.fetch(raw_url)
       uri, = UrlSafety.validate(raw_url)
 
       known_url = KnownSiteExtractor.thumbnail_url(uri)
-      return fetch_image(known_url) if known_url
+      image = fetch_image_or_nil(known_url) if known_url
+      return image if image
 
-      page = HttpFetcher.fetch(uri.to_s)
+      niconico_url = niconico_thumbnail_url(uri)
+      image = fetch_image_or_nil(niconico_url) if niconico_url
+      return image if image
+
+      page = HttpFetcher.fetch(uri.to_s, max_bytes: HTML_MAX_BYTES)
       metadata = HtmlMetadataExtractor.extract(page)
       raise GenerationFailed, 'サムネール画像が見つかりませんでした．' if metadata[:image_url].blank?
 
-      fetch_image(metadata[:image_url])
+      fetch_image!(metadata[:image_url])
     end
 
     def self.title(raw_url)
       uri, = UrlSafety.validate(raw_url)
-      HtmlMetadataExtractor.extract(HttpFetcher.fetch(uri.to_s))[:title]
+      HtmlMetadataExtractor.extract(
+          HttpFetcher.fetch(uri.to_s, max_bytes: HTML_MAX_BYTES))[:title]
     end
 
-    def self.fetch_image(url)
+    def self.fetch_image_or_nil(url)
+      return nil if url.blank?
+
       response = HttpFetcher.fetch(url)
-      unless response.content_type.downcase.start_with?('image/')
+      return nil unless allowed_image_content_type?(response.content_type)
+
+      response.body
+    rescue HttpFetcher::FetchTimeout
+      raise
+    rescue HttpFetcher::FetchFailed
+      nil
+    end
+
+    def self.fetch_image!(url)
+      response = HttpFetcher.fetch(url)
+      unless allowed_image_content_type?(response.content_type)
         raise GenerationFailed, 'サムネール画像が見つかりませんでした．'
       end
 
       response.body
     rescue HttpFetcher::FetchTimeout
       raise
+    rescue HttpFetcher::ResponseTooLarge
+      raise
     rescue HttpFetcher::FetchFailed
       raise GenerationFailed, 'サムネール画像を取得できませんでした．'
     end
 
-    private_class_method :fetch_image
+    def self.niconico_thumbnail_url(uri)
+      video_id = KnownSiteExtractor.niconico_video_id(uri)
+      return nil if video_id.blank?
+
+      response = HttpFetcher.fetch("https://ext.nicovideo.jp/api/getthumbinfo/#{ video_id }",
+                                   max_bytes: NICONICO_XML_MAX_BYTES)
+      xml = Nokogiri::XML(response.body)
+      return nil unless xml.at_xpath('/nicovideo_thumb_response/@status')&.value == 'ok'
+
+      xml.at_xpath('//thumbnail_url')&.text&.strip.presence
+    rescue HttpFetcher::FetchFailed, HttpFetcher::FetchTimeout => e
+      Rails.logger.info("preview_niconico_getthumbinfo_fallback #{ { url: uri.to_s,
+                                                                     video_id:,
+                                                                     error: e.class.name,
+                                                                     message: e.message }.to_json }")
+      nil
+    rescue Nokogiri::XML::SyntaxError => e
+      Rails.logger.info("preview_niconico_getthumbinfo_fallback #{ { url: uri.to_s,
+                                                                     video_id:,
+                                                                     error: e.class.name,
+                                                                     message: e.message }.to_json }")
+      nil
+    end
+
+    def self.allowed_image_content_type?(content_type)
+      mime_type = content_type.to_s.split(';', 2).first.downcase.strip
+      ALLOWED_IMAGE_CONTENT_TYPES.include?(mime_type)
+    end
+
+    private_class_method :fetch_image_or_nil, :fetch_image!,
+                         :niconico_thumbnail_url,
+                         :allowed_image_content_type?
   end
 end

backend/app/services/preview/url_safety.rb

@@ -40,6 +40,8 @@ module Preview
       end
 
       [uri, parsed_addresses.map(&:to_s)]
+    rescue Resolv::ResolvError
+      raise UnsafeUrl, 'URL のホストを解決できません．'
     rescue URI::InvalidURIError, IPAddr::InvalidAddressError
       raise UnsafeUrl, 'URL が不正です．'
     end

