hometown/app/helpers/jsonld_helper.rb

# frozen_string_literal: true

module JsonLdHelper
  include ContextHelper

  def equals_or_includes?(haystack, needle)
    haystack.is_a?(Array) ? haystack.include?(needle) : haystack == needle
  end

  def equals_or_includes_any?(haystack, needles)
    needles.any? { |needle| equals_or_includes?(haystack, needle) }
  end

  def first_of_value(value)
    value.is_a?(Array) ? value.first : value
  end

  def uri_from_bearcap(str)
    if str&.start_with?('bear:')
      Addressable::URI.parse(str).query_values['u']
    else
      str
    end
  end

  # The url attribute can be a string, an array of strings, or an array of objects.
  # The objects could include a mimeType. Not-included mimeType means it's text/html.
  def url_to_href(value, preferred_type = nil)
    single_value = begin
      if value.is_a?(Array) && !value.first.is_a?(String)
        value.find { |link| preferred_type.nil? || ((link['mimeType'].presence || 'text/html') == preferred_type) }
      elsif value.is_a?(Array)
        value.first
      else
        value
      end
    end

    if single_value.nil? || single_value.is_a?(String)
      single_value
    else
      single_value['href']
    end
  end

  def as_array(value)
    if value.nil?
      []
    elsif value.is_a?(Array)
      value
    else
      [value]
    end
  end

  def value_or_id(value)
    value.is_a?(String) || value.nil? ? value : value['id']
  end

  def supported_context?(json)
    !json.nil? && equals_or_includes?(json['@context'], ActivityPub::TagManager::CONTEXT)
  end

  def unsupported_uri_scheme?(uri)
    uri.nil? || !uri.start_with?('http://', 'https://')
  end

  def invalid_origin?(url)
    return true if unsupported_uri_scheme?(url)

    needle   = Addressable::URI.parse(url).host
    haystack = Addressable::URI.parse(@account.uri).host

    !haystack.casecmp(needle).zero?
  end

  def canonicalize(json)
    graph = RDF::Graph.new << JSON::LD::API.toRdf(json, documentLoader: method(:load_jsonld_context))
    graph.dump(:normalize)
  end

  def compact(json)
    compacted = JSON::LD::API.compact(json.without('signature'), full_context, documentLoader: method(:load_jsonld_context))
    compacted['signature'] = json['signature']
    compacted
  end

  # Patches a JSON-LD document to avoid compatibility issues on redistribution
  #
  # Since compacting a JSON-LD document against Mastodon's built-in vocabulary
  # means other extension namespaces will be expanded, malformed JSON-LD
  # attributes lost, and some values “unexpectedly” compacted this method
  # patches the following likely sources of incompatibility:
  # - 'https://www.w3.org/ns/activitystreams#Public' being compacted to
  #   'as:Public' (for instance, pre-3.4.0 Mastodon does not understand
  #   'as:Public')
  # - single-item arrays being compacted to the item itself (`[foo]` being
  #   compacted to `foo`)
  #
  # It is not always possible for `patch_for_forwarding!` to produce a document
  # deemed safe for forwarding. Use `safe_for_forwarding?` to check the status
  # of the output document.
  #
  # @param original [Hash] The original JSON-LD document used as reference
  # @param compacted [Hash] The compacted JSON-LD document to be patched
  # @return [void]
  def patch_for_forwarding!(original, compacted)
    original.without('@context', 'signature').each do |key, value|
      next if value.nil? || !compacted.key?(key)

      compacted_value = compacted[key]
      if value.is_a?(Hash) && compacted_value.is_a?(Hash)
        patch_for_forwarding!(value, compacted_value)
      elsif value.is_a?(Array)
        compacted_value = [compacted_value] unless compacted_value.is_a?(Array)
        return if value.size != compacted_value.size

        compacted[key] = value.zip(compacted_value).map do |v, vc|
          if v.is_a?(Hash) && vc.is_a?(Hash)
            patch_for_forwarding!(v, vc)
            vc
          elsif v == 'https://www.w3.org/ns/activitystreams#Public' && vc == 'as:Public'
            v
          else
            vc
          end
        end
      elsif value == 'https://www.w3.org/ns/activitystreams#Public' && compacted_value == 'as:Public'
        compacted[key] = value
      end
    end
  end

