Add ActivityPub inbox (#4216)

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests

app/lib/activitypub/activity/create.rb

@@ -0,0 +1,148 @@
+# frozen_string_literal: true
+
+class ActivityPub::Activity::Create < ActivityPub::Activity
+  def perform
+    return if delete_arrived_first?(object_uri) || unsupported_object_type?
+
+    status = Status.find_by(uri: object_uri)
+
+    return status unless status.nil?
+
+    ApplicationRecord.transaction do
+      status = Status.create!(status_params)
+
+      process_tags(status)
+      process_attachments(status)
+    end
+
+    resolve_thread(status)
+    distribute(status)
+
+    status
+  end
+
+  private
+
+  def status_params
+    {
+      uri: @object['id'],
+      url: @object['url'],
+      account: @account,
+      text: text_from_content || '',
+      language: language_from_content,
+      spoiler_text: @object['summary'] || '',
+      created_at: @object['published'] || Time.now.utc,
+      reply: @object['inReplyTo'].present?,
+      sensitive: @object['sensitive'] || false,
+      visibility: visibility_from_audience,
+      thread: replied_to_status,
+      conversation: conversation_from_uri(@object['_:conversation']),
+    }
+  end
+
+  def process_tags(status)
+    return unless @object['tag'].is_a?(Array)
+
+    @object['tag'].each do |tag|
+      case tag['type']
+      when 'Hashtag'
+        process_hashtag tag, status
+      when 'Mention'
+        process_mention tag, status
+      end
+    end
+  end
+
+  def process_hashtag(tag, status)
+    hashtag = tag['name'].gsub(/\A#/, '').mb_chars.downcase
+    hashtag = Tag.where(name: hashtag).first_or_initialize(name: hashtag)
+
+    status.tags << hashtag
+  end
+
+  def process_mention(tag, status)
+    account = account_from_uri(tag['href'])
+    account = ActivityPub::FetchRemoteAccountService.new.call(tag['href']) if account.nil?
+    return if account.nil?
+    account.mentions.create(status: status)
+  end
+
+  def process_attachments(status)
+    return unless @object['attachment'].is_a?(Array)
+
+    @object['attachment'].each do |attachment|
+      next if unsupported_media_type?(attachment['mediaType'])
+
+      href             = Addressable::URI.parse(attachment['url']).normalize.to_s
+      media_attachment = MediaAttachment.create(status: status, account: status.account, remote_url: href)
+
+      next if skip_download?
+
+      media_attachment.file_remote_url = href
+      media_attachment.save
+    end
+  end
+
+  def resolve_thread(status)
+    return unless status.reply? && status.thread.nil?
+    ActivityPub::ThreadResolveWorker.perform_async(status.id, @object['inReplyTo'])
+  end
+
+  def conversation_from_uri(uri)
+    return nil if uri.nil?
+    return Conversation.find_by(id: TagManager.instance.unique_tag_to_local_id(uri, 'Conversation')) if TagManager.instance.local_id?(uri)
+    Conversation.find_by(uri: uri) || Conversation.create!(uri: uri)
+  end
+
+  def visibility_from_audience
+    if equals_or_includes?(@object['to'], ActivityPub::TagManager::COLLECTIONS[:public])
+      :public
+    elsif equals_or_includes?(@object['cc'], ActivityPub::TagManager::COLLECTIONS[:public])
+      :unlisted
+    elsif equals_or_includes?(@object['to'], @account.followers_url)
+      :private
+    else
+      :direct
+    end
+  end
+
+  def audience_includes?(account)
+    uri = ActivityPub::TagManager.instance.uri_for(account)
+    equals_or_includes?(@object['to'], uri) || equals_or_includes?(@object['cc'], uri)
+  end
+
+  def replied_to_status
+    return if @object['inReplyTo'].blank?
+    @replied_to_status ||= status_from_uri(@object['inReplyTo'])
+  end
+
+  def text_from_content
+    if @object['content'].present?
+      @object['content']
+    elsif language_map?
+      @object['contentMap'].values.first
+    end
+  end
+
+  def language_from_content
+    return nil unless language_map?
+    @object['contentMap'].keys.first
+  end
+
+  def language_map?
+    @object['contentMap'].is_a?(Hash) && !@object['contentMap'].empty?
+  end
+
+  def unsupported_object_type?
+    @object.is_a?(String) || !%w(Article Note).include?(@object['type'])
+  end
+
+  def unsupported_media_type?(mime_type)
+    mime_type.present? && !(MediaAttachment::IMAGE_MIME_TYPES + MediaAttachment::VIDEO_MIME_TYPES).include?(mime_type)
+  end
+
+  def skip_download?
+    return @skip_download if defined?(@skip_download)
+    @skip_download ||= DomainBlock.find_by(domain: @account.domain)&.reject_media?
+  end
+end