backend/db/migrate/20260713000000_create_post_url_sanitisation_rules.rb

@@ -0,0 +1,61 @@
+class CreatePostUrlSanitisationRules < ActiveRecord::Migration[8.0]
+  class PostUrlSanitisationRule < ActiveRecord::Base
+    self.table_name = 'post_url_sanitisation_rules'
+  end
+
+  def up
+    create_table :post_url_sanitisation_rules, id: :integer, primary_key: :priority do |t|
+      t.string :source_pattern, null: false
+      t.string :replacement, null: false
+      t.timestamps
+      t.datetime :discarded_at
+
+      t.index :source_pattern, unique: true
+      t.index :discarded_at
+    end
+
+    now = Time.current
+
+    PostUrlSanitisationRule.insert_all!([
+        { priority: 10,
+          source_pattern: '\Ahttps?://youtu\.be/([^/?#]+)(?:[?#].*)?\z',
+          replacement: 'https://www.youtube.com/watch?v=\1',
+          created_at: now,
+          updated_at: now },
+        { priority: 20,
+          source_pattern: '\Ahttps?://(?:www\.|m\.)?youtube\.com/live/([^/?#]+)(?:[?#].*)?\z',
+          replacement: 'https://www.youtube.com/watch?v=\1',
+          created_at: now,
+          updated_at: now },
+        { priority: 30,
+          source_pattern: '\Ahttps?://(?:www\.|m\.)?youtube\.com/shorts/([^/?#]+)(?:[?#].*)?\z',
+          replacement: 'https://www.youtube.com/watch?v=\1',
+          created_at: now,
+          updated_at: now },
+        { priority: 40,
+          source_pattern: '\Ahttps?://(?:www\.|m\.)?youtube\.com/embed/([^/?#]+)(?:[?#].*)?\z',
+          replacement: 'https://www.youtube.com/watch?v=\1',
+          created_at: now,
+          updated_at: now },
+        { priority: 50,
+          source_pattern:
+              '\Ahttps?://(?:www\.|m\.)?youtube\.com/watch\?(?:[^#&]+&)*v=([^&#]+)(?:[&#].*)?\z',
+          replacement: 'https://www.youtube.com/watch?v=\1',
+          created_at: now,
+          updated_at: now },
+        { priority: 60,
+          source_pattern: '\Ahttps?://nico\.ms/([^/?#]+)(?:[?#].*)?\z',
+          replacement: 'https://www.nicovideo.jp/watch/\1',
+          created_at: now,
+          updated_at: now },
+        { priority: 70,
+          source_pattern: '\Ahttps?://(?:www\.)?nicovideo\.jp/watch/([^?#/]+)(?:[?#].*)?\z',
+          replacement: 'https://www.nicovideo.jp/watch/\1',
+          created_at: now,
+          updated_at: now }])
+  end
+
+  def down
+    drop_table :post_url_sanitisation_rules
+  end
+end

backend/db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.0].define(version: 2026_07_05_000000) do
+ActiveRecord::Schema[8.0].define(version: 2026_07_13_000000) do
   create_table "active_storage_attachments", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.string "name", null: false
     t.string "record_type", null: false
@@ -331,6 +331,16 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_05_000000) do
     t.index ["tag_id"], name: "index_post_tags_on_tag_id"
   end
 
+  create_table "post_url_sanitisation_rules", primary_key: "priority", id: :integer, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.string "source_pattern", null: false
+    t.string "replacement", null: false
+    t.datetime "created_at", null: false
+    t.datetime "updated_at", null: false
+    t.datetime "discarded_at"
+    t.index ["discarded_at"], name: "index_post_url_sanitisation_rules_on_discarded_at"
+    t.index ["source_pattern"], name: "index_post_url_sanitisation_rules_on_source_pattern", unique: true
+  end
+
   create_table "post_versions", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "post_id", null: false
     t.integer "version_no", null: false