  # Tests whether a JSON-LD compaction is deemed safe for redistribution,
  # that is, if it doesn't change its meaning to consumers that do not actually
  # handle JSON-LD, but rely on values being serialized in a certain way.
  #
  # See `patch_for_forwarding!` for details.
  #
  # @param original [Hash] The original JSON-LD document used as reference
  # @param compacted [Hash] The compacted JSON-LD document to be patched
  # @return [Boolean] Whether the patched document is deemed safe
  def safe_for_forwarding?(original, compacted)
    original.without('@context', 'signature').all? do |key, value|
      compacted_value = compacted[key]
      return false unless value.class == compacted_value.class

      if value.is_a?(Hash)
        safe_for_forwarding?(value, compacted_value)
      elsif value.is_a?(Array)
        value.zip(compacted_value).all? do |v, vc|
          v.is_a?(Hash) ? (vc.is_a?(Hash) && safe_for_forwarding?(v, vc)) : v == vc
        end
      else
        value == compacted_value
      end
    end
  end

  def fetch_resource(uri, id_is_known, on_behalf_of = nil)
    unless id_is_known
      json = fetch_resource_without_id_validation(uri, on_behalf_of)

      return if !json.is_a?(Hash) || unsupported_uri_scheme?(json['id'])

      uri = json['id']
    end

    json = fetch_resource_without_id_validation(uri, on_behalf_of)
    json.present? && json['id'] == uri ? json : nil
  end

  def fetch_resource_without_id_validation(uri, on_behalf_of = nil, raise_on_temporary_error = false)
    on_behalf_of ||= Account.representative

    build_request(uri, on_behalf_of).perform do |response|
      raise Mastodon::UnexpectedResponseError, response unless response_successful?(response) || response_error_unsalvageable?(response) || !raise_on_temporary_error

      body_to_json(response.body_with_limit) if response.code == 200 && valid_activitypub_content_type?(response)
    end
  end

  def valid_activitypub_content_type?(response)
    return true if response.mime_type == 'application/activity+json'

    # When the mime type is `application/ld+json`, we need to check the profile,
    # but `http.rb` does not parse it for us.
    return false unless response.mime_type == 'application/ld+json'

    response.headers[HTTP::Headers::CONTENT_TYPE]&.split(';')&.map(&:strip)&.any? do |str|
      str.start_with?('profile="') && str[9...-1].split.include?('https://www.w3.org/ns/activitystreams')
    end
  end

  def body_to_json(body, compare_id: nil)
    json = body.is_a?(String) ? Oj.load(body, mode: :strict) : body

    return if compare_id.present? && json['id'] != compare_id

    json
  rescue Oj::ParseError
    nil
  end

  def merge_context(context, new_context)
    if context.is_a?(Array)
      context << new_context
    else
      [context, new_context]
    end
  end

  def response_successful?(response)
    (200...300).cover?(response.code)
  end

  def response_error_unsalvageable?(response)
    response.code == 501 || ((400...500).cover?(response.code) && ![401, 408, 429].include?(response.code))
  end

  def build_request(uri, on_behalf_of = nil)
    Request.new(:get, uri).tap do |request|
      request.on_behalf_of(on_behalf_of) if on_behalf_of
      request.add_headers('Accept' => 'application/activity+json, application/ld+json')
    end
  end

  def load_jsonld_context(url, _options = {}, &_block)
    json = Rails.cache.fetch("jsonld:context:#{url}", expires_in: 30.days, raw: true) do
      request = Request.new(:get, url)
      request.add_headers('Accept' => 'application/ld+json')
      request.perform do |res|
        raise JSON::LD::JsonLdError::LoadingDocumentFailed unless res.code == 200 && res.mime_type == 'application/ld+json'

        res.body_with_limit
      end
    end

    doc = JSON::LD::API::RemoteDocument.new(json, documentUrl: url)

    block_given? ? yield(doc) : doc
  end
end