backend/db/seeds.rb

@@ -8,6 +8,43 @@
 #     MovieGenre.find_or_create_by!(name: genre_name)
 #   end
 
+post_url_sanitisation_rules = [
+  { priority: 10,
+    source_pattern: '\Ahttps?://youtu\.be/([^/?#]+)(?:[?#].*)?\z',
+    replacement: 'https://www.youtube.com/watch?v=\1' },
+  { priority: 20,
+    source_pattern: '\Ahttps?://(?:www\.|m\.)?youtube\.com/live/([^/?#]+)(?:[?#].*)?\z',
+    replacement: 'https://www.youtube.com/watch?v=\1' },
+  { priority: 30,
+    source_pattern: '\Ahttps?://(?:www\.|m\.)?youtube\.com/shorts/([^/?#]+)(?:[?#].*)?\z',
+    replacement: 'https://www.youtube.com/watch?v=\1' },
+  { priority: 40,
+    source_pattern: '\Ahttps?://(?:www\.|m\.)?youtube\.com/embed/([^/?#]+)(?:[?#].*)?\z',
+    replacement: 'https://www.youtube.com/watch?v=\1' },
+  { priority: 50,
+    source_pattern:
+        '\Ahttps?://(?:www\.|m\.)?youtube\.com/watch\?(?:[^#&]+&)*v=([^&#]+)(?:[&#].*)?\z',
+    replacement: 'https://www.youtube.com/watch?v=\1' },
+  { priority: 60,
+    source_pattern: '\Ahttps?://nico\.ms/([^/?#]+)(?:[?#].*)?\z',
+    replacement: 'https://www.nicovideo.jp/watch/\1' },
+  { priority: 70,
+    source_pattern: '\Ahttps?://(?:www\.)?nicovideo\.jp/watch/([^?#/]+)(?:[?#].*)?\z',
+    replacement: 'https://www.nicovideo.jp/watch/\1' }
+]
+
+post_url_sanitisation_rule_scope = PostUrlSanitisationRule.unscoped
+
+post_url_sanitisation_rules.each do |attributes|
+  priority = attributes.fetch(:priority)
+  source_pattern = attributes.fetch(:source_pattern)
+
+  next if post_url_sanitisation_rule_scope.exists?(priority:)
+  next if post_url_sanitisation_rule_scope.exists?(source_pattern:)
+
+  post_url_sanitisation_rule_scope.create!(attributes)
+end
+
 material_sync_source_uri = ENV['MATERIAL_SYNC_SOURCE_URI']
 material_sync_source_file_id = ENV['MATERIAL_SYNC_SOURCE_FILE_ID']
 

backend/spec/db/seeds_spec.rb

@@ -0,0 +1,65 @@
+require 'rails_helper'
+
+RSpec.describe 'database seeds' do
+  before do
+    PostUrlSanitisationRule.unscoped.delete_all
+  end
+
+  it 'registers the initial post URL sanitisation rules' do
+    load_seeds
+
+    urls = {
+      'https://youtu.be/abc123?si=share' => youtube_url('abc123'),
+      'https://www.youtube.com/live/abc123?t=10' => youtube_url('abc123'),
+      'https://youtube.com/shorts/abc123?feature=share' => youtube_url('abc123'),
+      'https://m.youtube.com/embed/abc123' => youtube_url('abc123'),
+      'https://youtube.com/watch?feature=share&v=abc123&t=10' => youtube_url('abc123'),
+      'https://nico.ms/sm123?from=share#fragment' => nico_url('sm123'),
+      'https://www.nicovideo.jp/watch/sm123?ref=share#fragment' => nico_url('sm123')
+    }
+
+    expect(PostUrlSanitisationRule.count).to eq(7)
+    urls.each do |url, canonical_url|
+      expect(PostUrlSanitisationRule.sanitise(url)).to eq(canonical_url)
+    end
+  end
+
+  it 'does not overwrite or restore an existing rule' do
+    rule = PostUrlSanitisationRule.create!(
+      priority: 10,
+      source_pattern: '\Ahttps://example\.com/custom\z',
+      replacement: 'https://example.com/replacement'
+    )
+    rule.discard!
+    original_attributes = rule.reload.attributes
+
+    2.times { load_seeds }
+
+    persisted_rule = PostUrlSanitisationRule.unscoped.find(10)
+    expect(persisted_rule.attributes).to eq(original_attributes)
+    expect(PostUrlSanitisationRule.unscoped.count).to eq(7)
+  end
+
+  it 'does not duplicate a rule moved to another priority' do
+    source_pattern = '\Ahttps?://youtu\.be/([^/?#]+)(?:[?#].*)?\z'
+    PostUrlSanitisationRule.create!(
+      priority: 80,
+      source_pattern:,
+      replacement: 'https://example.com/custom/\1'
+    )
+
+    load_seeds
+
+    rules = PostUrlSanitisationRule.unscoped
+    expect(rules.where(source_pattern:).count).to eq(1)
+    expect(rules.find(80).replacement).to eq('https://example.com/custom/\1')
+  end
+
+  def load_seeds
+    load Rails.root.join('db/seeds.rb')
+  end
+
+  def youtube_url(video_id) = "https://www.youtube.com/watch?v=#{ video_id }"
+
+  def nico_url(video_id) = "https://www.nicovideo.jp/watch/#{ video_id }"
+end

backend/spec/models/post_spec.rb

@@ -0,0 +1,68 @@
+require 'rails_helper'
+
+RSpec.describe Post, type: :model do
+  before do
+    PostUrlSanitisationRule.unscoped.delete_all
+  end
+
+  describe 'URL normalisation' do
+    it 'normalises the HTTP URL before applying sanitisation rules' do
+      PostUrlSanitisationRule.create!(
+        priority: 10,
+        source_pattern: '\\Ahttps://example\\.com/videos/([^/]+)\\z',
+        replacement: 'https://example.com/watch/\\1'
+      )
+
+      post = described_class.create!(
+        title: 'normalised URL',
+        url: '  https://EXAMPLE.com/videos/123/  '
+      )
+
+      expect(post.url).to eq('https://example.com/watch/123')
+    end
+
+    it 'does not normalise an unchanged URL when another attribute changes' do
+      post = create(:post)
+      post.update_column(:url, 'https://EXAMPLE.com/unchanged/')
+
+      post.update!(title: 'updated title')
+
+      expect(post.reload.url).to eq('https://EXAMPLE.com/unchanged/')
+    end
+
+    it 'validates the sanitised URL length' do
+      path = 'a' * 375
+
+      PostUrlSanitisationRule.create!(
+        priority: 10,
+        source_pattern: '\\Ahttps://example\\.com/(a+)\\z',
+        replacement: 'https://example.com/\\1\\1'
+      )
+
+      post = described_class.new(
+        title: 'long URL',
+        url: "https://example.com/#{ path }"
+      )
+
+      expect(post).to be_invalid
+      expect(post.errors.details.fetch(:url)).to include(
+        error: :too_long,
+        count: 768
+      )
+    end
+
+    it 'validates uniqueness after sanitisation' do
+      PostUrlSanitisationRule.create!(
+        priority: 10,
+        source_pattern: '\\Ahttps://example\\.com/alias\\z',
+        replacement: 'https://example.com/canonical'
+      )
+      create(:post, url: 'https://example.com/canonical')
+
+      post = described_class.new(title: 'duplicate URL', url: 'https://example.com/alias')
+
+      expect(post).to be_invalid
+      expect(post.errors.details.fetch(:url)).to include(error: :taken, value: post.url)
+    end
+  end
+end

backend/spec/models/post_url_sanitisation_rule_spec.rb

@@ -0,0 +1,265 @@
+require 'rails_helper'
+
+RSpec.describe PostUrlSanitisationRule, type: :model do
+  before do
+    described_class.unscoped.delete_all
+  end
+
+  describe 'validations' do
+    it 'requires a source pattern' do
+      rule = described_class.new(priority: 10, source_pattern: nil, replacement: '')
+
+      expect(rule).to be_invalid
+      expect(rule.errors.details.fetch(:source_pattern)).to eq([{ error: :blank }])
+    end
+
+    it 'requires a unique source pattern' do
+      described_class.create!(priority: 10, source_pattern: 'source', replacement: 'first')
+      rule = described_class.new(
+        priority: 20,
+        source_pattern: 'source',
+        replacement: 'second'
+      )
+
+      expect(rule).to be_invalid
+      expect(rule.errors.details.fetch(:source_pattern)).to include(
+        error: :taken,
+        value: 'source'
+      )
+    end
+
+    it 'rejects an invalid regexp' do
+      rule = described_class.new(priority: 10, source_pattern: '[', replacement: '')
+
+      expect(rule).to be_invalid
+      expect(rule.errors[:source_pattern]).to include('変な正規表現だね〜（笑）')
+    end
+  end
+
+  describe '.sanitise' do
+    it 'applies each active rule once in priority order' do
+      described_class.create!(priority: 30, source_pattern: 'c', replacement: 'd')
+      described_class.create!(priority: 10, source_pattern: 'a', replacement: 'aa')
+      described_class.create!(priority: 20, source_pattern: 'aa', replacement: 'c')
+      discarded = described_class.create!(
+        priority: 5,
+        source_pattern: '.',
+        replacement: 'discarded'
+      )
+      discarded.discard!
+
+      expect(described_class.sanitise('a')).to eq('d')
+    end
+
+    it 'canonicalises the initial YouTube and Nico URL forms' do
+      create_initial_rules
+
+      urls = {
+        'https://youtu.be/abc123?si=share' => youtube_url('abc123'),
+        'https://www.youtube.com/live/abc123?t=10' => youtube_url('abc123'),
+        'https://youtube.com/shorts/abc123?feature=share' => youtube_url('abc123'),
+        'https://m.youtube.com/embed/abc123' => youtube_url('abc123'),
+        'https://youtube.com/watch?feature=share&v=abc123&t=10' => youtube_url('abc123'),
+        'https://nico.ms/sm123?from=share#fragment' => nico_url('sm123'),
+        'https://www.nicovideo.jp/watch/sm123?ref=share#fragment' => nico_url('sm123')
+      }
+
+      urls.each do |url, canonical_url|
+        expect(described_class.sanitise(url)).to eq(canonical_url)
+      end
+    end
+  end
+
+  describe '.apply!' do
+    it 'locks posts in ID order and updates through temporary URLs' do
+      first = create(:post, url: 'https://example.com/source/1')
+      second = create(:post, url: 'https://example.com/source/2')
+      create_source_rule
+      sql = capture_sql { described_class.apply! }
+
+      lock_sql = sql.find { _1.match?(/SELECT .*posts.*FOR UPDATE/i) }
+      expect(lock_sql).to match(/ORDER BY .*posts.*id.* ASC/i)
+      expect(sql.grep(/post-url-sanitising\.invalid/).size).to eq(2)
+      expect(first.reload.url).to eq('https://example.com/canonical/1')
+      expect(second.reload.url).to eq('https://example.com/canonical/2')
+    end
+
+    it 'loads and compiles rules only once' do
+      create(:post, url: 'https://example.com/source/1')
+      create(:post, url: 'https://example.com/source/2')
+      create_source_rule
+
+      expect(described_class).to receive(:rules).once.and_call_original
+
+      described_class.apply!
+    end
+
+    it 'does not change post versions, version_no, or updated_at' do
+      post = create(:post, url: 'https://example.com/source/1')
+      post.update_columns(version_no: 7, updated_at: 1.day.ago)
+      original_updated_at = post.reload.updated_at
+      create_source_rule
+
+      expect { described_class.apply! }.not_to change(PostVersion, :count)
+
+      post.reload
+      expect(post.url).to eq('https://example.com/canonical/1')
+      expect(post.version_no).to eq(7)
+      expect(post.updated_at).to eq(original_updated_at)
+    end
+
+    invalid_urls = {
+      'a blank URL' => '',
+      'an unparseable URL' => 'https://[',
+      'a non-HTTP URL' => 'ftp://example.com/file',
+      'an HTTP URL without a host' => 'https:/path'
+    }
+
+    invalid_urls.each do |description, sanitised_url|
+      it "rolls back every update when sanitisation produces #{ description }" do
+        valid_post = create(:post, url: 'https://example.com/source/1')
+        invalid_post = create(:post, url: 'https://example.com/invalid')
+        create_source_rule
+        described_class.create!(
+          priority: 20,
+          source_pattern: '\\Ahttps://example\\.com/invalid\\z',
+          replacement: sanitised_url
+        )
+
+        expect { described_class.apply! }
+            .to raise_error(described_class::InvalidUrlError) { |error|
+          expect(error.invalid_rows).to eq([
+            { post_id: invalid_post.id,
+              original_url: 'https://example.com/invalid',
+              sanitised_url: }
+          ])
+        }
+        expect(valid_post.reload.url).to eq('https://example.com/source/1')
+        expect(invalid_post.reload.url).to eq('https://example.com/invalid')
+      end
+    end
+
+    it 'rolls back every update when sanitisation produces a URL longer than 768 characters' do
+      path = 'a' * 375
+      original_url = "https://example.com/#{ path }"
+      sanitised_url = "https://example.com/#{ path }#{ path }"
+      valid_post = create(:post, url: 'https://example.com/source/1')
+      invalid_post = create(:post, url: original_url)
+      create_source_rule
+      described_class.create!(
+        priority: 20,
+        source_pattern: '\\Ahttps://example\\.com/(a+)\\z',
+        replacement: 'https://example.com/\\1\\1'
+      )
+
+      expect { described_class.apply! }
+          .to raise_error(described_class::InvalidUrlError) { |error|
+        expect(error.invalid_rows).to eq([
+          { post_id: invalid_post.id,
+            original_url:,
+            sanitised_url: }
+        ])
+      }
+      expect(valid_post.reload.url).to eq('https://example.com/source/1')
+      expect(invalid_post.reload.url).to eq(original_url)
+    end
+
+    it 'rejects sanitised URL collisions case-insensitively without changing posts' do
+      source = create(:post, url: 'https://example.com/source/Foo')
+      canonical = create(:post, url: 'https://example.com/canonical/foo')
+      create_source_rule
+
+      expect { described_class.apply! }
+          .to raise_error(described_class::UrlConflictError) { |error|
+        expect(error.conflicts).to contain_exactly(
+          { url: 'https://example.com/canonical/Foo',
+            post_id: source.id,
+            original_url: 'https://example.com/source/Foo' },
+          { url: 'https://example.com/canonical/foo',
+            post_id: canonical.id,
+            original_url: 'https://example.com/canonical/foo' }
+        )
+      }
+      expect(source.reload.url).to eq('https://example.com/source/Foo')
+      expect(canonical.reload.url).to eq('https://example.com/canonical/foo')
+      expect(Post.count).to eq(2)
+    end
+
+    it 'converts a persisted URL constraint race into UrlConflictError' do
+      post = create(:post, url: 'https://example.com/source/1')
+      create_source_rule
+      conflict = { url: 'https://example.com/canonical/1',
+                   post_id: post.id,
+                   original_url: post.url }
+      database_error = ActiveRecord::RecordNotUnique.new('duplicate URL')
+      allow_any_instance_of(ActiveRecord::Relation)
+          .to receive(:update_all).and_raise(database_error)
+      allow(described_class).to receive(:build_persisted_conflicts).and_return([conflict])
+
+      expect { described_class.apply! }
+          .to raise_error(described_class::UrlConflictError) { |error|
+        expect(error.conflicts).to eq([conflict])
+        expect(error.cause).to equal(database_error)
+      }
+      expect(post.reload.url).to eq('https://example.com/source/1')
+    end
+
+    it 're-raises an unidentified RecordNotUnique error' do
+      post = create(:post, url: 'https://example.com/source/1')
+      create_source_rule
+      database_error = ActiveRecord::RecordNotUnique.new('another unique constraint')
+      allow_any_instance_of(ActiveRecord::Relation)
+          .to receive(:update_all).and_raise(database_error)
+      allow(described_class).to receive(:build_persisted_conflicts).and_return([])
+
+      expect { described_class.apply! }.to raise_error(database_error)
+      expect(post.reload.url).to eq('https://example.com/source/1')
+    end
+  end
+
+  def capture_sql
+    statements = []
+    subscriber = lambda do |_name, _start, _finish, _id, payload|
+      binds = payload.fetch(:binds, []).map { _1.value_for_database.to_s }
+      statements << ([payload.fetch(:sql)] + binds).join(' ')
+    end
+    ActiveSupport::Notifications.subscribed(subscriber, 'sql.active_record') { yield }
+    statements
+  end
+
+  def create_source_rule
+    described_class.create!(
+      priority: 10,
+      source_pattern: '\\Ahttps://example\\.com/source/(.+)\\z',
+      replacement: 'https://example.com/canonical/\\1'
+    )
+  end
+
+  def create_initial_rules
+    rules = [
+      ['\\Ahttps?://youtu\\.be/([^/?#]+)(?:[?#].*)?\\z', youtube_url('\\1')],
+      ['\\Ahttps?://(?:www\\.|m\\.)?youtube\\.com/live/([^/?#]+)(?:[?#].*)?\\z',
+       youtube_url('\\1')],
+      ['\\Ahttps?://(?:www\\.|m\\.)?youtube\\.com/shorts/([^/?#]+)(?:[?#].*)?\\z',
+       youtube_url('\\1')],
+      ['\\Ahttps?://(?:www\\.|m\\.)?youtube\\.com/embed/([^/?#]+)(?:[?#].*)?\\z',
+       youtube_url('\\1')],
+      ['\\Ahttps?://(?:www\\.|m\\.)?youtube\\.com/watch\\?(?:[^#&]+&)*' \
+       'v=([^&#]+)(?:[&#].*)?\\z', youtube_url('\\1')],
+      ['\\Ahttps?://nico\\.ms/([^/?#]+)(?:[?#].*)?\\z', nico_url('\\1')],
+      ['\\Ahttps?://(?:www\\.)?nicovideo\\.jp/watch/([^?#/]+)(?:[?#].*)?\\z',
+       nico_url('\\1')]
+    ]
+    rules.each_with_index do |(source_pattern, replacement), index|
+      described_class.create!(
+        priority: (index + 1) * 10,
+        source_pattern:,
+        replacement:
+      )
+    end
+  end
+
+  def youtube_url(video_id) = "https://www.youtube.com/watch?v=#{ video_id }"
+
+  def nico_url(video_id) = "https://www.nicovideo.jp/watch/#{ video_id }"
+end

backend/spec/requests/preview_spec.rb

@@ -1,28 +1,46 @@
-require "rails_helper"
+require 'rails_helper'
 
 
-RSpec.describe "Preview", type: :request do
-  describe "GET /preview/title" do
-    it "401 unless logged in" do
+RSpec.describe 'Preview', type: :request do
+  describe 'GET /preview/title' do
+    it '401 unless logged in' do
       sign_out
-      get "/preview/title", params: { url: "example.com" }
+      get '/preview/title', params: { url: 'example.com' }
       expect(response).to have_http_status(:unauthorized)
     end
 
-    it "400 when url blank" do
-      sign_in_as(create(:user))
-      get "/preview/title", params: { url: "" }
+    it '403 when logged in as guest' do
+      sign_in_as(create(:user, :guest))
+      get '/preview/title', params: { url: 'example.com' }
+      expect(response).to have_http_status(:forbidden)
+    end
+
+    it '400 when url blank' do
+      sign_in_as(create(:user, :member))
+      get '/preview/title', params: { url: '' }
       expect(response).to have_http_status(:bad_request)
     end
 
-    it "returns parsed title (stubbing URI.open)" do
-      sign_in_as(create(:user))
-      fake_html = "<html><head><title>  Hello  </title></head></html>"
-      allow(URI).to receive(:open).and_return(StringIO.new(fake_html))
+    it 'returns parsed title' do
+      sign_in_as(create(:user, :member))
+      allow(Preview::ThumbnailFetcher)
+          .to receive(:title)
+          .with('example.com')
+          .and_return('Hello')
 
-      get "/preview/title", params: { url: "example.com" }
+      get '/preview/title', params: { url: 'example.com' }
       expect(response).to have_http_status(:ok)
-      expect(json["title"]).to eq("Hello")
+      expect(json['title']).to eq('Hello')
+    end
+
+    it '413 when fetched response is too large' do
+      sign_in_as(create(:user, :member))
+      allow(Preview::ThumbnailFetcher)
+          .to receive(:title)
+          .and_raise(Preview::HttpFetcher::ResponseTooLarge, '外部データが大きすぎます．')
+
+      get '/preview/title', params: { url: 'example.com' }
+      expect(response).to have_http_status(:payload_too_large)
     end
   end
 end

backend/spec/services/preview/http_fetcher_spec.rb

@@ -0,0 +1,23 @@
+require 'rails_helper'
+
+RSpec.describe Preview::HttpFetcher do
+  describe '.fetch' do
+    it 'raises FetchFailed when redirect location is invalid' do
+      redirect = Net::HTTPFound.new('1.1', '302', 'Found')
+      redirect['location'] = 'http://[invalid'
+
+      allow(Preview::UrlSafety).to receive(:validate)
+          .with('https://example.com/page')
+          .and_return([URI.parse('https://example.com/page'), ['203.0.113.10']])
+      allow(described_class).to receive(:request)
+          .and_return(redirect)
+      allow(Rails.logger).to receive(:warn)
+
+      expect {
+        described_class.fetch('https://example.com/page')
+      }.to raise_error(Preview::HttpFetcher::FetchFailed)
+      expect(Rails.logger).to have_received(:warn)
+          .with(/invalid_redirect_location/)
+    end
+  end
+end

backend/spec/services/preview/thumbnail_fetcher_spec.rb

@@ -0,0 +1,51 @@
+require 'rails_helper'
+
+RSpec.describe Preview::ThumbnailFetcher do
+  describe '.fetch' do
+    it 'rejects svg thumbnails' do
+      page = Preview::HttpFetcher::Response.new(
+          '<meta property="og:image" content="https://example.com/thumb.svg">',
+          'text/html',
+          'https://example.com/page')
+      svg = Preview::HttpFetcher::Response.new(
+          '<svg></svg>',
+          'image/svg+xml',
+          'https://example.com/thumb.svg')
+
+      allow(Preview::UrlSafety).to receive(:validate)
+          .and_return([URI.parse('https://example.com/page'), ['203.0.113.10']])
+      allow(Preview::HttpFetcher).to receive(:fetch)
+          .with('https://example.com/page', max_bytes: described_class::HTML_MAX_BYTES)
+          .and_return(page)
+      allow(Preview::HttpFetcher).to receive(:fetch)
+          .with('https://example.com/thumb.svg')
+          .and_return(svg)
+
+      expect {
+        described_class.fetch('https://example.com/page')
+      }.to raise_error(Preview::ThumbnailFetcher::GenerationFailed)
+    end
+
+    it 'accepts allowed image content type with parameters' do
+      page = Preview::HttpFetcher::Response.new(
+          '<meta property="og:image" content="https://example.com/thumb.jpg">',
+          'text/html',
+          'https://example.com/page')
+      image = Preview::HttpFetcher::Response.new(
+          'jpeg-bytes',
+          'image/jpeg; charset=binary',
+          'https://example.com/thumb.jpg')
+
+      allow(Preview::UrlSafety).to receive(:validate)
+          .and_return([URI.parse('https://example.com/page'), ['203.0.113.10']])
+      allow(Preview::HttpFetcher).to receive(:fetch)
+          .with('https://example.com/page', max_bytes: described_class::HTML_MAX_BYTES)
+          .and_return(page)
+      allow(Preview::HttpFetcher).to receive(:fetch)
+          .with('https://example.com/thumb.jpg')
+          .and_return(image)
+
+      expect(described_class.fetch('https://example.com/page')).to eq('jpeg-bytes')
+    end
+  end
+end

backend/spec/services/preview/url_safety_spec.rb

@@ -0,0 +1,15 @@
+require 'rails_helper'
+
+RSpec.describe Preview::UrlSafety do
+  describe '.validate' do
+    it 'raises UnsafeUrl when DNS resolution fails' do
+      allow(Resolv).to receive(:getaddresses)
+          .with('missing.example')
+          .and_raise(Resolv::ResolvError)
+
+      expect {
+        described_class.validate('https://missing.example')
+      }.to raise_error(Preview::UrlSafety::UnsafeUrl)
+    end
+  end
+end

backend/spec/tasks/nico_export_spec.rb

@@ -62,7 +62,7 @@ RSpec.describe 'nico:export' do
 
     it 'deduplicates video ids' do
       create_post('https://www.nicovideo.jp/watch/sm12345')
-      create_post('https://www.nicovideo.jp/watch/sm12345?from=1')
+      create_post('https://sp.nicovideo.jp/watch/sm12345')
 
       expect(Open3).to receive(:capture3) do |_env, *args, **_kwargs|
         expect(args.drop(3)).to eq(['sm12345'])

frontend/src/index.css

@@ -239,6 +239,11 @@ body
   background: var(--top-nav-submenu-bg);
 }
 
+.top-nav-mobile-menu .top-nav-submenu
+{
+  background: var(--top-nav-mobile-active-bg);
+}
+
 .top-nav-mobile-menu
 {
   background: var(--top-nav-mobile-menu-bg);

frontend/src/index.test.ts

@@ -0,0 +1,22 @@
+import { readFileSync } from 'node:fs'
+import { resolve } from 'node:path'
+
+import { describe, expect, it } from 'vitest'
+
+
+const indexCss = readFileSync (
+    resolve (process.cwd (), 'src/index.css'),
+    'utf8')
+const compactIndexCss = indexCss.replace (/\s+/g, ' ')
+const mobileActiveRule = '.top-nav-mobile-active {'
+    + ' background: var(--top-nav-mobile-active-bg); }'
+const mobileSubmenuRule = '.top-nav-mobile-menu .top-nav-submenu {'
+    + ' background: var(--top-nav-mobile-active-bg); }'
+
+
+describe ('TopNav mobile menu colours', () => {
+  it ('uses one background variable for active rows and expanded submenus', () => {
+    expect (compactIndexCss).toContain (mobileActiveRule)
+    expect (compactIndexCss).toContain (mobileSubmenuRule)
+  })
+})

frontend/src/lib/settings.test.ts

@@ -89,8 +89,12 @@ describe ('settings', () => {
   })
 
   it ('derives TopNav colours without mixing mobile active backgrounds', () => {
-    expect (buildThemeTokens ('light', { }).topNavMobileActiveBackground).toBe (
+    const tokens = buildThemeTokens ('light', { })
+
+    expect (tokens.topNavMobileActiveBackground).toBe (
 	LIGHT_THEME_TOKENS.topNavRootBackgroundDesktop)
+    expect (tokens.topNavMobileActiveBackground).not.toBe (
+	tokens.topNavRootBackgroundMobile)
     expect (buildThemeTokens ('dark', { }).topNavMobileActiveBackground).toBe (
 	DARK_THEME_TOKENS.topNavActiveBackground)
   })